Click here to view an
animation of the endocrine system
The endocrine
system is a set of hormone secreting glands within the body of an animal. The
function of the endocrine system is homeostasis, communication and response to
stimuli. The endocrine system regulates the internal environment of the animal
for growth, survival and reproduction as well as allowing it to respond to
changes in its external environment.
The endocrine system’s glands 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.
The other communication method in the
body is the nervous system. Although there are differences between them, they
complement each other in many responses, e.g., response to danger.
The difference between
nervous and endocrine control are as follows:
1. Nervous response is
faster.
2. Nervous response is
shorter in duration.
3. Nervous response stops
quicker.
- Nervous response is
much more local.
- Nerve ‘messages’ are conducted electrically; endocrine ‘messages’
are carried chemically.
Hormones
Most hormones are made of protein. They are called
peptides. Peptides are short chains of amino acids; most hormones are
peptides. They are secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys.
Some hormones are steroid based. Steroids are lipids derived from
cholesterol. Testosterone is the male sex hormone. Estradiol, similar in
structure to testosterone, is responsible for many female sex characteristics.
Steroid hormones are secreted by the gonads, adrenal cortex, and placenta.
Hormones are usually slow to act but,
once they act, they remain active for long periods of time and, also, their effects
remain for a long time.
Endocrine Glands
There
are 10 endocrine glands. As stated previously, other organs such as the
stomach, intestines, kidneys, heart, brain, and placenta also make hormones.
Click
here to take an online quiz on the location of the endocrine glands
The
Pituitary Gland
Click here to view an
animation of the pituitary gland
The
pituitary gland is often called the master gland. That is because the
pituitary gland produces hormones that regulate other endocrine glands. Some
hormones produced by the pituitary gland are:
1.
Follicle
Stimulating Hormone (FSH): Will be
discussed in a later Chapter of the syllabus.
2.
Luteinising
Hormone (LH): Will be discussed in
a later Chapter of the syllabus.
3.
Growth Hormone
(GH): Causes body cells to absorb
amino acids and form protein for growth. The main function is to cause the
elongation of bones.
4.
Prolactin: stimulates milk formation by the breast after the
birth of the baby.
5.
Oxytocin: stimulates muscle contraction of uterus during
birth, stimulates muscle contraction in the milk ducts during breast-feeding.
6.
Antidiuretic Hormone (ADH): causes
increased water reabsorption by kidneys.
7.
Thyroid
Stimulating Hormone (TSH):
Combines with iodine at the thyroid gland to produce thyroxine.
Overproduction
of GH causes gigantism and underproduction causes dwarfism.
The Hypothalamus
The
hypothalamus links the nervous system with the endocrine
system. It produces hormones that control the pituitary gland’s responses
to messages from the brain and other hormones. Some these hormones, called releasing
hormones, stimulate the pituitary gland to make other hormones. Others,
called release inhibiting hormones, prevent the production of pituitary
hormones.
An
example is growth hormone releasing factor. This causes the production
of growth hormone (GH) by the pituitary gland.
The
Pineal Gland
This
gland is in the brain. One hormone produced there is melatonin. Synthesis and
release of melatonin is stimulated by darkness and inhibited by light.
But even without visual cues, the level of melatonin in the blood rises and
falls on a daily (circadian)
cycle with peak levels occurring in the wee hours of the morning. Melatonin is
readily available in drug stores and health food stores, and it has become
quite popular. Ingesting even modest doses of melatonin raises the melatonin
level in the blood to as much as 100 times greater than normal. These levels
appear to promote going to sleep and thus help, insomnia to hasten recovery
from jet lag, and to not to have dangerous side effects.
The
Thyroid Gland
The thyroid gland produces the hormone called thyroxin.
Thyroxin controls the rate of all the body’s internal reactions. In other
words, thyroxin controls the rate of the body’s metabolism.
Physical
conditions related to abnormal thyroid function are:
Hypothyroidism-
Under Production of Thyroxine
1.
Cretinism- Under production of thyroxin in young
children. This results in low metabolic rates and results in retarded
physical and mental development.
2.
Myxoedema- Under
production of thyroxin in adults.
Characteristics are tiredness, lack of energy, slow mental and physical
activity, and weight gain.
3.
Goitre-
Swelling of the thyroid caused by
myxoedema.
Goitre
In cases of low
production of thyroxine tablets are available to increase the thyroxine in the
body. Since thyroxine needs iodine to be produced iodine is also
administered to boost thyroxine levels.
Thyroxine Excess (Hyperthyroidism)
Thyroxine secretion is above normal. This causes a raised
level of metabolism. Symptoms of over production of thyroxin are bulging eyes,
weight loss heat production, nervousness, irritability, and anxiety. This condition
is called Grave’s Disease. Corrective measures for Grave’s Disease are:
1.
Drugs to suppress thyroid activity
2. Surgically
remove part of the gland
3. Use
radioactive iodine to destroy some of the gland.
The
Parathyroids
There are 4
parathyroid glands. They are located within the thyroid gland. The
hormone they produce is called parathormone. This hormone stimulates the
release of calcium from the bones. That is why we must continue to include
calcium in our diet even when our bones are fully grown.
The Thymus
Gland
This gland is located behind the breastbone. It produces the hormone thymosin.
This hormone causes white blood cells (lymphocytes) to become
mature and active. These blood cells, as previously discussed in the Blood web page, are involved in the body’s immune
system.
The
Adrenal Glands
Click here to view an
animation of the adrenal glands
The adrenal glands are located on top of each kidney. They
secrete the hormone called adrenaline (also called epinephrine).
This hormone prepares the body for stress and is released when we are
frightened or feel stress. It does the following:
1.
Increases blood flow to the heart, muscles, and brain.
2.
Reduces blood flow to the kidneys. This helps reduce blood loss if
we are cut. It causes us to get pale.
3.
Opens the bronchioles allowing us to get more air.
4.
Increases glucose levels in the blood.
5.
Increases heartbeat rate.
6.
Increase muscular contraction and strength.
7.
Increases mental alertness.
Pancreas
As discussed in the Human Nutrition web page the pancreas secretes pancreatic
juice for the digestive system.
In addition, the pancreas produces the hormone called insulin.
This hormone is produced in groups of cells called Islets of
Langerhans. 
Insulin is needed because it reduces blood
glucose levels in the blood. It causes cells, especially fat and muscle cells,
to absorb glucose from the blood. The glucose is needed for cellular
respiration or converted into glycogen. The glycogen is stored in the liver or
the muscles for future use in cellular respiration.
Diabetes is a serious condition that results from
1 of 2 causes. In type 1 diabetes, the pancreas no longer makes insulin
and therefore blood glucose cannot enter the cells to be used for energy. In type
2 diabetes, either the pancreas does not make enough insulin or the body is
unable to use insulin correctly. Symptoms of diabetes are high glucose levels
in the blood and urine, the production of large amounts of urine, severe
thirst, loss of weight, and tiredness.
Injections of insulin, which are
taken daily, the control of carbohydrate intake, exercise, and weight control
treat diabetes.
Anabolic Steroids
Anabolic steroids are hormone supplements that habe been used. They build up muscle, speed up recovery of muscle from injury, and help strengthen bones. There are many serious side effects such as liver and adrenal gland failure, infertility, impotence, and the development of male characteristics in females that can result if they are misused. They are also, sometimes given to animals to promote increased lean muscle (meat) production. This practice is banned in the EU.
Control of Thyroxine Level
Control of thyroxine level as well as many other hormones is done by negative
feedback. If the thyroxine level is normal the pituitary gland is inhibited
from releasing thyroid stimulating hormone (TSH). As a result, no
further thyroxine is produced. When thyroxine levels are low the
pituitary gland produces TSH. This causes more thyroxine to be produced by the
thyroid gland.
An Example of negative feedback in the
role of the thyroid in maintaining body temperature at 37°C.:
- The hypothalamus of
the brain detects a drop in blood temperature.
- The hypothalamus
stimulates the pituitary to secrete TSH (thyroid-stimulating hormone).
- This hormone
stimulates the thyroid to increase its secretion of thyroxine.
- The higher
concentration of thyroxine increases metabolism and heat production
increases.
- The blood is warmed
back to normal temperature.
OR:
- Hypothalamus
detecting raised blood temperature and reduces its stimulation of the
pituitary.
- High thyroxine levels
inhibiting the release of TSH from the pituitary.
- The increased level
of thyroxine leads to the limitation or reduction of its secretion.
- Body’s metabolism
slows down as a result of less thyroxin. The body’s temperature goes down.
Review Chart of Major Hormonal Glands
|
Where the Hormone is Produced |
Hormone(s) Secreted |
Hormone Function |
|
Adrenal Glands |
Adrenalin |
Causes Emergency Responses
(fight/flight) |
|
Pituitary Gland |
Growth hormone |
Affects growth and development;
stimulates protein production |
|
Pancreas |
Insulin |
Lowers blood sugar levels;
stimulates metabolism of glucose, protein, and fat |
Hypothalamus
|
Growth Hormone
Releasing Factor |
Causes growth hormone
to be made |
|
Pineal Gland |
Melatonin |
Controls body rhythms |
|
Parathyroid Glands |
Parathyroid hormone (Parathormone) |
Affects bone formation and
excretion of calcium and phosphorus |
|
Thyroid |
Thyroxine |
Controls Metabolism |
|
Thymus |
Thymosin |
Matures white blood cells |
