Lesson Proper for Week 13

THE APPENDICULAR SKELETON

The appendicular skeleton includes all bones of the upper and lower limbs, plus the bones that
attach each limb to the axial skeleton. There are 126 bones in the appendicular skeleton of an adult.

Bones of the Shoulder Girdle

Each shoulder girdle, or pectoral girdle, consists of two bones – a clavicle and a scapula.

 Clavicle. The clavicle, or collarbone, is a slender, doubly curved bone; it attaches to the
manubrium of the sternum medially and to the scapula laterally, where it helps to form the
shoulder joint; it acts as a brace to hold the arm away from the top of the thorax and helps
prevent shoulder dislocation.
 Scapulae. The scapulae, or shoulder blades, are triangular and commonly called
“wings” because they flare when we move our arms posteriorly.
  Parts of the scapula. Each scapula has a flattened body and two important processes-
the acromion and the coracoid.
 Acromion. The acromion is the enlarged end of the spine of the scapula and connects
with the clavicle laterally at the acromioclavicular joint.
 Coracoid. The beaklike coracoid process points over the top of the shoulder and anchors
some of the muscles of the arm; just medial to the coracoid process is the
large suprascapular notch, which serves as a nerve passageway.
 Borders of the scapula. The scapula has three borders- superior, medial (vertebral), and
lateral (axillary).
 Angles of the scapula. It also has three angles- superior, inferior, and lateral;
the glenoid cavity, a shallow socket that receives the head of the arm bone, is in the
lateral angle.
 Factors to free movement of the shoulder girdle. Each shoulder girdle attaches to
the axial skeleton at only one point- the sternoclavicular joint; the loose attachment of
the scapula allows it to slide back and forth against the thorax as muscles act; and, the
glenoid cavity is shallow, and the shoulder joint is poorly reinforced by ligaments.

Bones of the Upper Limb

Thirty separate bones form the skeletal framework of each upper limb; they form the foundations of
the arm, forearm, and hand.
Arm
The arm is formed by a single bone, the humerus, which is a typical long bone.

 Anatomical neck. Immediately inferior to the head is a slight constriction called

anatomical neck.
 Tubercles. Anterolateral to the head are two bony projections separated by
the intertubercular sulcus– the greater and lesser tubercles, which are sites
of muscle attachment.
 Surgical neck. Just distal to the tubercles is the surgical neck, so named because it is the
most frequently fractured part of the humerus.
 Deltoid tuberosity. In the midpoint of the shaft is a roughened area called the deltoid
tuberosity, where the large, fleshy deltoid muscle of the shoulder attaches.
 Radial groove. Nearby, the radial groove runs obliquely down the posterior aspect of the
shaft; this groove marks the course of the radial nerve, an important nerve of the upper
limb.
 Trochlea and capitulum. At the distal end of the humerus is the medial trochlea, which
looks somewhat like a spool, and the lateral ball-like capitulum; both of these processes
articulate with the bones of the forearm.
  Fossa. Above the trochlea anteriorly is a depression, the coronoid fossa; on the
posterior surface is the olecranon fossa; these two depressions, which are flanked
by medial and lateral epicondyles, allow the corresponding processes of the ulna to
move freely when the elbow is bent and extended.

Forearm
Two bones, the radius, and the ulna, form the skeleton of the forearm.

 Radius. When the body is in the anatomical position, the radius is the lateral bone; that is,
it is on the thumb side of the forearm; when the hand is rotated so that the palm faces
backward, the distal end of the radius crosses over and ends up medial to the ulna.
 Radioulnar Joints. Both proximally and distally the radius and ulna articulate at small
radioulnar joints and the two bones are connected along their entire length by the
flexible interosseous membrane.
 Styloid process. Both the ulna and the radius have as styloid process at their distal end.
 Radial tuberosity. The disc-shaped head of the radius also forms a joint with the
capitulum of the humerus; just below the head is the radial tuberosity, where the tendon of
the biceps muscle attaches.
 Ulna. When the upper limb is in the anatomical position, the ulna is the medial bone (on
the little-finger side) of the forearm.
 Trochlear notch. On its proximal end are the coronoid process and the posterior
olecranon process, which are separated by the trochlear notch; together, these two
processes grip the trochlea of the humerus in a pliers-like joint.

Hand
The skeleton of the hand consists of carpals, the metacarpals, and the phalanges.

 Carpal bones. The eight carpal bones, arranged in two irregular rows of four bones each,
form the part of the hand called carpus, or, more commonly, the wrist; the carpals are
bound together by ligaments that restrict movements between them.
 Metacarpals. The metacarpals are numbered 1 to 5 from the thumb side of the hand to
the little finger; when the fist is clenched, the heads of the metacarpals become obvious as
the “knuckles“.
 Phalanges. The phalanges are the bones of the fingers; each hand contains 14
phalanges; there are three in each finger (proximal, middle, and distal), except in the
thumb, which has only two )proximal and distal.

Bones of the Pelvic Girdle

The pelvic girdle is formed by two coxal bones, or ossa coxae, commonly called hip bones.
 Pelvic girdle. The bones of the pelvic girdle are large and heavy, and they are attached
securely to the axial skeleton; bearing weight is the most important function of this girdle
because the total weight of the upper body rests on the bony pelvis.
 Sockets. The sockets, which receives the thigh bones, are deep and heavily reinforced by
ligaments that attach the limbs firmly to the girdle.
 Bony pelvis. The reproductive organs, urinary bladder, and part of the large intestine lie
within and are protected by the bony pelvis.
 Ilium. The ilium, which connects posteriorly with the sacrum at the sacroiliac joint, is a
large, flaring bone that forms most of the hip bone; when you put your hands on your hips,
they are resting over the alae, or winglike portions, of the ilia.
 Iliac crest. The upper edge of an ala, the iliac crest, is an important anatomical landmark
that is always kept in mind by those who give intramuscular injections; the iliac crest ends
anteriorly in the anterior superior iliac spine and posteriorly in the posterior superior
iliac spine.
 Ischium. The ischium is the “sit-down” bone, so called because it forms the most inferior
part of the coxal bone.
 Ischial tuberosity. The ischial tuberosity is a roughened area that receives weight when
you are sitting.
 Ischial spine. The ischial spine, superior to the tuberosity, is another important
anatomical landmark, particularly in pregnant women, because it narrows the outlet of the
pelvis through which the baby must pass during the birth process.
 Greater sciatic notch. Another important structural feature of the ischium is the greater
sciatic notch, which allows blood vessels and the large sciatic nerve to pass from the pelvis
posteriorly into the thigh.
 Pubis. The pubis, or pubic bone, is the most anterior part of the coxal bone.
 Obturator foramen. An opening that allows blood vessels and nerves to pass into the
anterior part of the thigh.
 Pubic symphysis. The pubic bones of each hip bones fuse anteriorly to form a
cartilaginous joint, the pubic symphysis.
 Acetabulum. The ilium, ischium, and pubis fuse at a deep socket called the acetabulum,
which means “vinegar cup”; the acetabulum receives the head of the thigh bone.
 False pelvis. The false pelvis is superior to the true pelvis; it is the area medial to the
flaring portions of the ilia.
 True pelvis. The true pelvis is surrounded by bone and lies inferior to the flaring parts of
the ilia and the pelvic brim; the dimensions of the true pelvis of the woman are very
important because they must be large enough to allow the infant’s head to pass during
childbirth.
 Outlet and inlet. The dimensions of the cavity, particularly the outlet (the inferior
opening of the pelvis measured between the ischial spines, and the inlet (superior opening
between the right and left sides of the pelvic brim) are critical, and thus they are carefully
measured by the obstetrician.
Bones of the Lower Limbs
The lower limbs carry the total body weight when we are erect; hence, it is not surprising that the
bones forming the three segments of the lower limbs (thigh, leg, and foot) are much thicker and
stronger than the comparable bones of the upper limb.

Thigh
The femur, or thigh bone, is the only bone in the thigh; it is the heaviest, strongest bone in the body.

 Parts. Its proximal end has a ball-like head, a neck, and greater and lesser
trochanters (separated anteriorly by the intertrochanteric line and posteriorly by
the intertrochanteric crest).
 Gluteal tuberosity. These markings and the gluteal tuberosity, located on the shaft, all
serve as sites for muscle attachment.
 Head. The head of the femur articulates with the acetabulum of the hip bone in a deep,
secure socket.
 Neck. However, the neck of the femur is a common fracture site, especially in old age.
 Lateral and medial condyles. Distally on the femur are the lateral and medial condyles,
which articulate with the tibia below; posteriorly these condyles are separated by the
deep intercondylar fossa.
 Patellar surface. Anteriorly on the distal femur is the smooth patellar surface, which
forms a joint with the patella, or kneecap.

Leg
Connected along their length by an interosseous membrane, two bones, the tibia and fibula, form
the skeleton of the leg.
  Tibia. The tibia, or shinbone, is larger and more medial; at the proximal end, the medial
and lateral condyles articulate with the distal end of the femur to form the knee joint.
 Tibial tuberosity. The patellar (kneecap) ligament attaches to the tibial tuberosity, a
roughened area on the anterior tibial surface.
 Medial malleolus. Distally, a process called medial malleolus forms the inner bulge of the
ankle.
 Anterior border. The anterior surface of the tibia is a sharp ridge, the anterior border,
that is unprotected by the muscles; thus, it is easily felt beneath the skin.
 Fibula. The fibula, which lies along the tibia and forms joints with it both proximally and
distally, is thin and sticklike; the fibula has no part in forming the knee joint.
 Lateral malleolus. Its distal end, the lateral malleolus, forms the outer part of the ankle.

Foot
The foot, composed of the tarsals, metatarsals, and phalanges, has two important functions.; it
supports our body weight and serves as a lever that allows us to propel our bodies forward when we
walk and run.

 Tarsus. the tarsus, forming the posterior half of the foot, is composed of seven tarsal
bones.
 Calcaneus and Talus. Body weight is carried mostly by the two largest tarsals, the
calcaneus, or heel bone, and the talus (ankle), which lies between the tibia and the
calcaneus.
 Metatarsals. Five metatarsals form the sole.
 Phalanges. 14 phalanges form the toes; each toe has three phalanges, except the great
toe, which has two.
 Arches. The bones in the foot are arranged to form three strong arches: two longitudinal
(medial and lateral) and one transverse.

JOINTS
Joints, also called articulations, have two functions: they hold the bones together securely, but also
give the rigid skeleton mobility.

 Classification. Joints are classified in two ways- functionally and structurally.

 Functional classification. The functional classification focuses on the amount of
movement the joint allows.
 Types of functional joints. There are synarthroses or immovable
joints; amphiarthroses, or slightly movable joints, and diarthrosis, or freely movable
joints.
 Diarthroses. Freely movable joints predominate in the limbs, where mobility is important.
 Synarthroses and amphiarthroses. Immovable and slightly movable joints are
restricted mainly to the axial skeleton, where firm attachments and protection of internal
organs are priorities.
  Structural classification. Structurally, there are fibrous, cartilaginous,
and synovial joints; these classifications are based on whether fibrous tissue, cartilage, or
a joint cavity separates the bony regions at the joint.

Fibrous Joints
In fibrous joints, the bones are united by fibrous tissue.

 Examples. The best examples of this type of joint are the sutures of the skull; in sutures,
the irregular edges of the bones interlock and are bound tightly together by connective
tissue fibers, allowing essentially no movement.
 Syndesmoses. In syndesmoses, the connecting fibers are longer than those of sutures;
thus the joint has more “give”; the joint connecting the distal ends of the tibia and fibula is
a syndesmosis.

Cartilaginous Joints
In cartilaginous joints, the bone ends are connected by cartilage.

 Examples. Examples of this joint type that are slightly movable are the pubic
symphysis of the pelvis and the intervertebral joints of the spinal column, where the
articulating bone surfaces are connected by pads (discs) of fibrocartilage.
 Synarthrotic cartilaginous joints. The hyaline cartilage epiphyseal plates of growing
long bones and the cartilaginous joints between the first ribs and the sternum are
immovable cartilaginous joints.

Synovial Joints
Synovial joints are joints in which the articulating bone ends are separated by a joint cavity containing
a synovial fluid; they account for all joints of the limbs.
  Articular cartilage. Articular cartilage covers the ends of the bones forming the joints.
 Fibrous articular capsule. The joint surfaces are enclosed by a sleeve or a capsule of
fibrous connective tissue, and their capsule is lined with a smooth synovial
membrane (the reason these joints are called synovial joints).
 Joint cavity. The articular capsule encloses a cavity, called the joint cavity, which
contains lubricating synovial fluid.
 Reinforcing ligaments. The fibrous capsule is usually reinforced with ligaments.
 Bursae. Bursae are flattened fibrous sacs lined with synovial membrane and containing a
thin film of synovial fluid; they are common where ligaments, muscles, skin, tendons, or
bones rub together.
 Tendon sheath. A tendon sheath is essentially an elongated bursa that wraps completely
around a tendon subjected to friction, like a bun around a hotdog.

TYPES OF SYNOVIAL JOINTS BASED ON SHAPE

The shapes of the articulating bone surfaces determine what movements are allowed at a joint; based
on such shapes, our synovial joints can be classified as plane, hinge, pivot, condyloid, saddle, and ball-
and-socket joints.
  Plane joint. In a plane joint, the articular surfaces are essentially flat, and only short
slipping or gliding movements are allowed; the movements of plane joints are nonaxial,
that is, gliding does not involve rotation around any axis; the intercarpal joints of the wrist
are best examples of plane joints.
 Hinge joint. In a hinge joint, the cylindrical end of one bone fits into a trough-shaped
surface on another bone; angular movement is allowed in just one plane, like a mechanical
hinge; hinge joints are classified as uniaxial; they allow movement in only one axis, and
examples are the elbow joint, ankle joint, and the joints between the phalanges of the
fingers.
 Pivot joint. In a pivot joint, the rounded end of one bone fits into a sleeve or ring of bone;
because the rotating bone can turn only around its long axis, pivot joints are also uniaxial
joints; the proximal radioulnar joint and the joint between the atlas and the dens of the
axis are examples.
 Condyloid joint. In a condyloid joint, the egg-shaped articular surface fits into an oval
concavity in another; condyloid joints allow the moving bone to travel (1) from side to side
and (2) back and forth but the bone cannot rotate around its long axis; movement occurs
around two axes, hence these are biaxial joints.
 Saddle joints. In saddle joints, each articular surface has both convex and concave areas,
like a saddle; these biaxial joints allow essentially the same movements as condyloid
joints; the best examples of saddle joints are the carpometacarpal joints in the thumb.
 Ball-and-socket joint. In a ball-and-socket joint, the spherical head of one bone fits into
a round socket in another; these multiaxial joints allow movement in all axes, including
rotation, and are the most freely moving synovial joints; the shoulder and hip are
examples.

Lesson Proper for Week 14

MUSCULAR SYSTEM
Functions of the Muscular System
Producing movement is a common function of all muscle types, but skeletal muscle plays three
other important roles in the body as well.

1. Producing movement. Mobility of the body as a whole reflects the activity of the
skeletal muscles, which are responsible for all locomotion; they enable us to
respond quickly to changes in the external environment.
2. Maintaining posture. We are rarely aware of the skeletal muscles that maintain
body posture, yet they function almost continuously, making one tiny adjustment
after another so that we can maintain an erect or seated posture despite the never-
ending downward pull of gravity.
3. Stabilizing joints. As the skeletal muscles pull on bones to cause movements, they
also stabilize the joints of the skeleton; muscle tendons are extremely important in
reinforcing and stabilizing joints that have poorly fitting articulating surfaces.
 4. Generating heat. The fourth function of muscle, generation of body heat, is a by-
product of muscle activity; as ATP is used to power muscle contraction, nearly
three-quarters of its energy escape as heat and this heat is vital in maintaining
normal body temperature.

Anatomy of the Muscular System

Microscopic Anatomy of Skeletal Muscle
Skeletal muscle cells are multinucleate.

 Sarcolemma. Many oval nuclei can be seen just beneath the plasma membrane,
which is called the sarcolemma in muscle cells.
 Myofibrils. The nuclei are pushed aside by long ribbonlike organelles, the
myofibrils, which nearly fill the cytoplasm.
 Light and dark bands. Alternating dark and light bands along the length of the
perfectly aligned myofibrils give the muscle cell as a whole its striped appearance.
 Sarcomeres. The myofibrils are actually chains of tiny contractile units called
sarcomeres, which are aligned end to end like boxcars in a train along the length of
the myofibrils.
  Myofilaments. There are two types of threadlike protein myofilaments within each of
our “boxcar” sarcomeres.

 Thick filaments. The larger, thick filaments, also called myosin filaments, are made
mostly of bundled molecules of the protein myosin, but they also contain ATPase
enzymes, which split ATP to generate the power for muscle contraction.

 Cross bridges. Notice that the midparts of the thick filaments are smooth, but their
ends are studded with thick projections; these projections, or myosin beads, are
called cross bridges when they link the thick and thin filaments together during
contraction.
 Thin filaments. The thin filaments are composed of the contractile protein
called actin, plus some regulatory proteins that play a role in allowing (or preventing)
myosin-bead binding to actin; the thin filaments, also called actin filaments, are
anchored to the Z disc (a disclike membrane).
 Sarcoplasmic reticulum. Another very important muscle fiber organelle is the
sarcoplasmic reticulum, a specialized smooth endoplasmic reticulum; the
interconnecting tubules and sacs of the SR surround each and every myofibril just as
the sleeve of a loosely crocheted sweater surrounds your arm, and its major role is to
store calcium and to release it on demand.

Muscle Movements, Types, and Names

This section is a bit of a hodge-podge. It includes some topics that don’t really fit together, but
they don’t fit anywhere else any better.
Types of Body Movements
Every one of our 600-odd skeletal muscles is attached to bone, or to other connective tissue
structures, at no fewer than two points.
  Origin. One of these points, the origin, is attached to the immovable or less movable
bone.
 Insertion. The insertion is attached to the movable bone, and when the muscle
contracts, the insertion moves toward the origin.
 Flexion. Flexion is a movement, generally in the sagittal plane, that decrease the
angle of the joint and brings two bones closer together; it is a type of hinge joints, but
it is also common at ball-and-socket joints.
 Extension. Extension is the opposite of flexion, so it is a movement that increases the
angle, or the distance, between two bones or parts of the body.
 Rotation. Rotation is movement of a bone around a longitudinal axis; it is a common
movement of ball-and-socket joints.
 Abduction. Abduction is moving the limb away from the midline, or median plane,
of the body.
 Adduction. Adduction is the opposite of abduction, so it is the movement of a limb
toward the body midline.
 Circumduction. Circumduction is a combination of flexion, extension, abduction,
and adduction commonly seen in ball-and-socket joints; the proximal end is
stationary, and its distal end moves in a circle.

Special Movements
Certain movements do not fit into any of the previous categories and occur at only a few joints.

 Dorsiflexion and plantar flexion. Lifting the foot so that its superior surface
approaches the shin is called dorsiflexion, whereas depressing the foot is
called plantar flexion.
 Inversion and eversion. To invert the foot, turn the sole medially; to evert the foot,
turn the sole laterally.
 Supination and pronation. Supination occurs when the forearm rotates laterally so
that the palm faces anteriorly and the radius and ulna are parallel; pronation occurs
when the forearm rotates medially so that the palm faces posteriorly. Opposition. In
the palm of the hand, the saddle joint between metacarpal 1 and the carpals allows
opposition of the thumb.

Interactions of Skeletal Muscles in the Body

Muscles are arranged in such a way that whatever one muscle can do, other muscles can reverse.
Because of this, muscles are able to bring about an immense variety of movements.

 Prime mover. The muscle that has the major responsibility for causing a particular
movement is called the prime mover.
  Antagonists. Muscles that oppose or reverse a movement are antagonists; when a
prime mover is active, its antagonist is stretched and relaxed.
 Synergists. Synergists help prime movers by producing the same movement or by
reducing undesirable movements.
 Fixators. Fixators are specialized synergists; they hold a bone still or stabilize the
origin of a prime mover so all tension can be used to move the insertion bone.

Naming Skeletal Muscles

Like bones, muscles come in many shapes and sizes to suit their particular tasks in the body.

 Direction of the muscle fibers. When a muscle’s name includes the term rectus
(straight) its fibers run parallel to that imaginary line; the term oblique as part of a
muscle’s name tells you that the muscle fibers run obliquely (at a slant) to the
imaginary line.
 Relative size of the muscle. Such terms as maximus (largest) , minimus (smallest),
and longus (long) are often used in the names of muscles.
 Location of the muscle. Some muscles are named for the bone with which they are
associated; for example, the temporalis and frontalis muscles overlie the temporal and
frontal bones of the skull.
 Number of origins. When the term biceps, triceps, or quadriceps forms part of a
muscle name, one can assume that the muscle has two, three, or four origins.
 Location of the muscle’s origin and insertion. Occasionally, muscles are named for
their attachment sites.
 Shape of the muscle. Some muscles have a distinctive shape that helps to identify
them.
  Action of the muscle. When muscles are named for their actions, terms such as
flexor, extensor, and adductor appear in their names.

Arrangement of Fascicles
Skeletal muscles consists of fascicles, but fascicle arrangement vary, producing muscles with
different structures and functional properties.

 Circular. The pattern is circular when the fascicles are arranged in concentric rings;
circular muscles are typically found surrounding external body openings which they
close by contracting.
 Convergent. In convergent muscle, the fascicles converge toward a single insertion
tendon; such a muscle is triangular or fan-shaped.
 Parallel. In a parallel arrangement, the length of the fascicles run parallel to the long
axis of the muscle; these muscles are straplike; a modification of the parallel
arrangement, called fusiform, results in a spindle-shaped muscle with an expanded
belly.
 Pennate. In a pennate pattern, short fascicles attach obliquely to a central tendon; in
the extensor digitorium muscle of the leg, the fascicles insert into only one side of the
tendon and the muscle is unipennate; if the fascicles insert into opposite sides of the
tendon or from
 several different sides, the muscle is bipennate or multipennate.
Gross Anatomy of Skeletal Muscles
Only the most important muscles are described here because it is beyond our scope to describe
the hundreds of skeletal muscles of the human body.
Head and Neck Muscles
The head muscles are an interesting group because they have many specific functions but are
usually grouped into two large categories- facial muscles and chewing muscles.

Facial Muscles
There are 5 facial muscles:

 Frontalis. The frontalis, which covers the frontal bone, runs from the cranial
aponeurosis to the skin of the eyebrows, where it inserts; this muscle allows you to
raise your eyebrows and wrinkle your forehead; at the posterior end of the cranial
aponeurosis is the small occipitalis muscle.
 Orbicularis occuli. The orbicularis oculi has fibers that run in circles around the
eyes; it allows you to close your eyes, squint, blink, and wink.
 Orbicularis oris. The orbicularis oris is the circular muscle of the lips; because it
closes the mouth and protrudes the lips, it is often called the “kissing” muscle.
 Buccinator. The fleshy buccinator muscle runs horizontally across the cheek and
inserts into the orbicularis oris.
 Zygomaticus. The zygomaticus extends from the corner of the mouth to the
cheekbone; it is often referred to as the “smiling” muscle because it raises the corners
of the mouth upward.

Chewing Muscles
The buccinator muscle, which is a member of this group, is described with the facial muscles.

 Masseter. As it runs from the zygomatic process of the temporal bone to the
mandible, the masseter covers the angle of the lower jaw; this muscle closes the jaw
by elevating the mandible.
 Temporalis. The temporalis is a fan-shaped muscle overlying the temporal bone; it
inserts into the mandible and acts as a synergist of the masseter in closing the jaw.

Neck Muscles
For the most part, the neck muscles, which move the head and shoulder girdle, are small and
straplike. Only two neck muscles are considered here.

 Platysma. The platysma is a single, sheetlike muscle that covers the anterolateral
neck; its action is to pull the corners of the mouth inferiorly, producing a downward
sag of the mouth.
 Sternocleidomastoid. The paired sternocleidomastoid muscles are two-headed
muscles, one found on each side of the neck; when both sternocleidomastoid contract
together, they flex your neck.

Trunk Muscles
The trunk muscles include (1) those that move the vertebral column; (2) anterior thorax muscles,
which move the ribs, head, and arms; and (3) muscles of the abdominal wall, which help to move
the vertebral column and, most important, form the muscular “natural girdle” of the abdominal
body wall.

Anterior Muscles
The anterior muscles of the trunk include:
  Pectoralis major. The pectoralis major is a large, fan-shaped muscle covering the
upper part of the chest; this muscle forms the anterior wall of the axilla and acts to
adduct and flex the arm.
 Intercostal muscles. The intercostal muscles are deep muscles found between the
ribs; the external intercostals are important in breathing because they help you to
raise the rib cage when you inhale; the internal intercostals, which lie deep to the
external intercostals, depress the rib cage, which helps to move air out of
the lungs when you exhale forcibly.
 Muscles of the abdominal girdle. The anterior abdominal muscles (rectus
abdominis, ecternal and internal obliques, and transversus abdominis) form a “natural
girdle” that reinforces the body trunk; the paired straplike rectus abdominis muscles
are the most superficial muscles of the abdomen; the external oblique muscles are
paired superficial muscles that make up the lateral walls of the abdomen; the internal
oblique muscles are paired muscles deep to the external obliques; and
the transversus abdominis is the deepest muscle of the abdominal wall and has
fibers that run horizontally across the abdomen.

Posterior Muscles
The posterior muscles of the trunk include:

 Trapezius. The trapezius muscles are the most superficial muscles of the posterior
neck and upper trunk; the trapezius muscles extend the head; they also can elevate,
depress, adduct, and stabilize the scapula.
 Latissimus dorsi. The latissimus dorsi muscles are the two large, flat muscles that
cover the lower back; these are very important muscles when the arm must be
brought down in a power stroke.
 Erector spinae. The erector spinae group is the prime mover of back extension; these
muscles not only act as powerful back extensors but also provide resistance that helps
control the action of bending over at the waist.
 Quadratus lumborum. The fleshy quadratus lumborum muscles form part of the
posterior abdominal wall; acting separately, each muscle of the pair flexes the spine
laterally; acting together, they extend the lumbar spine.
 Deltoid. The deltoids are fleshy, triangle-shaped muscles that form the rounded shape
of the shoulders; the deltoids are the prime movers of arm abduction.

Muscles of the Upper Limb

The upper limb muscles fall into three groups. The first group arise from the shoulder girdle and
cross the shoulder joint to insert into the humerus. The second group causes movement at the
elbow joint. The third group includes the muscles of the forearm.
Muscles of the Humerus that Act on the Forearm
All anterior arm muscles cause elbow flexion. In order of decreasing strength, these are the
brachialis, biceps brachii, and brachioradialis.

 Biceps brachii. The biceps brachii is the most familiar muscle of the arm because it
bulges when the elbow is flexed; this muscle is the powerful prime mover for flexion
of the forearm and acts to supinate the forearm.
 Brachialis. The brachialis lies deep to the biceps muscle and is as important as the
biceps in the elbow portion; the brachialis lifts the ulna as as the biceps lift the radius.
 Brachioradialis. The brachioradialis is a fairly weak muscle that arises on the
humerus and inserts into the distal forearm.
 Triceps brachii. The triceps brachii is the only muscle fleshing out the posterior
humerus; being the powerful prime mover of elbow extension, it is the antagonist of
biceps brachii.
Muscles of the Lower Limb
Muscles that act on the lower limb cause movement at the hip, knee and foot joints. They are
among the largest and strongest muscle in the body and are specialized for walking and
balancing the body.

Muscles Causing Movement at the Hip Joint

Part of the muscles of the lower limb are the muscles at the hip joint.

 Gluteus maximus. The gluteus maximus is a superficial muscle of the hip that forms
most of the flesh of the buttock; it is a powerful hip extensor that acts to bring the
thigh in a straight line with the pelvis.
 Gluteus medius. The gluteus medius runs from the iliac to the femur, beneath the
gluteus maximus for most of its length; the gluteus medius is a hip abductor and is
important in steadying the pelvis during walking.
 Iliopsoas. The iliopsoas is a fused muscle composed of two muscles, the iliacus and
the psoas major; it is a prime mover of hip flexion and also acts to keep the upper
body from falling backward when we are standing erect.
 Adductor muscles. The muscles of the adductor group form the muscle mass at the
medial side of each thigh; as their name indicates, they adduct, or press, the thighs
together.

Muscles Causing Movement at the Knee Joint

The muscles of the lower limb that causes movement of the knee joint are:

 Hamstring group. The muscles forming the muscle mass of the posterior thigh are
the hamstrings; the group consists of three muscles, the biceps femoris,
 semimembranosus, and semitendinosus, which originate on the ischial tuberosity and
run down the thigh to insert on both sides of the proximal tibia.
 Sartorius. It is the most superficial muscle of the thigh; it acts as a synergist to bring
about the cross-legged position.
 Quadriceps group. The quadriceps group consists of four muscles- the rectus
femoris muscle and three vastus muscles– that flesh out the anterior thigh; the group
as a whole acts to extend the knee powerfully.

Muscles Causing Movement at the Ankle and Foot

There are 5 muscles that cause movement at the ankle and foot:

 Tibialis anterior. The tibialis anterior is a superficial muscle on the anterior leg; it
arises from the upper tibia and then parallels the anterior crest as it runs to the tarsal
bones.
 Extensor digitorum longus. Lateral to the tibialis anterior, the extensor digitorum
longus muscle arises from the lateral tibial condyle and proximal radius; it is a prime
mover of toe extension and a dorsiflexor of the foot.
 Fibularis muscles. The three fibularis muscles- longus, brevis, and tertius- are found
on the lateral part of the leg; the group as a whole plantar flexes and everts the foot.
 Gastrocnemius. The gastrocnemius muscle is a two-bellied muscle that forms the
curved half of the posterior leg; it is a prime mover for plantar flexion of the foot.
 Soleus. Deep to the gastrocnemius is the fleshy soleus muscle; because it arises from
the tibia and fibula, it does not affect knee movement.

PHYSIOLOGY OF THE MUSCULAR SYSTEM

Skeletal Muscle Activity
Muscle cells have some special functional properties that enable them to perform their duties.

Nerve Stimulus and the Action Potential

To contract, skeletal muscle cells must be stimulated by nerve impulse.

 Neurotransmitter. When a nerve impulse reaches the axon terminals, a chemical

referred to as the neurotransmitter is released; the specific neurotransmitter that
stimulate skeletal muscle cells is acetylcholine, or ACh.
 Temporary permeability. If enough acetylcholine is released, the sarcolemma at
that point becomes temporarily more permeable sodium ions, which rush into the
muscle cell, and to potassium ions, which diffuse out of the cell.
  Action potential. More channels in the sarcolemma open up to allow only sodium to
enter, which generates an electrical current called an action potential; once the action
is begun, it is unstoppable; it travels over the entire surface of the sarcolemma,
conducting the electrical impulse from one end of the cell to the other; the result id
contraction of the muscle cell.
 Break down of enzymes. Acetylcholine, which began the process of muscle
contraction, is broken down to acetic acid and choline by enzymes present on the
sarcolemma; for this reason, a single nerve impulse produces only one contraction;
the muscle cell relaxes until stimulated by the next round of acetylcholine release.

Mechanism of Muscle Contraction: The Sliding Filament Theory

When muscle fibers are activated by the nervous system, the myosin heads attach to binding sites
on the thin filaments, and the sliding begins.

 Relaxed muscle cell. In a relaxed muscle cell, the regulatory proteins forming part of
the actin myofilaments prevent myosin binding; when an action potential sweeps
along its sarcolemma and a muscle cell is excited, calcium ions are released from
intracellular storage areas.
 Contraction trigger. The flood of calcium acts as the final trigger for contraction,
because as calcium binds to the regulatory proteins on the actin filaments, they
change both their shape and their position on the thin filaments.
 Attachment. The physical attachment of myosin to actin “springs the trap”, causing
the myosin heads to snap toward the center of the sarcomere; because actin and
myosin are firmly bound to each other when this happens, the thin filaments are
slightly pulled toward the center of the sarcomere.

Lesson Proper for Week 15

NERVOUS AND LYMPHATIC SYSTEM

NERVOUS SYSTEM

The nervous system is the master controlling and communicating system of the body. Every thought,
action, and emotion reflects its activity. Its signaling device, or means of communicating with body
cells, is electrical impulses, which are rapid and specific and cause almost immediate responses.

Functions of the Nervous System

To carry out its normal role, the nervous system has three overlapping functions.
 1. Monitoring changes. Much like a sentry, it uses its millions of sensory receptors to
monitor changes occurring both inside and outside the body; these changes are called
stimuli, and the gathered information is called sensory input.
2. Interpretation of sensory input. It processes and interprets the sensory input and
decides what should be done at each moment, a process called integration.
3. Effects responses. It then effects a response by activating muscles or glands
(effectors) via motor output.
4. Mental activity. The brain is the center of mental activity, including consciousness,
thinking, and memory.
5. Homeostasis. This function depends on the ability of the nervous system to detect,
interpret, and respond to changes in the internal and external conditions. It can help
stimulate or inhibit the activities of other systems to help maintain a constant internal
environment.

Anatomy of the Nervous System

The nervous system does not work alone to regulate and maintain body homeostasis; the endocrine
system is a second important regulating system.

Organization of the Nervous System

We only have one nervous system, but, because of its complexity, it is difficult to consider all of its
parts at the same time; so, to simplify its study, we divide it in terms of its structures (structural
classification) or in terms of its activities (functional classification).

Structural Classification
The structural classification, which includes all of the nervous system organs, has two subdivisions- the
central nervous system and the peripheral nervous system.

 Central nervous system (CNS). The CNS consists of the brain and spinal cord, which
occupy the dorsal body cavity and act as the integrating and command centers of the
nervous system
 Peripheral nervous system (PNS). The PNS, the part of the nervous system outside the
CNS, consists mainly of the nerves that extend from the brain and spinal cord.

Functional Classification

The functional classification scheme is concerned only with PNS structures.

 Sensory division. The sensory, or afferent division, consists of nerves (composed of

nerve fibers) that convey impulses to the central nervous system from sensory receptors
located in various parts of the body.
 Somatic sensory fibers. Sensory fibers delivering impulses from the skin, skeletal
muscles, and joints are called somatic sensory fibers.
 Visceral sensory fibers. Those that transmit impulses from the visceral organs are
called visceral sensory fibers.
  Motor division. The motor, or efferent division carries impulses from the CNS to
effector organs, the muscles and glands; the motor division has two subdivisions:
the somatic nervous system and the autonomic nervous system.
 Somatic nervous system. The somatic nervous system allows us to consciously,
or voluntarily, control our skeletal muscles.
 Autonomic nervous system. The autonomic nervous system regulates events that are
automatic, or involuntary; this subdivision, commonly called involuntary nervous system,
has two parts: the sympathetic and parasympathetic, which typically bring about opposite
effects.

Nervous Tissue: Structure and Function

Even though it is complex, nervous tissue is made up of just two principal types of cells- supporting
cells and neurons.

Supporting Cells

Supporting cells in the CNS are “lumped together” as neuroglia, literally mean “nerve glue”.

 Neuroglia. Neuroglia include many types of cells that generally support, insulate, and
protect the delicate neurons; in addition, each of the different types of neuroglia, also
simply called either glia or glial cells,has special functions.
 Astrocytes. These are abundant, star-shaped cells that account for nearly half of the
neural tissue; astrocytes form a living barrier between the capillaries and neurons and play
a role in making exchanges between the two so they could help protect neurons from
harmful substances that might be in the blood.
 Microglia. These are spiderlike phagocytes that dispose of debris, including dead brain
cells and bacteria.
 Ependymal cells. Ependymal cells are glial cells that line the central cavities of the brain
and the spinal cord; the beating of their cilia helps to circulate the cerebrospinal fluid that
fills those cavities and forms a protective cushion around the CNS.
 Oligodendrocytes. These are glia that wrap their flat extensions tightly around the nerve
fibers, producing fatty insulating coverings called myelin sheaths.
 Schwann cells. Schwann cells form the myelin sheaths around nerve fibers that are
found in the PNS.
 Satellite cells. Satellite cells act as protective, cushioning cells.

Neurons

Neurons, also called nerve cells, are highly specialized to transmit messages (nerve impulses) from
one part of the body to another.
  Cell body. The cell body is the metabolic center of the neuron; it has a transparent
nucleus with a conspicuous nucleolus; the rough ER, called Nissl substance,
and neurofibrils are particularly abundant in the cell body.
 Processes. The armlike processes, or fibers, vary in length from microscopic to 3 to 4
feet; dendrons convey incoming messages toward the cell body, while axons generate
nerve impulses and typically conduct them away from the cell body.
 Axon hillock. Neurons may have hundreds of the branching dendrites, depending on the
neuron type, but each neuron has only one axon, which arises from a conelike region of
the cell body called the axon hillock.
 Axon terminals. These terminals contain hundreds of tiny vesicles, or membranous sacs
that contain neurotransmitters.
 Synaptic cleft. Each axon terminal is separated from the next neuron by a tiny gap called
synaptic cleft.
 Myelin sheaths. Most long nerve fibers are covered with a whitish, fatty material
called myelin, which has a waxy appearance; myelin protects and insulates the fibers and
increases the transmission rate of nerve impulses.
 Nodes of Ranvier. Because the myelin sheath is formed by many individual Schwann
cells, it has gaps, or indentations, called nodes of Ranvier.

Classification

Neurons may be classified either according to how they function or according to their structure.
  Functional classification. Functional classification groups neurons according to the
direction the nerve impulse is traveling relative to the CNS; on this basis, there
are sensory, motor, and association neurons.
 Sensory neurons. Neurons carrying impulses from sensory receptors to the CNS are
sensory, or afferent, neurons; sensory neurons keep us informed about what is happening
both inside and outside the body.
 Motor neurons. Neurons carrying impulses from the CNS to the viscera and/or muscles
and glands are motor, or efferent, neurons.
 Interneurons. The third category of neurons is known as the interneurons,
or association neurons; they connect the motor and sensory neurons in neural pathways.
 Structural classification. Structural classification is based on the number of processes
extending from the cell body.
 Multipolar neuron. If there are several processes, the neuron is a multipolar neuron;
because all motor and association neurons are multipolar, this is the most common
structural type.
 Bipolar neurons. Neurons with two processes- an axon and a dendrite- are called bipolar
neurons; these are rare in adults, found only in some special sense organs, where they act
in sensory processing as receptor cells.
 Unipolar neurons. Unipolar neurons have a single process emerging from the cell’s body,
however, it is very short and divides almost immediately into proximal (central) and distal
(peripheral) processes.

Central Nervous System

During embryonic development, the CNS first appears as a simple tube, the neural tube, which
extends down the dorsal median plan of the developing embryo’s body.

Brain
Because the brain is the largest and most complex mass of nervous tissue in the body, it is commonly
discussed in terms of its four major regions – cerebral hemispheres, diencephalon, brain stem, and
cerebellum.

Cerebral Hemispheres

The paired cerebral hemispheres, collectively called cerebrum, are the most superior part of the brain,
and together are a good deal larger than the other three brain regions combined.

 Gyri. The entire surface of the cerebral hemispheres exhibits elevated ridges of tissue
called gyri, separated by shallow grooves called sulci.
 Fissures. Less numerous are the deeper grooves of tissue called fissures, which separate
large regions of the brain; the cerebral hemispheres are separated by a single deep
fissure, the longitudinal fissure.
 Lobes. Other fissures or sulci divide each hemisphere into a number of lobes, named for
the cranial bones that lie over them.
 Regions of cerebral hemisphere. Each cerebral hemisphere has three basic regions: a
superficial cortex of gray matter, an internal white matter, and the basal nuclei.
 Cerebral cortex. Speech, memory, logical and emotional response, as well as
consciousness, interpretation of sensation, and voluntary movement are all functions of
neurons of the cerebral cortex.
 Parietal lobe. The primary somatic sensory area is located in the parietal lobe
posterior to the central sulcus; impulses traveling from the body’s sensory receptors are
localized and interpreted in this area.
 Occipital lobe. The visual area is located in the posterior part of the occipital lobe.
 Temporal lobe. The auditory area is in the temporal lobe bordering the lateral sulcus,
and the olfactory area is found deep inside the temporal lobe.
 Frontal lobe. The primary motor area, which allows us to consciously move our skeletal
muscles, is anterior to the central sulcus in the front lobe.
 Pyramidal tract. The axons of these motor neurons form the major voluntary motor tract-
the corticospinal or pyramidal tract, which descends to the cord.
  Broca’s area. A specialized cortical area that is very involved in our ability to speak,
Broca’s area, is found at the base of the precentral gyrus (the gyrus anterior to the central
sulcus).
 Speech area. The speech area is located at the junction of the temporal, parietal, and
occipital lobes; the speech area allows one to sound out words.
 Cerebral white matter. The deeper cerebral white matter is compose of fiber tracts
carrying impulses to, from, and within the cortex.
 Corpus callosum. One very large fiber tract, the corpus callosum, connect the cerbral
hemispheres; such fiber tracts are called commisures.
 Fiber tracts. Association fiber tracts connect areas within a hemisphere,
and projection fiber tracts connect the cerebrum with lower CNS centers.
 Basal nuclei. There are several islands of gray matter, called the basal nuclei, or basal
ganglia, buried deep within the white matter of the cerebral hemispheres; it helps
regulate the voluntary motor activities by modifying instructions sent to the skeletal
muscles by the primary motor cortex.

Diencephalon

The diencephalon, or interbrain, sits atop the brain stem and is enclosed by the cerebral hemispheres.

 Thalamus. The thalamus, which encloses the shallow third ventricle of the brain, is a relay
station for sensory impulses passing upward to the sensory cortex.
 Hypothalamus. The hypothalamus makes up the floor of the diencephalon; it is an
important autonomic nervous system center because it plays a role in the regulation of
body temperature, water balance, and metabolism; it is also the center for many drives
and emotions, and as such, it is an important part of the so-called limbic system or
“emotional-visceral brain”; the hypothalamus also regulates the pituitary gland and
produces two hormones of its own.
 Mammillary bodies. The mammillary bodies, reflex centers involved in olfaction (the
sense of smell), bulge from the floor of the hypothalamus posterior to the pituitary gland.
 Epithalamus. The epithalamus forms the roof of the third ventricle; important parts of the
epithalamus are the pineal body (part of the endocrine system) and the choroid
plexus of the third ventricle, which forms the cerebrospinal fluid.

Brain Stem

The brain stem is about the size of a thumb in diameter and approximately 3 inches long.

 Structures. Its structures are the midbrain, pons, and the medulla oblongata.
 Midbrain. The midbrain extends from the mammillary bodies to the pons inferiorly; it is
composed of two bulging fiber tracts, the cerebral peduncles, which convey descending
and ascending impulses.
  Corpora quadrigemina. Dorsally located are four rounded protrusions called the corpora
quadrigemina because they remind some anatomist of two pairs of twins; these bulging
nuclei are reflex centers involved in vision and hearing.
 Pons. The pons is a rounded structure that protrudes just below the midbrain, and this
area of the brain stem is mostly fiber tracts; however, it does have important nuclei
involved in the control of breathing.
 Medulla oblongata. The medulla oblongata is the most inferior part of the brain stem; it
contains nuclei that regulate vital visceral activities; it contains centers that control heart
rate, blood pressure, breathing, swallowing, and vomiting among others.
 Reticular formation. Extending the entire length of the brain stem is a diffuse mass of
gray matter, the reticular formation; the neurons of the reticular formation are involved in
motor control of the visceral organs; a special group of reticular formation neurons,
the reticular activating system (RAS), plays a role in consciousness and the
awake/sleep cycles.

Cerebellum

The large, cauliflower-like cerebellum projects dorsally from under the occipital lobe of the cerebrum.

 Structure. Like the cerebrum. the cerebellum has two hemispheres and a convoluted
surface; it also has an outer cortex made up of gray matter and an inner region of white
matter.
 Function. The cerebellum provides precise timing for skeletal muscle activity and controls
our balance and equilibrium.
 Coverage. Fibers reach the cerebellum from the equilibrium apparatus of the inner ear,
the eye, the proprioceptors of the skeletal muscles and tendons, and many other areas.

Protection of the Central Nervous System

Nervous tissue is very soft and delicate, and the irreplaceable neurons are injured by even the
slightest pressure, so nature has tried to protect the brain and the spinal cord by enclosing them
within bone (the skull and vertebral column), membranes (the meninges), and a watery cushion
(cerebrospinal fluid).

Meninges

The three connective tissue membranes covering and protecting the CNS structures are the meninges.
  Dura mater. The outermost layer, the leathery dura mater, is a double layered membrane
where it surrounds the brain; one of its layer is attached to the inner surface of the skull,
forming the periosteum (periosteal layer); the other, called the meningeal layer,
forms the outermost covering of the brain and continues as the dura mater of the spinal
cord.
 Falx cerebri. In several places, the inner dural membrane extends inward to form a fold
that attaches the brain to the cranial cavity, and one of these folds is the falx cerebri.
 Tentorium cerebelli. The tentorium cereberi separates the cerebellum from the
cerebrum.
 Arachnoid mater. The middle layer is the weblike arachnoid mater; its threadlike
extensions span the subarachnoid space to attach it to the innermost membrane.
 Pia mater. The delicate pia mater, the innermost meningeal layer, clings tightly to the
surface of the brain and spinal cord, following every fold.

Cerebrospinal Fluid

Cerebrospinal fluid (CSF) is a watery “broth” similar in its makeup to blood plasma, from which it
forms.

 Contents. The CSF contains less protein and more vitamin C, and glucose.
 Choroid plexus. CSF is continually formed from blood by the choroid plexuses; choroid
plexuses are clusters of capillaries hanging from the “roof” in each of the brain’s
ventricles.
 Function. The CSF in and around the brain and cord forms a watery cushion that protects
the fragile nervous tissue from blows and other trauma.
 Normal volume. CSF forms and drains at a constant rate so that its normal pressure and
volume (150 ml-about half a cup) are maintained.
 Lumbar tap. The CSF sample for testing is obtained by a procedure called lumbar
or spinal tap;because the withdrawal of fluid for testing decreases CSF fluid pressure, the
patient must remain in a horizontal position (lying down) for 6 to 12 hours after the
procedure to prevent an agonizingly painful “spinal headache”.

The Blood-Brain Barrier

No other body organ is so absolutely dependent on a constant internal environment as is the brain,
and so the blood-brain barrier is there to protect it.

 Function. The neurons are kept separated from bloodborne substances by the so-called
blood-brain barrier, composed of the least permeable capillaries in the whole body.
 Substances allowed. Of water-soluble substances, only water, glucose, and essential
amino acids pass easily through the walls of these capillaries.
 Prohibited substances. Metabolic wastes, such as toxins, urea, proteins, and most drugs
are prevented from entering the brain tissue.
 Fat-soluble substances. The blood-brain barrier is virtually useless against fats,
respiratory gases, and other fat-soluble molecules that diffuse easily through all plasma
membranes.
Spinal Cord

The cylindrical spinal cord is a glistening white continuation of the brain stem.

 Length. The spinal cord is approximately 17 inches (42 cm) long.

 Major function. The spinal cord provides a two-way conduction pathway to and from the
brain, and it is a major reflex center (spinal reflexes are completed at this level).
 Location. Enclosed within the vertebral column, the spinal cord extends from the foramen
magnum of the skull to the first or second lumbar vertebra, where it ends just below the
ribs.
 Meninges. Like the brain, the spinal cord is cushioned and protected by the meninges;
meningeal coverings do not end at the second lumbar vertebra but instead extend well
beyond the end of the spinal cord in the vertebral canal.
 Spinal nerves. In humans, 31 pairs of spinal nerves arise from the cord and exit from
the vertebral column to serve the body area close by.
  Cauda equina. The collection of spinal nerves at the inferior end of the vertebral canal is
called cauda equina because it looks so much like a horse’s tail.

Gray Matter of the Spinal Cord and Spinal Roots

The gray matter of the spinal cord looks like a butterfly or a letter H in cross section.

 Projections. The two posterior projections are the dorsal, or posterior, horns; the two
anterior projections are the ventral, or anterior, horns.
 Central canal. The gray matter surrounds the central canal of the cord, which contains
CSF.
 Dorsal root ganglion. The cell bodies of sensory neurons, whose fibers enter the cord by
the dorsal root, are found in an enlarged area called dorsal root ganglion; if the dorsal
root or its ganglion is damaged, sensation from the body area served will be lost.
 Dorsal horns. The dorsal horns contain interneurons.
 Ventral horns. The ventral horns of gray matter contain cell bodies of motor neurons of
the somatic nervous system, which send their axons out the ventral root of the cord.
 Spinal nerves. The dorsal and ventral roots fuse to form the spinal nerves.

White Matter of the Spinal Cord

White matter of the spinal cord is composed of myelinated fiber tracts- some running to higher
centers, some traveling from the brain to the cord, and some conducting impulses from one side of the
spinal cord to the other.

 Regions. Because of the irregular shape of the gray matter, the white matter on each side
of the cord is divided into three regions- the dorsal, lateral, and ventral columns; each
of the columns contains a number of fiber tracts made up of axon with the same
destination and function.
 Sensory tracts. Tracts conducting sensory impulses to the brain are sensory,
or afferent, tracts.
 Motor tracts. Those carrying impulses from the brain to skeletal muscles are motor,
or efferent, tracts.

Peripheral Nervous System

The peripheral nervous system consists of nerves and scattered groups of neuronal cell bodies
(ganglia) found outside the CNS.

Structure of a Nerve

A nerve is a bundle of neuron fibers found outside the CNS.

 Endoneurium. Each fiber is surrounded by a delicate connective tissue sheath, an

endoneurium.
 Perimeurium. Groups of fibers are bound by a coarser connective tissue wrapping, the
perineurium, to form fiber bundles, or fascicles.
  Epineurium. Finally, all the fascicles are bound together by a tough fibrous sheath, the
epineurium, to form the cordlike nerve.
 Mixed nerves. Nerves carrying both sensory and motor fibers are called mixed nerves.
 Sensory nerves. Nerves that carry impulses toward the CNS only are called sensory, or
afferent, nerves.
 Motor nerves. Those that carry only motor fibers are motor, or efferent, nerves.

Cranial Nerves
The 12 pairs of cranial nerves primarily serve the head and the neck.

 Olfactory. Fibers arise from the olfactory receptors in the nasal mucosa and synapse with
the olfactory bulbs; its function is purely sensory, and it carries impulses for the sense of
smell.
 Optic. Fibers arise from the retina of the eye and form the optic nerve; its function is
purely sensory, and carries impulses for vision.
 Oculomotor. Fibers run from the midbrain to the eye; it supplies motor fibers to four of
the six muscles (superior, inferior, and medial rectus, and inferior oblique) that direct the
 eyeball; to the eyelid; and to the internal eye muscles controlling lens shape and pupil
size.
 Trochlear. Fibers run from the midbrain to the eye; it supplies motor fibers for one
external eye muscle (superior oblique).
 Trigeminal. Fibers emerge from the pons and form three divisions that run to the face; it
conducts sensory impulses from the skin of the face and mucosa of the nose and mouth;
also contains motor fibers that activate the chewing muscles.
 Abducens. Fibers leave the pons and run to the eye; it supplies motor fibers to the lateral
rectus muscle, which rolls the eye laterally.
 Facial. Fibers leave the pons and run to the face; it activates the muscles of facial
expression and the lacrimal and salivary glands; carries sensory impulses from the taste
buds of the anterior tongue.
 Vestibulocochlear. fibers run from the equilibrium and hearing receptors of the inner ear
to the brain stem; its function is purely sensory; vestibular branch transmits impulses for
the sense of balance, and cochlear branch transmits impulses for the sense of hearing.
 Glossopharyngeal. Fibers emerge from the medulla and run to the throat; it supplies
motor fibers to the pharynx (throat) that promote swallowing and saliva production; it
carries sensory impulses from the taste buds of the posterior tongue and from pressure
receptors of the carotid artery.
 Vagus. Fibers emerge from the medulla and descend into the thorax and abdominal
cavity; the fibers carry sensory impulses from and motor impulses to the pharynx, larynx,
and the abdominal and thoracic viscera; most motor fibers are parasympathetic fibers that
promote digestive activity and help regulate heart activity.
 Accessory. Fiber arise from the medulla and superior spinal cord and travel to muscles of
the neck and back; mostly motor fiber that activate the sternocleidomastoid and trapezius
muscles.
 Hypoglossal. Fibers run from the medulla to the tongue; motor fibers control tongue
movements; sensory fibers carry impulses from the tongue.

Autonomic Nervous System

The autonomic nervous system (ANS) is the motor subdivision of the PNS that controls body activities
automatically.

 Composition. It is composed of a specialized group of neurons that regulate cardiac

muscle, smooth muscles, and glands.
 Function. At every moment, signals flood from the visceral organs into the CNS, and the
automatic nerves make adjustments as necessary to best support body activities.
 Divisions. The ANS has two arms: the sympathetic division and the parasympathetic
division. The parasympathetic division allows us to “unwind” and conserve energy. The
sympathetic division mobilizes the body during extreme situations, and is also called the
thoracolumbar division because its preganglionic neurons are in the gray matter of the
spinal cord from T1 through L2.
 Physiology of the Nervous System

The physiology of the nervous system involves a complex journey of impulses.

Nerve Impulse

Neurons have two major functional properties: irritability, the ability to respond to a stimulus and
convert it into a nerve impulse, and conductivity, the ability to transmit the impulse to other neurons,
muscles, or glands.

 Electrical conditions of a resting neuron’s membrane. The plasma membrane of a

resting, or inactive, neuron is polarized, which means that there are fewer positive ions
sitting on the inner face of the neuron’s plasma membrane than there are on its outer
surface; as long as the inside remains more negative than the outside, the neuron will stay
inactive.
 Action potential initiation and generation. Most neuron in the body are excited by
neurotransmitters released by other neurons; regardless what the stimulus is, the result is
always the same- the permeability properties of the cell’s plasma membrane change for a
very brief period.
 Depolarization. The inward rush of sodium ions changes the polarity of the neuron’s
membrane at that site, an event called depolarization.
 Graded potential. Locally, the inside is now more positive, and the outside is less
positive, a situation called graded potential.
 Nerve impulse. If the stimulus is strong enough, the local depolarization activates the
neuron to initiate and transmit a long-distance signal called action potential, also called a
nerve impulse; the nerve impulse is an all-or-none response; it is either propagated over
the entire axon, or it doesn’t happen at all; it never goes partway along an axon’s length,
nor does it die out with distance as do graded potential.
 Repolarization. The outflow of positive ions from the cell restores the electrical
conditions at the membrane to the polarized or resting, state, an event called
repolarization; until a repolarization occurs, a neuron cannot conduct another impulse.
 Saltatory conduction. Fibers that have myelin sheaths conduct impulses much faster
because the nerve impulse literally jumps, or leaps, from node to node along the length of
the fiber; this occurs because no electrical current can flow across the axon membrane
where there is fatty myelin insulation.

The Nerve Impulse Pathway

How the nerve impulse actually works is detailed below.

 Resting membrane electrical conditions. The external face of the membrane is

slightly positive; its internal face is slightly negative; the chief extracellular ion is sodium,
whereas the chief intracellular ion is potassium; the membrane is relatively permeable to
both ions.
 Stimulus initiates local depolarization. A stimulus changes the permeability of a
“patch” of the membrane, and sodium ions diffuse rapidly into the cell; this changes the
 polarity of the membrane (the inside becomes more positive; the outside becomes more
negative) at that site.
 Depolarization and generation of an action potential. If the stimulus is strong
enough, depolarization causes membrane polarity to be completely reversed and an action
potential is initiated.
 Propagation of the action potential. Depolarization of the first membrane patch
causes permeability changes in the adjacent membrane, and the events described in (b)
are repeated; thus, the action potential propagates rapidly along the entire length of the
membrane.
 Repolarization. Potassium ions diffuse out of the cell as the membrane permeability
changes again, restoring the negative charge on the inside of the membrane and the
positive charge on the outside surface; repolarization occurs in the same direction as
depolarization.

Autonomic Functioning

Body organs served by the autonomic nervous system receive fibers from both divisions.

 Antagonistic effect. When both divisions serve the same organ, they cause antagonistic
effects, mainly because their post ganglionic axons release different transmitters.
 Cholinergic fibers. The parasympathetic fibers called cholinergic fibers, release
acetylcholine.
 Adrenergic fibers. The sympathetic postganglionic fibers, called adrenergic fibers,
release norepinephrine.
 Preganglionic axons. The preganglionic axons of both divisions release acetylcholine.

Sympathetic Division

The sympathetic division is often referred to as the “fight-or-flight” system.

 Signs of sympathetic nervous system activities. A pounding heart; rapid, deep

breathing; cold, sweaty skin; a prickly scalp, and dilated pupils are sure signs sympathetic
nervous system activities.
 Effects. Under such conditions, the sympathetic nervous system increases heart rate,
blood pressure, and blood glucose levels; dilates the bronchioles of the lungs; and brings
about many other effects that help the individual cope with the stressor.
 Duration of the effect. The effects of sympathetic nervous system activation continue
for several minutes until its hormones are destroyed by the liver.
 Function. Its function is to provide the best conditions for responding to some threat,
whether the best response is to run, to see better, or to think more clearly.

Parasympathetic Division
The parasympathetic division is most active when the body is at rest and not threatened in any way.

 Function. This division, sometimes called the “resting-and-digesting” system, is chiefly

concerned with promoting normal digestion, with elimination of feces and urine, and with
conserving body energy, particularly by decreasing demands on the cardiovascular
system.
 Relaxed state. Blood pressure and heart and respiratory rates rate being regulated at
normal levels, the digestive tract is actively digesting food, and the skin is warm
(indicating that there is no need to divert blood to skeletal muscles or vital organs.
 Optical state. The eye pupils are constricted to protect the retinas from excessive
damaging light, and the lenses of the eye are “set” for close vision.

LYMPHATIC SYSTEM
When we mentally list off the names of the body’s organ systems, the lymphatic system is probably
not the first to come to mind. Yet without this quietly working system, our cardiovascular
system would stop working, and our immune system would be hopelessly impaired.

Functions of the Lymphatic System

The functions of the lymphatic system are:

1. Fluid balance. The lymphatic vessels transport back to the blood fluids that have
escaped from the blood vascular system. About 30 liters (L) of fluid pass from the blood
capillaries into the interstitial spaces each day, whereas only 27 L pass from the
interstitial spaces back into the blood capillaries. If the extra 3 L of interstitial fluid
remained in the interstitial spaces, edema would result, causing tissue damage and
eventually death. The remaining fluid enters the lymphatic capillaries, where the fluid is
called lymph.
2. Fat absorption. The lymphatic system absorbs fats and other substances from the
digestive tract. Lacteals are special lymphatic vessels located in the lining of the small
intestine. Fats enter the lacteals and pass through the lymphatic vessels to the venous
circulation.
3. House of the body’s defenses. The lymphoid tissues and organs house phagocytic
cells and lymphocytes, which play essential roles in body defense and resistance to
disease.

Anatomy of the Lymphatic System

The lymphatic system actually consists of two semi-independent parts: (1) a meandering network of
lymphatic vessels and (2) various lymphoid tissues and organs scattered throughout the body.

Lymphatic Vessels

The function of the lymphatic vessels is to form an elaborate drainage system that picks up excess
tissue fluid, now called lymph.
 Lymphatics. The lymphatic vessels, also called lymphatics, form a one-way system, and
lymph flows only toward the heart.
 Lymph capillaries. The microscopic, blind-ended lymph capillaries weave between the
tissue cells and blood capillaries in the loose connective tissues of the body and absorb the
leaked fluid.
 Minivalves. The edges of the endothelial cells forming their walls loosely overlap one
another, forming flaplike mini-valves that act as one-way swinging doors; the flaps,
anchored by fine collagen fibers to surrounding structures, gape open when the fluid
pressure is higher in the interstitial space, allowing fluid to enter the lymphatic capillary.

 Lymphatic collecting vessels. Lymph is transported from the lymph capillaries through
successively larger lymphatic vessels referred to as lymphatic collecting vessels, until it is
finally returned to the venous system through one of the two large ducts in the thoracic
region.
  Right lymphatic duct. The right lymphatic duct drains the lymph from the right arm and
the right side of the head and thorax.
 Thoracic duct. The large thoracic duct receives lymph from the rest of the body; both
ducts empty the lymph into the subclavian vein on their own side of the body.

Lymph Nodes

The lymph nodes in particular help protect the body by removing foreign material such as bacteria
and tumor cells from the lymphatic stream and by producing lymphocytes that function in the immune
response.

 Macrophages. Within the lymph nodes are macrophages, which engulf and destroy
bacteria, viruses, and other foreign substances in the lymph before it is returned to the
blood.
 Lymphocytes. Collections of lymphocytes (a type of white blood cell) are also
strategically located in the lymph nodes and respond to foreign substances in the
lymphatic stream.
 Size and shape. Lymph nodes vary in size and shape, but most are kidney-
shaped, less than 1 inch (approximately 2.5 cm) long, and “buried” in the connective
tissue that surrounds them.
 Trabeculae. Each node is surrounded by a fibrous capsule from which strands called
trabeculae extend inward to divide the node into a number of compartments.
 Cortex. The outer part of the node, the cortex, contains collections of lymphocytes
called follicles, many of which have dark-staining centers called germinal centers.
 Plasma cells. These centers enlarge when specific lymphocytes (the B cells) are
generating daughter cells called plasma cells, which release antibodies.
 T cells. The rest of the cortical cells are lymphocytes “in transit”, the so-called T cells that
circulate continuously between the blood, lymph nodes and lymphatic stream, performing
their surveillance role.
 Medulla. Phagocytic macrophages are located in the central medulla of the lymph node.
 Afferent lymphatic vessels. Lymph enters the convex side of a lymph node through the
afferent lymphatic vessels.
  Efferent lymphatic vessels. It then flows through a number of sinuses that cut through
the lymph node and finally exits from the node at its indented region, the hilum, via the
efferent lymphatic vessels.

Other Lymphoid Organs

Lymph nodes are just one of the many types of lymphoid organs in the body. Others are the spleen,
thymus gland, tonsils, and Peyer’s patches of the intestine, as well as bits of lymphoid tissue scattered
in the epithelial and connective tissues.

Spleen

The spleen is a soft, blood-rich organ that filters blood.

 Location. The spleen is located on the left side of the abdominal cavity, just beneath the
diaphragm, and curls around the anterior aspect of the stomach.
 Function. Instead of filtering lymph, the spleen filters and cleanses the blood of bacteria,
viruses, and other debris; it provides a site for lymphocyte proliferation and immune
surveillance, but its most important function is to destroy worn-out red blood cells and
return some of their breakdown products to the liver.
 Fetal spleen. In the fetus, the spleen is an important hematopoietic (blood cell-forming)
site, but as a rule only lymphocytes are produced by the adult spleen.

Thymus Gland

The thymus gland functions at peak levels only during youth.

 Location. The thymus gland is a lymphoid mass found low in the throat overlying the
heart.
 Functions. The thymus gland produces thymosin and others, that function in the
programming of certain lymphocytes so they can carry out their protective roles in the
body.

Tonsils

The tonsils are small masses of lymphoid tissue that ring the pharynx (the throat), where they are
found in the mucosa.

 Function. Their job is to trap and remove any bacteria or other foreign pathogens
entering the throat.
 Tonsilitis. They carry out this function so efficiently that sometimes they become
congested with bacteria and become red, swollen, and sore, a condition called tonsilitis.

Peyer’s Patches

Peyer’s patches resemble the look of the tonsils.

  Location. Peyer’s patches are found in the wall of the small intestine.
 Function. The macrophages of Peyer’s patches are in an ideal position to capture and
destroy bacteria (always present in tremendous numbers in the intestine), thereby
preventing them from penetrating the intestinal wall.
 Mucosa-associated lymphatic tissue. Peyer’s patches and the tonsils are part of the
collection of small lymphoid tissues referred to as mucosa-associated lymphatic tissue
(MALT); MALT acts as a sentinel to protect the upper respiratory and digestive tracts from
the never-ending attacks of foreign matter entering those cavities.

Lesson Proper for Week 16

CARDIOVASCULAR SYSTEM

The circulatory system is also called the cardiovascular system, where “cardi” refers to the heart, and
“vascular” refers to the blood vessels. So, these are the two key parts: the heart, which pumps blood,
and the blood vessels, which carry blood to the body and return it back to the heart again. Ultimately,
this is how nutrients like O2, or oxygen, get pushed out to the organs and tissues that need it, and how
waste like CO2, or carbon dioxide, which is the main byproduct of cellular respiration, gets removed.

Functions of the Heart

The functions of the heart are as follows:

1. Managing blood supply. Variations in the rate and force of heart contraction match
blood flow to the changing metabolic needs of the tissues during rest, exercise, and
changes in body position.
2. Producing blood pressure. Contractions of the heart produce blood pressure, which is
needed for blood flow through the blood vessels.
3. Securing one-way blood flow. The valves of the heart secure a one-way blood flow
through the heart and blood vessels.
4. Transmitting blood. The heart separates the pulmonary and systemic circulations,
which ensures the flow of oxygenated blood to tissues.

Anatomy of the Heart

The cardiovascular system can be compared to a muscular pump equipped with one-way valves and a
system of large and small plumbing tubes within which the blood travels.

Heart Structure and Functions

The modest size and weight of the heart give few hints of its incredible strength.
  Weight. Approximately the size of a person’s fist, the hollow, cone-shaped heart
weighs less than a pound.
 Mediastinum. Snugly enclosed within the inferior mediastinum, the medial cavity of the
thorax, the heart is flanked on each side by the lungs.
 Apex. It’s more pointed apex is directed toward the left hip and rests on the diaphragm,
approximately at the level of the fifth intercostal space.
 Base. Its broad posterosuperior aspect, or base, from which the great vessels of the body
emerge, points toward the right shoulder and lies beneath the second rib.
 Pericardium. The heart is enclosed in a double-walled sac called the pericardium and is
the outermost layer of the heart.
 Fibrous pericardium. The loosely fitting superficial part of this sac is referred to as the
fibrous pericardium, which helps protect the heart and anchors it to surrounding structures
such as the diaphragm and sternum.
 Serous pericardium. Deep to the fibrous pericardium is the slippery, two-layer serous
pericardium, where its parietal layer lines the interior of the fibrous pericardium.

Layers of the Heart

The heart muscle has three layers and they are as follows:

 Epicardium. The epicardium or the visceral and outermost layer is actually a part of the
heart wall.
 Myocardium. The myocardium consists of thick bundles of cardiac muscle twisted and
whirled into ringlike arrangements and it is the layer that actually contracts.
 Endocardium. The endocardium is the innermost layer of the heart and is a thin,
glistening sheet of endothelium hat lines the heart chambers.

Chambers of the Heart

The heart has four hollow chambers, or cavities: two atria and two ventricles.

 Receiving chambers. The two superior atria are primarily the receiving chambers, they
play a lighter role in the pumping activity of the heart.
  Discharging chambers. The two inferior, thick-walled ventricles are the discharging
chambers, or actual pumps of the heart wherein when they contract, blood is propelled out
of the heart and into the circulation.
 Septum. The septum that divides the heart longitudinally is referred to as either
the interventricular septum or the interatrial septum, depending on which chamber it
separates.

Associated Great Vessels

The great blood vessels provide a pathway for the entire cardiac circulation to proceed.

 Superior and inferior vena cava. The heart receives relatively oxygen-poor
blood from the veins of the body through the large superior and inferior vena cava and
pumps it through the pulmonary trunk.
 Pulmonary arteries. The pulmonary trunk splits into the right and left pulmonary
arteries, which carry blood to the lungs, where oxygen is picked up and carbon dioxide is
unloaded.
 Pulmonary veins. Oxygen-rich blood drains from the lungs and is returned to the left
side of the heart through the four pulmonary veins.
 Aorta. Blood returned to the left side of the heart is pumped out of the heart into the
aorta from which the systemic arteries branch to supply essentially all body tissues.

Heart Valves

The heart is equipped with four valves, which allow blood to flow in only one direction through the
heart chambers.

 Atrioventricular valves. Atrioventricular or AV valves are located between the atrial and
ventricular chambers on each side, and they prevent backflow into the atria when the
ventricles contract.
 Bicuspid valves. The left AV valve- the bicuspid or mitral valve, consists of two flaps, or
cusps, of endocardium.
 Tricuspid valve. The right AV valve, the tricuspid valve, has three flaps.
 Semilunar valve. The second set of valves, the semilunar valves, guards the bases of the
two large arteries leaving the ventricular chambers, thus they are known as the pulmonary
and aortic semilunar valves.
Cardiac Circulation Vessels

Although the heart chambers are bathed with blood almost continuously, the blood contained in the
heart does not nourish the myocardium.

 Coronary arteries. The coronary arteries branch from the base of the aorta and encircle
the heart in the coronary sulcus (atrioventricular groove) at the junction of the atria
and ventricles, and these arteries are compressed when the ventricles are contracting and
fill when the heart is relaxed.
 Cardiac veins. The myocardium is drained by several cardiac veins, which empty into an
enlarged vessel on the posterior of the heart called the coronary sinus.

Blood Vessels

Blood circulates inside the blood vessels, which form a closed transport system, the so-called vascular
system.

 Arteries. As the heart beats, blood is propelled into large arteries leaving the heart.
 Arterioles. It then moves into successively smaller and smaller arteries and then into
arterioles, which feed the capillary beds in the tissues.
 Veins. Capillary beds are drained by venules, which in turn empty into veins that finally
empty into the great veins entering the heart.

Tunics

Except for the microscopic capillaries, the walls of the blood vessels have three coats or tunics.

 Tunica intima. The tunica intima, which lines the lumen, or interior, of the vessels, is a
thin layer of endothelium resting on a basement membrane and decreases friction as
blood flows through the vessel lumen.
 Tunica media. The tunica media is the bulky middle coat which mostly consists of smooth
muscle and elastic fibers that constrict or dilate, making the blood pressure increase or
decrease.
  Tunica externa. The tunica externa is the outermost tunic composed largely of fibrous
connective tissue, and its function is basically to support and protect the vessels.

Major Arteries of the Systemic Circulation

The major branches of the aorta and the organs they serve are listed next in sequence from the heart.

Arterial Branches of the Ascending Aorta

The aorta springs upward from the left ventricle of heart as the ascending aorta.

 Coronary arteries. The only branches of the ascending aorta are the right and left
coronary arteries, which serve the heart.

Arterial Branches of the Aortic Arch

The aorta arches to the left as the aortic arch.

 Brachiocephalic trunk. The brachiocephalic trunk, the first branch off the aortic arch,
splits into the right common carotid artery and right subclavian artery.
 Left common carotid artery. The left common carotid artery is the second branch off
the aortic arch and it divides, forming the left internal carotid, which serves the brain,
and the left external carotid, which serves the skin and muscles of the head and neck.
 Left subclavian artery. The third branch of the aortic arch, the left subclavian artery,
gives off an important branch- the vertebral artery, which serves part of the brain.
  Axillary artery. In the axilla, the subclavian artery becomes the axillary artery.
 Brachial artery. the subclavian artery continues into the arm as the brachial artery,
which supplies the arm.
 Radial and ulnar arteries. At the elbow, the brachial artery splits to form the radial and
ulnar arteries, which serve the forearm.

Arterial Branches of the Thoracic Aorta

The aorta plunges downward through the thorax, following the spine as the thoracic aorta.

 Intercostal arteries. Ten pairs of intercostal arteries supply the muscles of the thorax
wall.

Arterial Branches of the Abdominal Aorta

Finally, the aorta passes through the diaphragm into the abdominopelvic cavity, where it becomes the
abdominal aorta.

 Celiac trunk. The celiac trunk is the first branch of the abdominal aorta and has three
branches: the left gastric artery supplies the stomach; the splenic artery supplies
the spleen, and the common hepatic artery supplies the liver.
 Superior mesenteric artery. The unpaired superior mesenteric artery supplies most of
the small intestine and the first half of the large intestine or colon.
 Renal arteries. The renal arteries serve the kidneys.
 Gonadal arteries. The gonadal arteries supply the gonads, and they are called ovarian
arteries in females while in males they are testicular arteries.
 Lumbar arteries. The lumbar arteries are several pairs of arteries serving the heavy
muscles of the abdomen and trunk walls.
 Inferior mesenteric artery. The inferior mesenteric artery is a small, unpaired artery
supplying the second half of the large intestine.
 Common iliac arteries. The common iliac arteries are the final branches of the
abdominal aorta.

Major Veins of the Systemic Circulation

Major veins converge on the venae cavae, which enter the right atrium of the heart.
Veins Draining into the Superior Vena Cava

Veins draining into the superior vena cava are named in a distal-to-proximal direction; that is, in the
same direction the blood flows into the superior vena cava.

 Radial and ulnar veins. The radial and ulnar veins are deep veins draining the forearm;
they unite to form the deep brachial vein, which drains the arm and empties into
the axillary vein in the axillary region.
 Cephalic vein. The cephalic vein provides for the superficial drainage of the lateral aspect
of the arm and empties into the axillary vein.
 Basilic vein. The basilic vein is a superficial vein that drains the medial aspect of the arm
and empties into the brachial vein proximally.
 Median cubital vein. The basilic and cephalic veins are joined at the anterior aspect of
the elbow by the median cubital vein, often chosen as the site for blood removal for the
purpose of blood testing.
 Subclavian vein. The subclavian vein receives venous blood from the arm through the
axillary vein and from the skin and muscles of the head through the external jugular
vein.
 Vertebral vein. The vertebral vein drains the posterior part of the head.
 Internal jugular vein. The internal jugular vein drains the dural sinuses of the brain.
 Brachiocephalic veins. The right and left brachiocephalic veins are large veins that
receive venous drainage from the subclavian, vertebral, and internal jugular veins on their
respective sides.
  Azygos vein. The azygos vein is a single vein that drains the thorax and enters the
superior vena cava just before it joins the heart.

Veins Draining into the Inferior Vena Cava

The inferior vena cava, which is much longer than the superior vena cava, returns blood to the heart
from all body regions below the diaphragm.

 Tibial veins. The anterior and posterior tibial veins and the fibular vein drain the leg;
the posterior tibial veins becomes the popliteal vein at the knee and then
the femoral vein in the thigh; the femoral vein becomes the external iliac vein as it
enters the pelvis.
 Great saphenous veins. The great saphenous veins are the longest veins in the body;
they begin at the dorsal venous arch in the foot and travel up the medial aspect of the
leg to empty into the femoral vein in the thigh.
 Common iliac vein. Each common iliac vein is formed by the union of the external iliac
vein and the internal iliac vein which drains the pelvis.
 Gonadal vein. The right gonadal vein drains the right ovary in females and the right
testicles in males; the left gonadal veins empties into the left renal veins superiorly.
 Renal veins. The right and left renal veins drain the kidneys.
 Hepatic portal vein. The hepatic portal vein is a single vein that drains the digestive
tract organs and carries this blood through the liver before it enters the systemic
circulation.
 Hepatic veins. The hepatic veins drain the liver.

Physiology of the Heart

As the heart beats or contracts, the blood makes continuous round trips- into and out of the heart,
through the rest of the body, and then back to the heart- only to be sent out again.

Intrinsic Conduction System of the Heart

The spontaneous contractions of the cardiac muscle cells occurs in a regular and continuous way,
giving rhythm to the heart.
Cardiac muscle cells. Cardiac muscle cells can and do contract spontaneously and independently,
even if all nervous connections are severed.

 Rhythms. Although cardiac muscles can beat independently, the muscle cells in the
different areas of the heart have different rhythms.
 Intrinsic conduction system. The intrinsic conduction system, or the nodal system,
that is built into the heart tissue sets the basic rhythm.
 Composition. The intrinsic conduction system is composed of a special tissue found
nowhere else in the body; it is much like a cross between a muscle and nervous tissue.
 Function. This system causes heart muscle depolarization in only one direction- from the
atria to the ventricles; it enforces a contraction rate of approximately 75 beats per minute
on the heart, thus the heart beats as a coordinated unit.
 Sinoatrial (SA) node. The SA node has the highest rate of depolarization in the
whole system, so it can start the beat and set the pace for the whole heart; thus the term
“pacemaker“.
 Atrial contraction. From the SA node, the impulse spread through the atria to the AV
node, and then the atria contract.
 Ventricular contraction. It then passes through the AV bundle, the bundle branches,
and the Purkinje fibers, resulting in a “wringing” contraction of the ventricles that begins
at the heart apex and moves toward the atria.
 Ejection. This contraction effectively ejects blood superiorly into the large arteries leaving
the heart.

The Pathway of the Conduction System

The conduction system occurs systematically through:

 SA node. The depolarization wave is initiated by the sinoatrial node.

 Atrial myocardium. The wave then successively passes through the atrial myocardium.
 Atrioventricular node. The depolarization wave then spreads to the AV node, and then
the atria contract.
 AV bundle. It then passes rapidly through the AV bundle.
 Bundle branches and Purkinje fibers. The wave then continues on through the right
and left bundle branches, and then to the Purkinje fibers in the ventricular walls, resulting
in a contraction that ejects blood, leaving the heart.

Blood Circulation Through the Heart

The right and left sides of the heart work together in achieving a smooth flowing blood circulation.
  Entrance to the heart. Blood enters the heart through two large veins, the inferior and
superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the
heart.
 Atrial contraction. As the atrium contracts, blood flows from the right atrium to the right
ventricle through the open tricuspid valve.
 Closure of the tricuspid valve. When the ventricle is full, the tricuspid valve shuts to
prevent blood from flowing backward into the atria while the ventricle contracts.
 Ventricle contraction. As the ventricle contracts, blood leaves the heart through the
pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.
 Oxygen-rich blood circulates. The pulmonary vein empties oxygen-rich blood from the
lungs into the left atrium of the heart.
 Opening of the mitral valve. As the atrium contracts, blood flows from your left atrium
into your left ventricle through the open mitral valve.
 Prevention of backflow. When the ventricle is full, the mitral valve shuts. This prevents
blood from flowing backward into the atrium while the ventricle contracts.
 Blood flow to systemic circulation. As the ventricle contracts, blood leaves the heart
through the aortic valve, into the aorta and to the body.

Capillary Exchange of Gases and Nutrients

Substances tend to move to and from the body cells according to their concentration gradients.

 Capillary network. Capillaries form an intricate network among the body’s cells such that
no substance has to diffuse very far to enter or leave a cell.
 Routes. Basically, substances leaving or entering the blood may take one of four routes
across the plasma membranes of the single layer of endothelial cells forming the capillary
wall.
 Lipid-soluble substances. As with all cells, substances can diffuse directly through
their plasma membranes if the substances are lipid-soluble.
  Lipid-insoluble substances. Certain lipid-insoluble substances may enter or leave the
blood and/or pass through the plasma membranes within vesicles, that is,
by endocytosis or exocytosis.
 Intercellular clefts. Limited passage of fluid and small solutes is allowed by intercellular
clefts (gaps or areas of plasma membrane not joined by tight junctions), so most of our
capillaries have intercellular clefts.
 Fenestrated capillaries. Very free passage of small solutes and fluid is allowed by
fenestrated capillaries, and these unique capillaries are found where absorption is a
priority or where filtration occurs.

 Complement fixation. Complement is the chief antibody ammunition used against

cellular antigens, and it is fixed (activated) during innate defenses; it is also activated very
efficiently when it binds to antibodies attached to cellular targets.
 Neutralization. Neutralization occurs when antibodies bind to specific sites on
bacterial exotoxins (toxic chemicals secreted by bacteria) or on viruses that can cause
cellular injury; in this way they block the harmful effects of the exotoxin or virus.
 Agglutination. When the cross-linking involves cell-bound antigens, the process causes
clumping of the foreign cells, a process called agglutination; this type of antigen-antibody
reaction occurs when mismatched blood is transfused and is the basis of tests used for
blood typing.
 Precipitation. When the cross-linking involves soluble antigenic molecules, the resulting
antigen-antibody complexes are so large that they become insoluble and settle out of
solution; this cross-linking reaction is more precisely called precipitation.

DIGESTIVE SYTEM

The easiest way to understand the digestive system is to divide its organs into two main categories.
The first group is the organs that make up the alimentary canal. Accessory digestive organs comprise
the second group and are critical for orchestrating the breakdown of food and the assimilation of its
nutrients into the body. Accessory digestive organs, despite their name, are critical to the function of
the digestive system.

Functions of the Digestive System

The functions of the digestive system are:

1. Ingestion. Food must be placed into the mouth before it can be acted on; this is an
active, voluntary process called ingestion.
2. Propulsion. If foods are to be processed by more than one digestive organ, they must
be propelled from one organ to the next; swallowing is one example of food movement
that depends largely on the propulsive process called peristalsis (involuntary,
alternating waves of contraction and relaxation of the muscles in the organ wall).
 3. Food breakdown: mechanical digestion. Mechanical digestion prepares food for
further degradation by enzymes by physically fragmenting the foods into smaller pieces,
and examples of mechanical digestion are: mixing of food in the mouth by the tongue,
churning of food in the stomach, and segmentation in the small intestine.
4. Food breakdown: chemical digestion. The sequence of steps in which the large food
molecules are broken down into their building blocks by enzymes is called chemical
digestion.
5. Absorption. Transport of digested end products from the lumen of the GI tract to
the blood or lymph is absorption, and for absorption to happen, the digested foods must
first enter the mucosal cells by active or passive transport processes.
6. Defecation. Defecation is the elimination of indigestible residues from the GI tract via
the anus in the form of feces.

Anatomy of the Digestive System

The organs of the digestive system can be separated into two main groups: those forming the
alimentary canal and the accessory digestive organs.

Organs of the Alimentary Canal

The alimentary canal, also called the gastrointestinal tract, is a continuous, hollow muscular tube that
winds through the ventral body cavity and is open at both ends. Its organs include the following:

Mouth

Food enters the digestive tract through the mouth, or oral cavity, a mucous membrane-lined cavity.

 Lips. The lips (labia) protect its anterior opening.

 Cheeks. The cheeks form its lateral walls.
  Palate. The hard palate forms its anterior roof, and the soft palate forms its posterior
roof.
 Uvula. The uvula is a fleshy finger-like projection of the soft palate, which extends
inferiorly from the posterior edge of the soft palate.
 Vestibule. The space between the lips and the cheeks externally and the teeth and gums
internally is the vestibule.
 Oral cavity proper. The area contained by the teeth is the oral cavity proper.
 Tongue. The muscular tongue occupies the floor of the mouth and has several bony
attachments- two of these are to the hyoid bone and the styloid processes of the skull.
 Lingual frenulum. The lingual frenulum, a fold of mucous membrane, secures the tongue
to the floor of the mouth and limits its posterior movements.
 Palatine tonsils. At the posterior end of the oral cavity are paired masses
of lymphatic tissue, the palatine tonsils.
 Lingual tonsil. The lingual tonsils cover the base of the tongue just beyond.

Pharynx

From the mouth, food passes posteriorly into the oropharynx and laryngopharynx.

 Oropharynx. The oropharynx is posterior to the oral cavity.

 Laryngopharynx. The laryngopharynx is continuous with the esophagus below; both of
which are common passageways for food, fluids, and air.

Esophagus

The esophagus or gullet, runs from the pharynx through the diaphragm to the stomach.

 Size and function. About 25 cm (10 inches) long, it is essentially a passageway that
conducts food by peristalsis to the stomach.
 Structure. The walls of the alimentary canal organs from the esophagus to the large
intestine are made up of the same four basic tissue layers or tunics.
 Mucosa. The mucosa is the innermost layer, a moist membrane that lines the cavity, or
lumen, of the organ; it consists primarily of a surface epithelium, plus a small amount of
connective tissue (lamina propria) and a scanty smooth muscle layer.
 Submucosa. The submucosa is found just beneath the mucosa; it is a soft connective
tissue layer containing blood vessels, nerve endings, lymph nodules, and lymphatic
vessels.
 Muscularis externa. The muscularis externa is a muscle layer typically made up of an
inner circular layer and an outer longitudinal layer of smooth muscle cells.
 Serosa. The serosa is the outermost layer of the wall that consists of a single layer of flat
serous fluid-producing cells, the visceral peritoneum.
 Intrinsic nerve plexuses. The alimentary canal wall contains two important intrinsic
nerve plexuses- the submucosal nerve plexus and the myenteric nerve plexus, both
 of which are networks of nerve fibers that are actually part of the autonomic nervous
system and help regulate the mobility and secretory activity of the GI tract organs.

Stomach

Different regions of the stomach have been named, and they include the following:

 Location. The C-shaped stomach is on the left side of the abdominal cavity, nearly hidden
by the liver and the diaphragm.
 Function. The stomach acts as a temporary “storage tank” for food as well as a site for
food breakdown.
 Cardiac region. The cardiac region surrounds the cardioesophageal sphincter,
through which food enters the stomach from the esophagus.
 Fundus. The fundus is the expanded part of the stomach lateral to the cardiac region.
 Body. The body is the midportion, and as it narrows inferiorly, it becomes the pyloric
antrum, and then the funnel-shaped pylorus.
 Pylorus. The pylorus is the terminal part of the stomach and it is continuous with the
small intestine through the pyloric sphincter or valve.
 Size. The stomach varies from 15 to 25 cm in length, but its diameter and volume
depend on how much food it contains; when it is full, it can hold about 4 liters (1 gallon)
of food, but when it is empty it collapses inward on itself.
 Rugae. The mucosa of the stomach is thrown into large folds called rugae when it is
empty.
  Greater curvature. The convex lateral surface of the stomach is the greater curvature.
 Lesser curvature. The concave medial surface is the lesser curvature.
 Lesser omentum. The lesser omentum, a double layer of peritoneum, extends from the
liver to the greater curvature.
 Greater omentum. The greater omentum, another extension of the peritoneum, drapes
downward and covers the abdominal organs like a lacy apron before attaching to the
posterior body wall, and is riddled with fat, which helps to insulate, cushion, and protect
the abdominal organs.
 Stomach mucosa. The mucosa of the stomach is a simple columnar epithelium
composed entirely of mucous cells that produce a protective layer of bicarbonate-rich
alkaline mucus that clings to the stomach mucosa and protects the stomach wall from
being damaged by acid and digested by enzymes.
 Gastric glands. This otherwise smooth lining is dotted with millions of deep gastric pits,
which lead into gastric glands that secrete the solution called gastric juice.
 Intrinsic factor. Some stomach cells produce intrinsic factor, a substance needed for the
absorption of vitamin b12 from the small intestine.
 Chief cells. The chief cells produce protein-digesting enzymes, mostly pepsinogens.
 Parietal cells. The parietal cells produce corrosive hydrochloric acid, which makes the
stomach contents acidic and activates the enzymes.
 Enteroendocrine cells. The enteroendocrine cells produce local hormones such
as gastrin, that are important to the digestive activities of the stomach.
 Chyme. After food has been processed, it resembles heavy cream and is called chyme.

Small Intestine

The small intestine is the body’s major digestive organ.

 Location. The small intestine is a muscular tube extending from the pyloric sphincter to
the large intestine.
 Size. It is the longest section of the alimentary tube, with an average length of 2.5 to 7
m (8 to 20 feet) in a living person.
  Subdivisions. The small intestine has three subdivisions: the duodenum, the jejunum,
and the ileum, which contribute 5 percent, nearly 40 percent, and almost 60 percent of
the small intestine, respectively.
 Ileocecal valve. The ileum meets the large intestine at the ileocecal valve, which joins
the large and small intestine.
 Hepatopancreatic ampulla. The main pancreatic and bile ducts join at the duodenum to
form the flasklike hepatopancreatic ampulla, literally, the ” liver-pacreatic-
enlargement”.
 Duodenal papilla. From there, the bile and pancreatic juice travel through the duodenal
papilla and enter the duodenum together.
 Microvilli. Microvilli are tiny projections of the plasma membrane of the mucosa cells that
give the cell surface a fuzzy appearance, sometimes referred to as the brush border; the
plasma membranes bear enzymes (brush border enzymes) that complete the digestion of
proteins and carbohydrates in the small intestine.
 Villi. Villi are fingerlike projections of the mucosa that give it a velvety appearance and
feel, much like the soft nap of a towel.
 Lacteal. Within each villus is a rich capillary bed and a modified lymphatic capillary called
a lacteal.
 Circular folds. Circular folds, also called plicae circulares, are deep folds of both
mucosa and submucosa layers, and they do not disappear when food fills the small
intestine.
 Peyer’s patches. In contrast, local collections of lymphatic tissue found in the submucosa
increase in number toward the end of the small intestine.

Large Intestine

The large intestine is much larger in diameter than the small intestine but shorter in length.

 Size. About 1.5 m (5 feet) long, it extends from the ileocecal valve to the anus.
 Functions. Its major functions are to dry out indigestible food residue by absorbing water
and to eliminate these residues from the body as feces.
  Subdivisions. It frames the small intestines on three sides and has the following
subdivisions: cecum, appendix, colon, rectum, and anal canal.
 Cecum. The saclike cecum is the first part of the large intestine.
 Appendix. Hanging from the cecum is the wormlike appendix, a potential trouble spot
because it is an ideal location for bacteria to accumulate and multiply.
 Ascending colon. The ascending colon travels up the right side of the abdominal cavity
and makes a turn, the right colic (or hepatic) flexure, to travel across the abdominal
cavity.
 Transverse colon. The ascending colon makes a turn and continuous to be the
transverse colon as it travels across the abdominal cavity.
 Descending colon. It then turns again at the left colic (or splenic) flexure, and
continues down the left side as the descending colon.
 Sigmoid colon. The intestine then enters the pelvis, where it becomes the S-shaped
sigmoid colon.
 Anal canal. The anal canal ends at the anus which opens to the exterior.
 External anal sphincter. The anal canal has an external voluntary sphincter, the
external anal sphincter, composed of skeletal muscle.
 Internal involuntary sphincter. The internal involuntary sphincter is formed by smooth
muscles.

Accessory Digestive Organs

Other than the intestines and the stomach, the following are also part of the digestive system:

Teeth

The role the teeth play in food processing needs little introduction; we masticate, or chew, by opening
and closing our jaws and moving them from side to side while continuously using our tongue to move
the food between our teeth.

 Function. The teeth tear and grind the food, breaking it down into smaller fragments.
  Deciduous teeth. The first set of teeth is the deciduous teeth, also called baby
teeth or milk teeth, and they begin to erupt around 6 months, and a baby has a full set
(20 teeth) by the age of 2 years.
 Permanent teeth. As the second set of teeth, the deeper permanent teeth, enlarge and
develop, the roots of the milk teeth are reabsorbed, and between the ages of 6 to 12 years
they loosen and fall out.
 Incisors. The chisel-shaped incisors are adapted for cutting.
 Canines. The fanglike canines are for tearing and piercing.
 Premolars and molars. Premolars (bicuspids) and molars have broad crowns with round
cusps ( tips) and are best suited for grinding.
 Crown. The enamel-covered crown is the exposed part of the tooth above the gingiva or
gum.
 Enamel. Enamel is the hardest substance in the body and is fairly brittle because it is
heavily mineralized with calcium salts.
 Root. The outer surface of the root is covered by a substance called cementum, which
attaches the tooth to the periodontal membrane (ligament).
 Dentin. Dentin, a bonelike material, underlies the enamel and forms the bulk of the tooth.
 Pulp cavity. It surrounds a central pulp cavity, which contains a number of structures
(connective tissue, blood vessels, and nerve fibers) collectively called the pulp.
 Root canal. Where the pulp cavity extends into the root, it becomes the root canal, which
provides a route for blood vessels, nerves, and other pulp structures to enter the pulp
cavity of the tooth.

Salivary Glands

Three pairs of salivary glands empty their secretions into the mouth.

 Parotid glands. The large parotid glands lie anterior to the ears and empty their
secretions into the mouth.
 Submandibular and sublingual glands. The submandibular and sublingual glands
empty their secretions into the floor of the mouth through tiny ducts.
 Saliva. The product of the salivary glands, saliva, is a mixture of mucus and serous fluids.
 Salivary amylase. The clear serous portion contains an enzyme, salivary amylase, in a
bicarbonate-rich juice that begins the process of starch digestion in the mouth.

Pancreas

Only the pancreas produces enzymes that break down all categories of digestible foods.
  Location. The pancreas is a soft, pink triangular gland that extends across the abdomen
from the spleen to the duodenum; but most of the pancreas lies posterior to the parietal
peritoneum, hence its location is referred to as retroperitoneal.
 Pancreatic enzymes. The pancreatic enzymes are secreted into the duodenum in an
alkaline fluid that neutralizes the acidic chyme coming in from the stomach.
 Endocrine function. The pancreas also has an endocrine function; it produces
hormones insulin and glucagon.

Liver

The liver is the largest gland in the body.

 Location. Located under the diaphragm, more to the right side of the body, it overlies and
almost completely covers the stomach.
 Falciform ligament. The liver has four lobes and is suspended from the diaphragm and
abdominal wall by a delicate mesentery cord, the falciform ligament.
 Function. The liver’s digestive function is to produce bile.
 Bile. Bile is a yellow-to-green, watery solution containing bile salts, bile
pigments, cholesterol, phospholipids, and a variety of electrolytes.
 Bile salts. Bile does not contain enzymes but its bile salts emulsify fats by physically
breaking large fat globules into smaller ones, thus providing more surface area for the fat-
digesting enzymes to work on.

Gallbladder

While in the gallbladder, bile is concentrated by the removal of water.

 Location. The gallbladder is a small, thin-walled green sac that snuggles in a shallow
fossa in the inferior surface of the liver.
 Cystic duct. When food digestion is not occurring, bile backs up the cystic duct and
enters the gallbladder to be stored.

Physiology of the Digestive System

Specifically, the digestive system takes in food (ingests it), breaks it down physically and chemically
into nutrient molecules (digests it), and absorbs the nutrients into the bloodstream, then, it rids the
body of indigestible remains (defecates).

Activities Occurring in the Mouth, Pharynx, and Esophagus

The activities that occur in the mouth, pharynx, and esophagus are food ingestion, food breakdown,
and food propulsion.

Food Ingestion and Breakdown

Once food is placed in the mouth, both mechanical and chemical digestion begin.

 Physical breakdown. First, the food is physically broken down into smaller particles by
chewing.
 Chemical breakdown. Then, as the food is mixed with saliva, salivary amylase begins
the chemical digestion of starch, breaking it down into maltose.
 Stimulation of saliva. When food enters the mouth, much larger amounts of saliva pour
out; however, the simple pressure of anything put into the mouth and chewed will also
stimulate the release of saliva.
 Passageways. The pharynx and the esophagus have no digestive function; they simply
provide passageways to carry food to the next processing site, the stomach.

Food Propulsion – Swallowing and Peristalsis

For food to be sent on its way to the mouth, it must first be swallowed.

 Deglutition. Deglutition, or swallowing, is a complex process that involves the

coordinated activity of several structures (tongue, soft palate, pharynx, and esophagus).
 Buccal phase of deglutition. The first phase, the voluntary buccal phase, occurs in the
mouth; once the food has been chewed and well mixed with saliva, the bolus (food mass)
is forced into the pharynx by the tongue.
 Pharyngeal-esophageal phase. The second phase, the involuntary pharyngeal-
esophageal phase, transports food through the pharynx and esophagus; the
parasympathetic division of the autonomic nervous system controls this phase and
promotes the mobility of the digestive organs from this point on.
 Food routes. All routes that the food may take, except the desired route distal into the
digestive tract, are blocked off; the tongue blocks off the mouth; the soft palate closes off
the nasal passages; the larynx rises so that its opening is covered by the flaplike epiglottis.
 Stomach entrance. Once food reaches the distal end of the esophagus, it presses
against the cardioesophageal sphincter, causing it to open, and food enters the stomach.

Activities of the Stomach

The activities of the stomach involve food breakdown and food propulsion.

Food Breakdown

The sight, smell, and taste of food stimulate parasympathetic nervous system reflexes, which increase
the secretion of gastric juice by the stomach glands

 Gastric juice. Secretion of gastric juice is regulated by both neural and hormonal factors.
  Gastrin. The presence of food and a rising pH in the stomach stimulate the stomach cells
to release the hormone gastrin, which prods the stomach glands to produce still more of
the protein-digesting enzymes (pepsinogen), mucus, and hydrochloric acid.
 Pepsinogen. The extremely acidic environment that hydrochloric acid provides is
necessary, because it activates pepsinogen to pepsin, the active protein-digesting
enzyme.
 Rennin. Rennin, the second protein-digesting enzyme produced by the stomach, works
primarily on milk protein and converts it to a substance that looks like sour milk.
 Food entry. As food enters and fills the stomach, its wall begins to stretch (at the same
time as the gastric juices are being secreted).
 Stomach wall activation. Then the three muscle layers of the stomach wall become
active; they compress and pummel the food, breaking it apart physically, all the while
continuously mixing the food with the enzyme-containing gastric juice so that the semifluid
chyme is formed.

Food Propulsion

Peristalsis is responsible for the movement of food towards the digestive site until the intestines.

 Peristalsis. Once the food has been well mixed, a rippling peristalsis begins in the upper
half of the stomach, and the contractions increase in force as the food approaches the
pyloric valve.
 Pyloric passage. The pylorus of the stomach, which holds about 30 ml of chyme, acts like
a meter that allows only liquids and very small particles to pass through the pyloric
sphincter; and because the pyloric sphincter barely opens, each contraction of the
stomach muscle squirts 3 ml or less of chyme into the small intestine.
 Enterogastric reflex. When the duodenum is filled with chyme and its wall is stretched, a
nervous reflex, the enterogastric reflex, occurs; this reflex “puts the brakes on” gastric
activity and slows the emptying of the stomach by inhibiting the vagus nerves and
tightening the pyloric sphincter, thus allowing time for intestinal processing to catch up.

Activities of the Small Intestine

The activities of the small intestine are food breakdown and absorption and food propulsion.

Food Breakdown and Absorption

Food reaching the small intestine is only partially digested.

 Digestion. Food reaching the small intestine is only partially digested; carbohydrate and
protein digestion has begun, but virtually no fats have been digested up to this point.
 Brush border enzymes. The microvilli of small intestine cells bears a few important
enzymes, the so-called brush border enzymes, that break down double sugars into simple
sugars and complete protein digestion.
 Pancreatic juice. Foods entering the small intestine are literally deluged with enzyme-
rich pancreatic juice ducted in from the pancreas, as well as bile from the liver; pancreatic
juice contains enzymes that, along with brush border enzymes, complete the digestion of
 starch, carry out about half of the protein digestion, and are totally responsible for fat
digestion and digestion of nucleic acids.
 Chyme stimulation. When chyme enters the small intestine, it stimulates the mucosa
cells to produce several hormones; two of these are secretin and cholecystokinin which
influence the release of pancreatic juice and bile.
 Absorption. Absorption of water and of the end products of digestion occurs all along the
length of the small intestine; most substances are absorbed through the intestinal cell
plasma membranes by the process of active transport.
 Diffusion. Lipids or fats are absorbed passively by the process of diffusion.
 Debris. At the end of the ileum, all that remains are some water, indigestible food
materials, and large amounts of bacteria; this debris enters the large intestine through the
ileocecal valve.

Food Propulsion
Peristalsis is the major means of propelling food through the digestive tract.

Peristalsis. The net effect is that the food is moved through the small intestine in much the same way
that toothpaste is squeezed from the tube.

 Constrictions. Rhythmic segmental movements produce local constrictions of the

intestine that mix the chyme with the digestive juices, and help to propel food through the
intestine.

Activities of the Large Intestine

The activities of the large intestine are food breakdown and absorption and defecation.

Food Breakdown and Absorption

What is finally delivered to the large intestine contains few nutrients, but that residue still has 12 to 24
hours more to spend there.

 Metabolism. The “resident” bacteria that live in its lumen metabolize some of the
remaining nutrients, releasing gases (methane and hydrogen sulfide) that contribute to
the odor of feces.
 Flatus. About 50 ml of gas (flatus) is produced each day, much more when certain
carbohydrate-rich foods are eaten.
 Absorption. Absorption by the large intestine is limited to the absorption of vitamin K,
some B vitamins, some ions, and most of the remaining water.
 Feces. Feces, the more or less solid product delivered to the rectum, contains undigested
food residues, mucus, millions of bacteria, and just enough water to allow their smooth
passage.

Propulsion of the Residue and Defecation

When presented with residue, the colon becomes mobile, but its contractions are sluggish or short-
lived.
  Haustral contractions. The movements most seen in the colon are haustral contractions,
slow segmenting movements lasting about one minute that occur every 30 minutes or so.
 Propulsion. As the haustrum fills with food residue, the distension stimulates its muscle
to contract, which propels the luminal contents into the next haustrum.
 Mass movements. Mass movements are long, slow-moving, but powerful contractile
waves that move over large areas of the colon three or four times daily and force the
contents toward the rectum.
 Rectum. The rectum is generally empty, but when feces are forced into it by mass
movements and its wall is stretched, the defecation reflex is initiated.
 Defecation reflex. The defecation reflex is a spinal (sacral region) reflex that causes the
walls of the sigmoid colon and the rectum to contract and anal sphincters to relax.
 Impulses. As the feces is forced into the anal canal, messages reach the brain giving us
time to make a decision as to whether the external voluntary sphincter should remain
open or be constricted to stop passage of feces.
 Relaxation. Within a few seconds, the reflex contractions end and rectal walls relax; with
the next mass movement, the defecation reflex is initiated again.

Lesson Proper for Week 17

URINARY SYSTEM

The kidney and urinary systems help the body to get rid of liquid waste called urea. They also
help to keep chemicals (such as potassium and sodium) and water in balance. Urea is produced
when foods containing protein (such as meat, poultry, and certain vegetables) are broken down in
the body. Urea is carried in the blood to the kidneys. This is where it is removed, along with
water and other wastes in the form of urine.

Functions of the Urinary System

The function of the kidneys are as follows:

1. Filter. Every day, the kidneys filter gallons of fluid from the bloodstream.
2. Waste processing. The kidneys then process this filtrate,
allowing wastes and excess ions to leave the body in urine while returning needed
substances to the blood in just the right proportions.
3. Elimination. Although the lungs and the skin also play roles in excretion, the
kidneys bear the major responsibility for eliminating nitrogenous wastes, toxins,
and drugs from the body.
 4. Regulation. The kidneys also regulate the blood’s volume and chemical makeup so
that the proper balance between water and salts and between acids and bases is
maintained.
5. Other regulatory functions. By producing the enzyme renin, they help
regulate blood pressure, and their hormone erythropoietin stimulates red blood
cell production in the bone marrow.
6. Conversion. Kidney cells also convert vitamin D to its active form.

Anatomy of the Urinary System

The urinary system consists of two kidneys, two ureters, a urinary bladder, and a urethra. The
kidneys alone perform the functions just described and manufacture urine in the process, while
the other organs of the urinary system provide temporary storage reservoirs for urine or serve as
transportation channels to carry it from one body region to another.
The Kidneys

The kidneys, which maintain the purity and constancy of our internal fluids, are perfect examples
of homeostatic organs.

 Location. These small, dark red organs with a kidney-bean shape lie against the
dorsal body wall in a retroperitoneal position (beneath the parietal peritoneum) in
the superior lumbar region; they extend from the T12 to the L3 vertebra, thus they
receive protection from the lower part of the rib cage.
 Positioning. Because it is crowded by the liver, the right kidney is positioned
slightly lower than the left.
 Size. An adult kidney is about 12 cm (5 inches) long, 6 cm (2.5 inches) wide, and 3
cm (1 inch) thick, about the size of a large bar of soap.
 Adrenal gland. Atop each kidney is an adrenal gland, which is part of the endocrine
system is a distinctly separate organ functionally.
 Fibrous capsule. A transparent fibrous capsule encloses each kidney and gives a
fresh kidney a glistening appearance.
 Perirenal fat capsule. A fatty mass, the perirenal fat capsule, surrounds each kidney
and acts to cushion it against blows.
 Renal fascia. The renal fascia, the outermost capsule, anchors the kidney and helps
hold it in place against the muscles of the trunk wall.

 Renal cortex. The outer region, which is light in color, is the renal cortex.
 Renal medulla. Deep to the cortex is a darker, reddish-brown area, the renal medulla.
 Renal pyramids. The medulla has many basically triangular regions with a striped
appearance, the renal, or medullary pyramids; the broader base of each pyramid faces
toward the cortex while its tip, the apex, points toward the inner region of the kidney.
 Renal columns. The pyramids are separated by extensions of cortex-like tissue, the
renal columns.
 Renal pelvis. Medial to the hilum is a flat, basinlike cavity, the renal pelvis, which is
continuous with the ureter leaving the hilum.
 Calyces. Extensions of the pelvis, calyces, form cup-shaped areas that enclose the
tips of the pyramid and collect urine, which continuously drains from the tips of the
pyramids into the renal pelvis.
 Renal artery. The arterial supply of each kidney is the renal artery, which divides
into segmental arteries as it approaches the hilum, and each segmental artery gives
off several branches called interlobar arteries.
 Arcuate arteries. At the cortex-medulla junction, interlobar arteries give off arcuate
arteries, which curve over the medullary pyramids.
 Cortical radiate arteries. Small cortical radiate arteries then branch off the arcuate
arteries and run outward to supply the cortical tissue.
Nephrons
Nephrons are the structural and functional units of the kidneys.

 Nephrons. Each kidney contains over a million tiny structures called nephrons, and
they are responsible for forming urine.
 Glomerulus. One of the main structures of a nephron, a glomerulus is a knot of
capillaries.
 Renal tubule. Another one of the main structures in a nephron is the renal tubule.
 Bowman’s capsule. The closed end of the renal tubule is enlarged and cup-shaped
and completely surrounds the glomerulus, and it is called the glomerular or
Bowman’s capsule.
 Podocytes. The inner layer of the capsule is made up of highly modified octopus-
like cells called podocytes.
 Foot processes. Podocytes have long branching processes called foot processes that
intertwine with one another and cling to the glomerulus.
 Collecting duct. As the tubule extends from the glomerular capsule, it coils and
twists before forming a hairpin loop and then again becomes coiled and twisted
before entering a collecting tubule called the collecting duct, which receives urine
from many nephrons.
 Proximal convoluted tubule. This is the part of the tubule that is near to the
glomerular capsule.
 Loop of Henle. The loop of Henle is the hairpin loop following the proximal
convoluted tubule.
 Distal convoluted tubule. After the loop of Henle, the tubule continues to coil and
twist before the collecting duct, and this part is called the distal convoluted tubule.
  Cortical nephrons. Most nephrons are called cortical nephrons because they are
located almost entirely within the cortex.
 Juxtamedullary nephrons. In a few cases, the nephrons are called juxtamedullary
nephrons because they are situated next to the cortex-medullary junction, and their
loops of Henle dip deep into the medulla.
 Afferent arteriole. The afferent arteriole, which arises from a cortical radiate artery,
is the “feeder vessel”.
 Efferent arteriole. The efferent arteriole receives blood that has passed through the
glomerulus.
 Peritubular capillaries. They arise from the efferent arteriole that drains the
glomerulus.

Ureters
The ureters do play an active role in urine transport.

 Size. The ureters are two slender tubes each 25 to 30 cm (10 to 12 inches) long and 6
mm (1/4 inch) in diameter.
 Location. Each ureter runs behind the peritoneum from the renal hilum to the
posterior aspect of the bladder, which it enters at a slight angle.
 Function. Essentially, the ureters are passageways that carry urine from the kidneys
to the bladder through contraction of the smooth muscle layers in their walls that
propel urine into the bladder by peristalsis and is prevented from flowing back by
small valve-like folds of bladder mucosa that flap over the ureter openings.

Urinary Bladder
The urinary bladder is a smooth, collapsible, muscular sac that stores urine temporarily.

 Location. It is located retroperitoneally in the pelvis just posterior to the symphysis

pubis.
 Function. The detrusor muscles and the transitional epithelium both make the
bladder uniquely suited for its function of urine storage.
 Trigone. The smooth triangular region of the bladder base outlined by these three
openings is called the trigone, where infections tend to persist.
 Detrusor muscles. The bladder wall contains three layers of smooth muscle,
collectively called the detrusor muscle, and its mucosa is a special type of
epithelium, transitional epithelium.

Urethra
The urethra is a thin-walled tube that carries urine by peristalsis from the bladder to the outside
of the body.

 Internal urethral sphincter. At the bladder-urethral junction, a thickening of the

smooth muscle forms the internal urethral sphincter, an involuntary sphincter that
keeps the urethra closed when the urine is not being passed.
 External urethral sphincter. A second sphincter, the external urethral sphincter, is
fashioned by skeletal muscle as the urethra passes through the pelvic floor and
is voluntarily controlled.
 Female urethra. The female urethra is about 3 to 4 cm (1 1/2 inches) long, and its
external orifice, or opening, lies anteriorly to the vaginal opening.
 Male urethra. In me, the urethra is approximately 20 cm (8 inches) long and has
three named regions: the prostatic, membranous, and spongy (penile) urethrae; it
opens at the tip of the penis after traveling down its length.

Physiology of the Urinary System

Every day, the kidneys filter gallons of fluid from the bloodstream. The normal physiology that
takes place in the urinary system are as follows:
Urine Formation
Urine formation is a result of three processes:

 Glomerular filtration. Water and solutes smaller than proteins are forced through
the capillary walls and pores of the glomerular capsule into the renal tubule.
 Tubular reabsorption. Water, glucose, amino acids, and needed ions are transported
out of the filtrate into the tubule cells and then enter the capillary blood.
  Tubular secretion. Hydrogen, potassium, creatinine, and drugs are removed from the
peritubular blood and secreted by the tubule cells into the filtrate.

Characteristics of Urine
In 24 hours, the marvelously complex kidneys filter some 150 to 180 liters of blood plasma
through their glomeruli into the tubules.

 Daily volume. In 24 hours, only about 1.0 to 1.8 liters of urine are produced.
 Components. Urine contains nitrogenous wastes and unneeded substances.
 Color. Freshly voided urine is generally clear and pale to deep yellow.
 Odor. When formed, urine is sterile and slightly aromatic, but if allowed to stand, it
takes on an ammonia odor caused by the action of bacteria on the urine solutes.
 pH. Urine pH is usually slightly acidic (around 6), but changes in body metabolism
and certain foods may cause it to be much more acidic or basic.
 Specific gravity. Whereas the specific gravity of pure water is 1.0, the specific
gravity of urine usually ranges from 1.001 to 1.035.
 Solutes. Solutes normally found in urine include sodium and potassium ions, urea,
uric acid, creatinine, ammonia, bicarbonate ions, and various other ions.

Micturition
Micturition or voiding is the act of emptying the bladder.

 Accumulation. Ordinarily, the bladder continues to collect urine until about 200 ml
have accumulated.
 Activation. At about this point, stretching of the bladder wall activates stretch
receptors.
 Transmission. Impulses transmitted to the sacral region of the spinal cord and then
back to the bladder via the pelvic splanchnic nerves cause the bladder to go into
reflex contractions.
 Passage. As the contractions become stronger, stored urine is forced past the internal
urethral sphincter into the upper part of the urethra.
 External sphincter. Because the lower external sphincter is skeletal muscle and
voluntarily controlled, we can choose to keep it closed or it can be relaxed so that
urine is flushed from the body.

FEMALE REPRODUCTIVE SYSTEM

Women have the responsibility of bringing forth life into the world, hence the creation and the
function of the female reproductive system. This system performs a miracle from the conception
of life until the birth of the growing life within, and it is only proper to be introduced to the main
characters and supporting roles of this play.

Internal Structures
Ovaries

 The ovaries are the ultimate life-maker for the females.

 For its physical structure, it has an estimated length of 4 cm and width of 2 cm and is
1.5 cm thick. It appears to be shaped like an almond. It looks pitted, like a raisin, but
is grayish white in color.
 It is located proximal to both sides of the uterus at the lower abdomen.
 For its function, the ovaries produce, mature, and discharge the egg cells or ova.
 Ovarian function is for the maturation and maintenance of the secondary sex
characteristics in females.
 It also has three divisions: the protective layer of epithelium, the cortex, and the
central medulla.

Fallopian Tubes

 The fallopian tubes serve as the pathway of the egg cells towards the uterus.
  It is a smooth, hollow tunnel that is divided into four parts: the interstitial, which is 1
cm in length; the isthmus, which is2 cm in length; the ampulla, which is 5 cm in
length; and the infundibular, which is 2 cm long and shaped like a funnel.
 The funnel has small hairs called the fimbria that propel the ovum into the fallopian
tube.
 The fallopian tube is lined with mucous membrane, and underneath is the connective
tissue and the muscle layer.
 The muscle layer is responsible for the peristaltic movements that propel the ovum
forward.
 The distal ends of the fallopian tubes are open, making a pathway for conception to
occur.

Uterus

 The uterus is described as a hollow, muscular, pear-shaped organ.

 It is located at the lower pelvis, which is posterior to the bladder and anterior to the
rectum.
 The uterus has an estimated length of 5 to 7 cm and width of 5 cm. it is 2.5 cm deep
in its widest part.
 For non-pregnant women, it is approximately 60g in weight.
 Its function is to receive the ovum from the fallopian tube and provide a place for
implantation and nourishment.
 It also gives protection for the growing fetus.
 It is divided into three: the body, the isthmus, and the cervix. f
 The body forms the bulk of the uterus, being the uppermost part. This is also the part
that expands to accommodate the growing fetus.
 The isthmus is just a short connection between the body and the cervix. This is the
portion that is cut during a cesarean section.
 The cervix lies halfway above the vagina, and the other half extends into the vagina.
It has an internal and external cervical os, which is the opening into the cervical
canal.

External Structures
Mons Veneris

 The mons veneris is a pad of fat tissues over the symphysis pubis.
 It has a covering of coarse, curly hairs, the pubic hair.
 It protects the pubic bone from trauma.

Labia Minora

 The labia minora is a spread of two connective tissue folds that are pinkish in color.
  The internal surface is composed of mucous membrane and the external surface is
skin.
 It contains sebaceous glands all over the area.

Labia Majora

 Lateral to the labia minora are two folds of fat tissue covered by loose connective
tissue and epithelium, the labia majora.
 Its function is to protect the external genitalia and the distal urethra and vagina from
trauma.
 It is covered in pubic hair that serves as additional protection against harmful bacteria
that may enter the structure.

Vestibule

 It is a smooth, flattened surface inside the labia wherein the openings to the urethra
and the vagina arise.

Clitoris

 The clitoris is a small, circular organ of erectile tissue at the front of the labia minora.
 The prepuce, a fold of skin, serves as its covering.
 This is the center for sexual arousal and pleasure for females because it is highly
sensitive to touch and temperature.

Skene’s Glands

 Also called as paraurethral glands, they are found lateral to the urethral meatus and
have ducts that open into the urethra.
 The secretions from this gland lubricate the external genitalia during coitus.

Bartholin’s Gland

 Also called bulbovaginal gland, this is another gland responsible for the lubrication of
the external genitalia during coitus.
 It has ducts that open into the distal vagina.
 Both of these glands secretions are alkaline to help the sperm survive in the vagina.

Fourchette

 This is a ridge of tissue which is formed by the posterior joining of the labia minora
and majora.
 During episiotomy, this is the tissue that is cut to enlarge the vaginal opening.
Perineal Body

 This is a muscular area that stretches easily during childbirth.

 Most pregnancy exercises such as Kegel’s and squatting are done to strengthen the
perineal body to allow easier expansion during childbirth and avoid tearing the tissue.

Hymen

 This covers the opening of the vagina.

 It is tough, elastic, semicircle tissue torn during the first sexual intercourse.

MALE REPRODUCTIVE SYSTEM

In cooperation with the women, men were also given the task of supplying generations upon
generations of brethren for mankind’s race. They are also equipped with miracle-inducing parts
that enable them to propagate with the woman and bring forth life into the world.

Internal Structures

Epididymis
  This is a tightly coiled tube that is responsible for conducting the sperm from the
tubule to the vas deferens.
 It has a length of approximately 20 feet long.
 Some sperm are stored in the epididymis, along with the semen.
 The sperm takes an estimated 12 to 20 days of travel along the epididymis, and a total
of 64 days to reach maturity.

Vas Deferens

 The function of the vas deferens is to carry the sperm through the inguinal canal from
the epididymis into the abdominal cavity where it will end at the seminal vesicles and
the ejaculatory duct.
 It is a hollow tube that is protected by a thick fibrous coating and surrounded by
arteries and veins.

Seminal Vesicles

 These are two convoluted pouches along the lower portion of the posterior surface of
the bladder.
 The seminal vesicles secrete a liquid that is viscous and alkaline and has high protein,
sugar, and prostaglandin content, which makes the sperm increasingly motile.

Ejaculatory Ducts

 These ducts pass through the prostate gland to join the seminal vesicles and the
urethra.

Prostate Gland

 This is a chestnut-sized gland that is situated below the bladder.

 It secretes a thin, alkaline fluid that adds protection to the sperm from being
immobilized by the low pH level of the urethra.
 The urethra passes through its center like a doughnut.

Bulbourethral Glands

 Also called as Cowper’s gland, these glands also secrete alkaline fluid to counteract
the acidic environment in the urethra.
 These are twp glands located at either side of the prostate gland and seminal vesicles
and empty through the short ducts towards the urethra.
 Semen is a product of 60% from the prostate gland, 30% from the seminal vesicles,
5% from the epididymis, and 5% from the bulbourethral glands.

Urethra
  This structure passes through the prostate gland towards the shaft and glans penis.
 It is a hollow tube from the base of the bladder and lined with mucous membrane.
 It has a length of approximately 8 inches or 18 to 20 cm.

External Structures
Scrotum

 The scrotum is responsible for the support of the testes and it regulates the
temperature of the sperm.
 It is a rugated, muscular, skin-covered pouch over the perineum.
 To promote the production and viability of the sperm, the scrotum contracts towards
the body during a very cold weather and relaxes away from the body during a hot
weather.

Testes

 In each scrotum lies two oval-shaped glands called the testes.

 These are 2 to 3 cm in width and are encapsulated in a protective, white fibrous
capsule.
 Several lobules are contained in each testis, which also contains Leydig’s cells that
produce testosterone and seminiferous tubules that produce spermatozoa.
 In most men, one testis is slightly lower than the other to prevent trauma and easily
sit or do any muscular activity.

Penis

 The penis has three parts: two are called the corpus cavernosa, and the other is the
corpus spongiosum.
 These erectile tissues also contain the urethra, making the penis an outlet for both
urinary and reproductive functions.
 Erection of the penis is stimulated by the parasympathetic nerve innervations, and
the blood supply for the penis is from the penile artery.
 The glans, a sensitive, bulging ridge of tissue, is located at the distal part of the penis.
 The prepuce, which is a retractable casing of skin, protects the glans at birth. It is also
the part that is surgically removed during circumcision.

Human reproductive system, organ system by which humans reproduce and bear live offspring.
Provided all organs are present, normally constructed, and functioning properly, the essential
features of human reproduction are:
1) liberation of an ovum, or egg, at a specific time in the reproductive cycle,
(2) internal fertilization of the ovum by spermatozoa, or sperm cells,
(3) transport of the fertilized ovum to the uterus, or womb,
(4) implantation of the blastocyst, the early embryo developed from the fertilized ovum, in the
wall of the uterus,
(5) formation of a placenta and maintenance of the unborn child during the entire period of
gestation, (6) birth of the child and expulsion of the placenta, and
(7) suckling and care of the child, with an eventual return of the maternal organs to virtually their
original state.

For this biological process to be carried out, certain organs and structures are required in both the
male and the female. The source of the ova (the female germ cells) is the female ovary; that of
spermatozoa (the male germ cells) is the testis. In females, the two ovaries are situated in the
pelvic cavity; in males, the two testes are enveloped in a sac of skin, the scrotum, lying below
and outside the abdomen. Besides producing the germ cells, or gametes, the ovaries and testes
are the source of hormones that cause full development of secondary sexual characteristics and
also the proper functioning of the reproductive tracts. These tracts comprise the fallopian tubes,
the uterus, the vagina, and associated structures in females and the penis, the sperm channels
(epididymis, ductus deferens, and ejaculatory ducts), and other related structures and glands in
males. The function of the fallopian tube is to convey an ovum, which is fertilized in the tube, to
the uterus, where gestation (development before birth) takes place. The function of the male
ducts is to convey spermatozoa from the testis, to store them, and, when ejaculation occurs, to
eject them with secretions from the male glands through the penis.

At copulation, or sexual intercourse, the erect penis is inserted into the vagina, and spermatozoa
contained in the seminal fluid (semen) are ejaculated into the female genital tract. Spermatozoa
then pass from the vagina through the uterus to the fallopian tube to fertilize the ovum in the
outer part of the tube. Females exhibit a periodicity in the activity of their ovaries and uterus,
which starts at puberty and ends at the menopause. The periodicity
is manifested by menstruation at intervals of about 28 days; important changes occur in the
ovaries and uterus during each reproductive, or menstrual, cycle. Periodicity, and subsequently
menstruation, is suppressed during pregnancy and lactation.