Human Body Systems
Animal organs are usually composed of more than one cell type. Organs perform a
certain function. Most organs have functions in only one organ system. Organ
systems are composed of organs, and perform a major function for the organism.
Digestive System
Multicellular organisms need to digest their food and filter the waste and the
nutrients. The digestive system uses mechanical and chemical methods to break
food down into nutrient molecules that can be absorbed into the blood. There are a
few steps commonly involved to correctly digest:
1. movement: propels food through the digestive system
2. secretion: release of digestive juices in response to a specific stimulus
3. digestion: breakdown of food into molecular components small enough to cross
the plasma membrane
4. absorption: passage of the molecules into the body's interior and their passage
throughout the body
5. elimination: removal of undigested food and wastes
Circulatory System
Multicellular animals do not have most of their cells in
contact with the external environment and so have
developed circulatory systems to transport nutrients,
oxygen, carbon dioxide and metabolic wastes.
Components of the circulatory system include
• blood: a
connective
tissue of
liquid plasma
and cells
• heart: a
muscular
pump to
move the
blood
• blood vessels:
arteries,
capillaries
and veins
that deliver blood to all tissues
Respiratory System
The primary function of the respiratory
system is to supply the blood with oxygen in
order for the blood to deliver oxygen to all
parts of the body. The respiratory system
does this through breathing. When we
breathe, we inhale oxygen and exhale
carbon dioxide. This exchange of gases is
the respiratory system's means of getting
oxygen to the blood. Among four-legged
animals, the respiratory system generally
includes tubes, such as the bronchi, used to
carry air to the lungs, where gas exchange
takes place. A diaphragm pulls air in and pushes it out. Respiratory systems of
various types are found in a wide variety of organisms. Even trees have respiratory
systems.
The Nervous System
The Endocrine System
The endocrine system is a collection of
glands that secrete chemical messages we
call hormones. These signals are passed through the blood to arrive at a target
organ, which has cells possessing the appropriate receptor. Exocrine glands (not
part of the endocrine system) secrete products that are passed outside the body.
Sweat glands, salivary glands, and digestive glands are examples of exocrine
glands.
Hormone Gland Target Tissue Function
Origin
Adrenocorticotropi Pituitary Adrenal cortex Triggers secretion of hydrocortisone from
c gland the adrenal gland
(anterior)
Growth hormone Pituitary Throughout body Stimulates growth and development
gland
(anterior)
Follicle-stimulating Pituitary Sex glands Stimulates female egg maturation and
hormone gland male sperm production
(anterior)
Luteinizing Pituitary Sex glands Stimulates female ovulation and male
hormone gland secretion of testosterone
(anterior)
Prolactin Pituitary Mammary glands Stimulates milk production in the breasts
gland after childbirth
(anterior)
Thyroid- Pituitary Thyroid gland Triggers secretion of thyroid hormones
stimulating gland
hormone (anterior)
Melanocyte- Pituitary Melanin-producing Controls skin pigmentation
stimulating gland cells
hormone (anterior)
Antidiuretic Pituitary Kidneys Regulates water retention and blood
hormone gland pressure
(posterior)
Oxytocin Pituitary Uterus Triggers contraction of the uterus during
gland labor
(posterior) Mammary glands Stimulates milk letdown for breast-
feeding after childbirth
Melatonin Pineal Unclear, although May affect skin pigmentation; may
gland possible target sites regulate biorhythms (awake/sleep
are pigment cells patterns) and prevent jet lag
and sex organs
Calcitonin Thyroid Bones Controls the level of calcium in the blood
gland by depositing it in the bones
Thyroid hormone Thyroid Throughout body Increases the body's metabolic rate;
gland promotes normal growth and
development
Parathyroid Parathyroid Bones, intestines, Regulates calcium level in blood
hormone glands and kidneys
Thymosin Thymus White blood cells Promotes the growth and development of
white blood cells, helping the body fight
infection
Aldosterone Adrenal Kidneys Regulates sodium and potassium levels in
gland the blood to control blood pressure
Hydrocortisone Adrenal Throughout body Plays key role in stress response;
gland increases blood glucose levels and
mobilizes fat stores; reduces
inflammatation
Epinephrine Adrenal Muscles and blood Increases blood pressure, heart and
gland vessels metabolic rate, and blood sugar levels;
dilates blood vessels. Also released
during exercise
Norepinephrine Adrenal Muscles and blood Increases blood pressure and heart rate;
gland vessels constricts blood vessels
Glucagon Pancreas Liver Stimulates the breakdown of glycogen
(stored carbohydrate) into glucose (blood
sugar); regulates glucose blood level
Insulin Pancreas Throughout body Regulates blood glucose levels; increases
storage of glycogen; facilitates glucose
intake by body cells
Estrogen Ovaries Female reproductive Causes sexual development and growth;
system maintains proper functioning of female
reproductive system
Progesterone Ovaries Mammary glands Prepares uterus for pregnancy
Uterus
Testosterone Testes Throughout body Causes sexual development and growth
spurt; maintains proper functioning of
male reproductive system
Erythropoietin Kidney Bone Marrow Produces red blood cells
A reflex is an involuntary, or automatic, action that your body does in response to
something - without you even having to think about it. You don't decide to kick
your leg, it just kicks. There are many types of reflexes and every healthy person
has them. In fact, we're born with most of them.
Reflexes protect your body from things that can harm it. For example, if you put
your hand on a hot stove, a reflex causes you to immediately remove your hand
before the message, "Hey, this is hot!" even gets to your brain.
The three types of neurons are arranged in circuits and networks, the simplest of
which is the reflex arc.
In a simple reflex arc, such as the knee jerk, a stimulus is detected by a receptor
cell, which synapses with a sensory neuron. The sensory neuron carries the
impulse from site of the stimulus to the central nervous system (the brain or spinal
cord), where it synapses with an interneuron. The interneuron synapses with a
motor neuron, which carries the nerve impulse out to an effector, such as a
muscle, which responds by contracting.
