FORM FOUR

BIOLOGY

3.1 CHAPTER THREE: ENDOCRINE SYSTEM

The endocrine system is a network of glands and organs in the body

that produce, store, and release hormones. Hormones are chemical
messengers that regulate various functions, including growth,
metabolism, reproduction, mood, and homeostasis (the body's internal
balance).

Both endocrine system and nervous system are responsible for many
activities taking place in our bodies including detecting, processing and
bringing about responses to ensure good health and achieve homeostasis
for an individual or organism

Similarities between endocrine and nervous system

 Both involve transmission of message triggered by a stimulus to bring

about a response
 Both provide means of communication within the body of an organism
 The target organs of hormones are like the effector organs of nervous
system
 Both bring about survival response
 Both involve chemical transmission

The difference between endocrine and nervous systems have

been summarizer

Table 6: Difference between endocrine and nervous systems

1Endocrine system Nervous system

Chemical substance evokes a Nerve impulse evokes a response
response
Responses are slow but affect several Response is quick, specific and
parts of the body localized
Chemical transmitted through blood Impulse is transmitted through the
nerve fibres
Effects are long-lasting Effects are rapid and short - lived
Responses takes place involuntarily Responses takes place both
voluntarily and involuntarily

Glands are essential organs found throughout the body. They produce
and release substances that perform certain functions. There are two
types of glands; endocrine and exocrine glands.
Exocrine glands are glands that have ducts and secrete chemical
substances into ducts that help in releasing the secretions. Most of
exocrine glands release the secretions to the outside of the body whereas
others release the secretions to the outside of the gland onto a surface
still within the body. It can also be acid that these glands synthesize and
secrete their products onto a surface a directly or through a tube/duct.

Exocrine glands secrete mucus, ions, water, enzymes sweat and other
substances.

Endocrine glands are specialized organs that store and secrete chemical
substances into the bloodstream or tissues of the body. Endocrine glands
are ductless glands found inside of human body which release hormones
directly into the blood.

Important exocrine glands include:

a. Salivary glands

b. Sweat glands

c. Pancreas

d. Liver, etc.

Table 7: Difference between exocrine and endocrine glands

Exocrine glands Endocrine glands

Secretes enzymes Secretes hormones
Secretes the products through ducts Secretes the products directly into
blood
Ducts glands Ductless gland
Mostly controls short term activities Mostly controls long term activities of
of the body the body
Examples: salivary and sweat glands Examples: pituitary glands and
thyroid glands
The endocrine contains two words derived from Greek "endo" means
within or inside, and "crinis" means to secrete

In general, endocrine system is defined the collection of glands that

produce hormones which regulate, growth and development, metabolism,
reproduction tissue function, mood, sleep and other things.

Endocrine gland. Hormone. blood stream.

Target

A hormone is a chemical released by a cell or a gland in one part of the

body that sends out messages that affect cells in other parts of the
organism.
Enzymes are biological catalysts that speed up chemical reactions in
living organisms without being consumed in the process

How hormones works

Hormones can only impact cells that have receptors which are particular
to them. Cells can have receptors for multiple hormones at the same time.
The more receptors for a particular hormone that a cell shows, the more
sensitive to that hormone the cell will be.

Whenever, a cell exhibits more receptors to a hormone, this is called up

regulation, but when a cell decreases its number of receptors for a
particular hormone, this is called down-regulation.

Steroids

The steroids are biologically active organic compounds. Steroids are

mostly made up of four rings which are arranged in a specific molecular
configuration.

Steroid hormones

These hormones are made of lipids, such as cholesterol and phospholipids.

Steroid hormones are fat soluble, so that they can diffuse across the
plasma membrane of target cells and bind with receptors in the cytoplasm
of the cell.

Examples of steroid hormones include:

 Androgens
 Mineralocorticoids
 Glucocorticoids
 Oestrogens
 Progesterone

Non-steroid hormones

These hormones are made of amino acids. Non steroid hormones are not
fat soluble, so that they cannot diffuse across the plasma membrane of
target cells. But a non-steroid hormone binds to a receptor on the cell
membrane.

Examples of steroid hormones include:

1. Insulin

2. Thyroid hormones

General features of endocrine glands

  They are ductless glands whose secretions are released directly
from gland into the bloodstream.
 Their secretions are called hormones
 The hormones are highly specific
 Responsible for chemical co-ordination of body

The main endocrine glands are:

a. Pituitary gland

b. Thyroid gland

c. Adrenal glands

d. Pancreas

e. Ovaries (females)

f. Testes (males)

pituitary gland

The pituitary gland is attached to the hypothalamus at the floor of the

Cerebrum. It is also known as the master gland because it secretes a
number hormones which controls the activity of particular glands

The pituitary gland is divided in to two lobes; anterior pituitary lobe

and posterior pituitary lobe. It is good to identify which hormones are
secreted from anterior Pituitary lobe and posterior pituitary lobe.

The pituitary gland secretes:

 The number of hormones- ACTH, TSH, FSH and LH (see table) which
either stimulate or inhibit other endocrine glands.
 Prolactin, growth hormone, oxytocin, antidiuretic hormone which
acts on target organs which are not endocrine glands.

Hypothalamus

a. It is located at the base of the brain, above the pituitary gland

and acts as a link of nervous and endocrine systems.

b. Has certain neurons which secrete hormones (neurosecretory)

that strictly control the secretion of hormones from the anterior
pituitary gland

c. Greatly influences the pituitary gland.

Hypothalamus produces Gonadotropin-releasing hormone (GnRH). The

hormone (GnRH) in tum, regulates the secretion of LH (luteinizing
hormone) and FSH (follicle stimulating hormone) from the anterior
pituitary. The FSH and LH are also as gonad - stimulating hormones as
they stimulate the gonads (testes and ovaries) to secrete sex hormones.

Table 8: Hormones of the pituitary gland which regulate

endocrine glands

Endocrine Hormones Major Functions

glands target(organs
)
Pituitary glands Thyroid Thyroid glands Stimulates the
anterior lobe stimulating thyroid gland to
hormones (TSH) produce
thyroxin
Adrenocorticotro Adrenal gland -Stimulates
pic hormone (cortex) adrenal cortex
(ACTH) to produce
corticoid
hormone
- Regulates the
growth of the
adrenal cortex
Follicle Ovary and -initiates
stimulating testis cyclical
hormones(FSH) changes in the
ovaries
-initiates sperm
formations in
the testes
Luteinizing Ovary and -causes
hormones (LH) testis ovulation and
development of
follicle in to
corpus luteum
-Stimulates the
secretion of
testosterone
from interstitial
cells in the
testes
Growth Liver, adipose -controls
hormones (GH) tissue protein , lipids
and carbon
hydrates
metabolism
-promotes
growth of
skeleton and
muscles
Prolactin (PRL) Mammary Stimulates milk
 glands production by
the Mammy
glands
Pituitary gland Oxytocin Ovary -Stimulates
posterior lobe smooth
muscles of the
uterus to
contact during
labor
-facilitates the
ejection of the
milk from the
breast during
nursing
Antidiuretics Kidney Stimulates the
hormone (ADH) kidney to
or vasopressin reabsorb water
and produce
less urine

The thyroid gland

The thyroid gland is located in the neck in front of trachea. It produces

hormone The thyroid sin. Thyroxin contains iodine and is secreted almost
all the time, in quite small amounts. It is secreted throughout the life

Function of thyroxin

a. It speeds up cell metabolic rate specially respiration in

mitochondria

b. Stimulates normal physical growth and mental development

Effects of thyroxin imbalance

1. Thyroxin deficiency causes:

a. Cretinism (congenital hypothyroidism) in children - defective

development

b. Myxedema (severe hypothyroidism) in adults-sluggishness of

metabolism, swelling of subcutaneous tissue, disrupted mental and
sexual activities.

Excessive release of thyroxin causes:

a. A high metabolic rate

b. An increased rate of heart beat

 c. Higher activity and insomnia (restlessness /sleeplessness)

d. Goiter (abnormally enlarged thyroid gland) in the neck and

eyeballs protrude

Parathyroid glands

Parathyroid glands contain four small glands which are located behind
the thyroids in the neck. They produce a hormone called parathyroid
hormone, which helps maintain an appropriate balance of calcium in the
bloodstream and in tissues that depend on calcium for proper functioning.

If these glands produce excess parathyroid hormones, they can cause

negative effects such as brittle bones and kidney stones.

Adrenal glands

There are two adrenal glands located above each kidney. Each consists of
an outer adrenal cortex and a central adrenal medulla.

1. Adrenal medulla

Adrenal medulla produces adrenaline also called epinephrine which is

the main hormone in the fight-or-flight response in our body. The adrenal
medulla receives nerve impulses from the central nervous system (CNS)
through the spinal nerves and in response, secretes adrenaline
(epinephrine) and noradrenaline (norepinephrine).

Functions of adrenaline hormone

It prepares the body for stressful situation of flight or fight brought on by:

a. Raising respiration and heart beat rate

b. Increasing blood flow to muscle and brain

c. Stimulating the conversion of glycogen to glucose

Effects of adrenaline imbalance

Excessive adrenaline hormone:

1. Raises blood pressure

2. Raises blood glucose level

3. Causes glucose to be present in the urine

2. Adrenal cortex produces over 30 different steroid hormones

collectively called corticosteroids or corticoid hormones. These include;
cortisol, aldosterone and androgen.

Functions of cortisol hormone

 a Raises blood glucose level by stimulating the conversion lipids and
proteins

to glucose

b Are produced in response to stress

Effects of cortisol imbalance

1. Cortisol deficiency cause Addison's disease - weight loss, fatigue,

weak muscles, low blood pressure, darkening of the skin, etc.

2. Excessive cortisol causes Cushing's syndrome weak muscles,

gains weight, osteoporosis, poor skin healing, etc.

Functions aldosterone hormone

 Regulates blood osmotic pressure by reabsorbing Na and K+ in the

kidneys to retain water

Effects of aldosterone imbalance

1. Aldosterone deficiency decreases Na and increases K, more water

is excreted and blood pressure drops

2. Excessive aldosterone increases Na', decreases K, body retains

excessive water and blood pressure Increases.

Functions androgen hormone

 Regulates the Maturity of sex organs and development of secondary

sexual characteristics

Effects of androgen Imbalance

1. Androgen deficiency causes undeveloped sex organs in adults

2. Excessive androgen leads to early physical maturity in children

the pancreas

The pancreas is located slightly behind and beneath the stomach. The
pancreas is made up of numerous small groups of cells called islets of
Langerhans (simply, Islets). Pancreatic islets contain two major types of
cells, namely: beta cells and alpha cells, each of which produces a
different endocrine product. B-cells of the islets of Langerhans secrete
insulin and o-cells secrete glucagon. Insulin lowers blood glucose level
while glucagon raises it.

Function of insulin hormone

Insulin lowers blood glucose level by stimulating:

a. Glucose storage as glycogen (in muscle and liver), fats (in

adipose tis and protein

b. Oxidation of glucose in cell respiration

Effects of insulin imbalance

1. Insulin deficiency causes:

c. Elevated blood glucose levels

d. Glucose to be excreted in the urine (diabetes mellitus)

2. Excess of Insulin:

a. Causes low blood glucose levels

b. Causes Weakness, light-headedness, heart beat becomes rapid

and irregular.

Function of glucagon hormone

 Glucagon hormone raises blood glucose level by stimulating the

conversion of glycogen to glucose.

Effects of glucagon imbalance

1. Deficiency of glucagon makes a person weak and lack energy

2. When glycogen becomes excess, it causes a person to be over

active.

Gonads (reproductive organs)

Gonads are the gamete-producing organs. They consist of ovaries and

testes. Ovaries are located on both sides of the pelvic area whereas testes
are located in the scrotum.

The ovaries produce the female hormones oestrogen and progesterone

and testes produce the male hormone testosterone.

Function of oestrogen hormone

a. Regulates the development of secondary sexual characteristics,

e.g.. enlargement of breasts and pelvic area, growth of hair on
armpit and pubic area

b. Stimulates thickening of the uterus during ovulation.

Effects of oestrogen imbalance

 1. Oestrogen deficiency hinders normal development of secondary
sexual characteristics

2. Excessive oestrogen causes tenderness of breasts, water

retention, Irritability, headache, visual disturbance and difficult in
memory

Function of progesterone hormone

 Progesterone hormone prepares the uterus for pregnancy.

Effects of progesterone imbalance

1. Progesterone deficiency causes miscarriages

2. Excessive progesterone hiders the contraction of the uterus

during birth

Function of testosterone hormone

 Testosterone hormone regulates the development of secondary

sexual characteristics, e.g. growth of beard and moustache, growth
of pubic hair deepening voice, etc,

Effects of testosterone Imbalance

1. Testosterone deficiency during puberty hinders the development

of secondary sexual characteristics

2. Testosterone deficiency after puberty causes impotence, also

called erectile dysfunction (inability in a man to have an erection or
orgasm), a loss of muscle tone (how strong and firm the muscle is),
increased body and reduced bone mass.

Regulation of hormonal secretion

The secretion of a hormone may be regulated by:

1. Another hormone

2. The level of certain substances (metabolites)

3. The nervous system

Hormonal imbalance

1. Hormonal balance is important for the coordination and proper

functioning of all the physiological activities in our body

2. Hormonal imbalance occurs when a particular hormone is either

produced in excess or deficient in the body.

Hormonal imbalance of growth hormone

1. Deficiency of growth hormone:

a. In children, the deficiency of growth hormone causes pituitary

dwarfism.

b. Adult with growth hormone deficiency may have diminished lean

body mass, poor bone density and number of physical and
physiological symptoms.

2. Excessive secretion of growth hormone:

a. When growth hormone is secreted in excess in young children or

adolescent, it causes gigantism.

b. In adults, it causes acromegaly (whereby new bone tissue grows

in the body's extremities) - the jaw protrudes, the hands and feet
become enlarged, etc.

3.2 HOMEOSTASIS

The word homeostasis is derived from Greek words; homeo meaning

same and stasis meaning position. The process of maintaining of a
constant internal environment in living organisms is called homeostasis.
Internal environment of an organism is the immediate surroundings of the
body cells.

Such factors or conditions which should be kept constant include:

1. Glucose concentration

2. Carbon dioxide concentration

3. Temperature

4. PH and

5. Water content

Mechanism of homeostasis

Different systems in the body such as circulatory, excretory, endocrine

and nervous systems work in a coordinated way to bring about
homeostasis Homeostasis works on a feedback mechanism. There are two
types of feedback mechanism referred to as:

1. Negative feedback and

2. Positive feedback

Negative feedback
Negative feedback is when a factor such as sugar level or temperature
drops below or rises above the normal. It is detected and a corrective
action taken This is said to be negative feedback

Positive feedback

Positive feedback is where unchanged below or above normal is not

corrected Instead, it leads to further undesirable changes. This
mechanism is found many biological systems and plays an important role
during labor and childbirth, blood clotting, menstrual cycle among.

Temperature regulation

in many animals, the body temperature is not constant but varies with
temperature of surrounding air or water. Animals whose body temperature
varies are known as variable temperature animals or poikilotherms
(ectoderms). Fishes, amphibians and reptiles are variable temperature
animals. They are less active when cold than when hot.

Animals which maintain constant body temperature are called constant

temperature animals or homeotherms (endotherms). Birds and mammals
are homeotherms.

Factors affecting temperature regulation

The important factors which affect temperature regulation are:

1. The difference in Temperature between the body and its

surrounding medium (air and water)

2. The surface area to volume ratio i.e., the surface area of the
exposed the surrounding medium (air and water)

3. Whether or not the body has insulating material around it

4. Whether the surrounding is flowing past the body or not

5. The physical state of the surface of the body e.g., whether it is

moist hence whether it can lose liquid by evaporation

6. Whether the body possess any mechanism for generating heat

Temperature regulation in humans

Man is homoeothermic. The body temperature of man is controlled by the

Hypothalamus. Slight changes in temperature results in an activation
corrective mechanisms.

The heat loss and heat gain are through relatively hairless skin by
radiation convection and conduction under conditions of low temperature.
Heat is lost rapidly from fingers, toes, ears and nose (body extremities).
Too much heat loss for a longer time produce "frost bite". The tissues can
die due to lack oxygen, food and heat.

Babies lose heat quickly because of their high surface area control
methods (loss or gain) could be adaptations to volume ratio. Heat
physiological methods or behavioral

1. Physiological methods include:

a. Fat insulation

b. Vasodilation and vasoconstriction

c. Evaporation by sweating

d. Panting

e. Metabolic activity (increase or decrease)

2. Behavioral methods include:

a. Moving to a shade

b. Moving to water

c. Clothing

d. Aggregation

e. Basking etc.

Maintenance of blood glucose levels

The levels of glucose in the bloodstream must be maintained in order for

humans to stay healthy. All cells in the body use glucose as their "fuel,"
but the amount of glucose required by each cell varies and is dependent
on the cell's functional activity.

The control of glucose levels in the blood, however, is the most difficult
homeostatic system known. Blood glucose levels normally vary between
70-80mg per 100ml of blood following a period without food. But after a
meal, it reaches 110-120mg glucose per 100ml of blood.

When glucose levels are at their highest, it is converted to glycogen and

stored Insulin, glucagon, growth hormone, adrenaline, thyroxin, and
glucocorticoids are the hormones that regulate blood glucose levels.
Except for insulin, ach inhibits glycogenolysis (the creation of glucose from
stored glycogen all hormones induce glycogenesis (the production of
glucose from stored glycogen) (the production of glycogen from glucose in
the blood).
When blood glucose levels are high, the pancreas produces insulin, which
acts to lower blood glucose levels; this is an example of negative feedback