Chemical coordination
is a fundamental process that involves the use of chemical signals to
coordinate and regulate the functions of various parts of an organism.
Hormones, which are chemical messengers, are released from one part of the body
and transported to another part, where they bind to specific receptor proteins
on target cells and initiate a response.
The endocrine system,
consisting of glands such as the pituitary, thyroid, and adrenal glands, is
responsible for chemical coordination in animals. The hormones produced by
these glands have diverse effects on the body, including growth regulation,
metabolism control, and behavior influence.
The endocrine system
and nervous system work in collaboration to coordinate the functions of organs
and tissues in the body. While the nervous system rapidly transmits signals
through electrical impulses, the endocrine system typically acts more slowly,
with the effects of hormones taking minutes, hours, or even days to manifest.
Chemical coordination
is critical for maintaining homeostasis, which is the balance of internal
conditions in the body. Hormones regulate several physiological processes,
including body temperature, blood pressure, and blood sugar levels, in addition
to playing a significant role in the stress response, reproductive functions,
and immune system.
Disruptions in chemical
coordination can cause various health issues, such as hormonal imbalances,
diabetes, and thyroid disorders. Hormone replacement therapy and other
treatments may be used to rectify these imbalances and restore proper chemical
coordination in the body.
Hormones are chemical
messengers, which regulate and control a number of activities of the body.
Hormones are produced by structures called endocrine glands. These are ductless
glands, which pour their secretion directly into blood. The study of endocrine
glands and hormonal control is called endocrinology.
Hormones
A hormone is a
regulatory chemical. It has the following properties:
(a) It travels in the
blood.
(b) It has its effect
at a site different from the site where it is made, called the target, hence
the term messenger.
(c) It fits precisely
into receptor molecules in the target, like a key in a lock. It is, therefore,
specific for a particular target.
(d) It is a small
soluble organic molecule.
(e) It is effective in
low concentration.
Types
of Hormones
The two main types of
hormone are:
(1) Steroid hormones
which are lipids made from cholesterol, e.g. estrogen, testosterone, cortisone.
(2) Non-steroid
hormones are all synthesized from amino acids and there are three main classes
of these substances:
(a)
The amine hormones are modified versions of single amino acids e.g., thyroxin,
epinephrine and norepinephrine.
(b)
The peptide hormones are short chains of amino acid as few as three,
antidiuretic hormones or vasopressin, oxytocin, thyrotropin.
(c)
The protein hormones are made of polypeptide having as many as 200 amino acids
e.g. insulin and glucagon.
Endocrine
Glands
Some of the endocrine
glands are exclusively endocrine in function and some have both endocrine and
exocrine function. The figure shows the locations of most of the human
endocrine glands.
The following endocrine
glands are present in man
(1) Thyroid
(2) Parathyroid
(3) Pancreas
(4) Adrenal Gland
(5) Gonads
(6) Glandular
Epithelium
(7) Pineal
(8) Thymus
(9) Pituitary Gland
Thyroid
Gland
The thyroid gland is a
large gland located in the neck where it attaches to the trachea below the
larynx. The thyroid gland has two lobes one on each side of the junction
between the larynx and trachea. A bridge of thyroid tissue called isthmus connects
the two lobes. The thyroid gland is made of spherical thyroid cells filled with
stored thyroid hormone.
The hormone produced by
thyroid called thyroxin (thyroxin) which has two forms. Thyroxin is usually
secreted as T4 (tetraiodothyronine) which contains four iodine atoms, but
eventually this form is converted to
T3 (triiodothyronine), the active form of the hormone. The two hormones act
essentially the same way. Another hormone secreted by thyroid, is called
calcitonin.
The thyroid is active
continuously but produces higher levels of secretions during period of rapid
growth and sexual maturation and in stress situations such as cold and hunger.
Thyroxin (T4 and T3) increases the metabolic rate. It does not have one target
organ; instead, it stimulates all organs of body to metabolize at a faster
rate. More glucose is broken down with the release of heat and more ATF is
generated. They also act in conjunctions with somatotropin (growth hormone
secreted by anterior pituitary) in bringing about growth and act directly on
brain cells causing them to differentiate (in amphibians, they affect the
process of metamorphosis. If secretion of thyroid is deficient, the tadpole of
frog does not metamorphose to develop into frog, but instead grows into a large
size tadpole.
Hypothyroidism
It is a general term
for the condition in which too little thyroxin is produced. It can cause the
following:
Cretinism
If there is a great
deficiency of thyroxin in early childhood, a disease called cretinism occurs.
The child develops into a midget called cretin. Cretinism results in mental
retardation and irregular development of bones and muscles. The skin is dry,
eyelids are puffy, hair is brittle and the shoulders sag. There is also failure
to develop sexuality. Ordinarily cretinism cannot be cured but early diagnosis
and treatment with thyroxin may arrest the disease before the nervous system is
damaged.
Myxedema
The occurrence of
hypothyroidism in the adult produces the condition known as Myxedema
(myxoedema). It is characterized by lethargy, weight gain, loss of hair, slower
pulse rate, lower body temperature and thickness and puffiness of hand and
skin.
Goiter
The disease goiter is
the enlargement of the thyroid gland itself. Goiter cause an increase in the
size of the entire throat and neck. The thyroid has to work harder to produce a
sufficient amount of thyroxin. Thus, it enlarges. It is common in mountainous
areas due to lack of iodine in soil and water.
Hyperthyroidism
It is also known as
Graves' diseases. It is a condition in which too much thyroxin is produced.
Symptoms:
These include nervousness, irritability, increased heart rate and blood
pressure, weakness, weight loss, rapid use of oxygen at rest and bulging eyes
(exophthalmic goiter) causes in part by increased fluid behind eyes. This can
lead to cardiac failure if prolonged.
Cause:
The cause of Graves' disease is the production of an abnormal body protein,
which continuously stimulates the thyroid to excessive secretions.
Treatment:
Drug therapy that inhibits thyroxin production and the administration of
radioactive iodine has successfully replaced surgery as treatment for
hyperthyroidism.
Calcitonin
High calcium ions
concentration in the blood causes stimulation of the synthesis and release of
calcitonin, low level of calcium ions suppress its manufacture, it helps to
regulate the blood calcium level in the blood and opposes the action of
parathyroid hormone. Calcitonin lowers blood calcium (Ca++) by increasing the
buildup of bone. Excess or deficiency leads to a disturbance of calcium
metabolism with its associated effects on nerve, skeleton, muscles, blood etc.
Parathyroid
Located on the thyroid
gland are two pairs of very small structures called the parathyroid glands?
They secrete the hormone parathormone. Low levels of blood calcium ions
stimulate the parathyroid directly to increase parathormone production whereas
high levels of calcium ions suppress its release. It controls balance of
calcium ions and phosphate in body.
Deficiency:
Too much Ca++ ions move from the blood to the bones, the muscles and nerves do
not receive enough calcium ions. As a result, they become very sensitive to
stimuli. Spasm (contractions and relaxations of muscles) and even death may
occur, in case of severe deficiency.
Overproduction:
It causes Ca++ ions to leave the bones to the blood and causes weakening of the
skeletal system similar to rickets. Nerve and muscle do not respond well to
stimuli. Over activity also causes massive formation of kidney stone. Both
conditions may be fatal.
Removal:
Small as the parathyroid glands are, their removal invariably causes death.
Pancreas |
Pancreas
Pancreas is composed of
two types of tissue. Exocrine tissue produces and secretes digestive juices that
go by way of ducts to the small intestine. Endocrine tissue called pancreatic
Islets (island) of Langerhans named after Paul Langerhans who discovered them.
These cells are under the control of pituitary hormones STH (Somatotropin
hormone) and ACTH (adrenocorticotrophic hormone) and responds directly to the
level of blood glucose. Islets of Langerhans secrete two hormones Insulin and
glucagon. Both are proteins. Insulin is secreted by the alpha cells, which are
large in number, and glucagon is secreted by cells, which are less in number.
Insulin
Insulin is a small
protein composed of 51 amino acids. Insulin is secreted when the level of blood
sugar rises, such as right after a meal. The most important effect of insulin
is to facilitate glucose transport across cell membrane. Insulin depresses
blood glucose level in various ways
(a) Increases the rate
of conversion of glucose to glycogen, called glycogenosis, which takes place
mainly in the liver and muscle.
(b) Increase in the
rate of uptake of glucose by cells.
(c) Increase in the use
of glucose rather than other substance.
(d) Increase in the
conversion of glucose into lipids and proteins.
(e) Decrease production
of glucose.
Diabetes
Mellitus
A deficiency in insulin
production leads to the metabolic disease known as diabetes mellitus. The
symptoms include:
(1) Sugar in Urine.
(2) Frequent Copious
Urination.
(3) Abnormal Thirst.
(4) Rapid Weight Loss
(5) General Weakness
(6) Drowsiness and Fatigue.
Hypoglycemia
If excess of insulin is
produced, the utilization of sugar is too great and its level falls in the
blood that upsets nerve and muscle functioning.
Glucagon
It is a protein
composed of 29 amino acids. Its role is to increase blood glucose level. Its
main target is the liver. Glucagon stimulates the conversion of glycogen to
glucose (glucogenesis). It also stimulates the breakdown of proteins and fats
to glucose and conversion of lactic acid to glucose (gluconeogenesis). Glucose
abnormalities seem rare as endocrine disorders. Tumors on beta cells will cause
excess glucagon secretions and consequent high blood glucose level.
Adrenal
Glands
Each of the two adrenal
glands rests on a kidney. Each adrenal gland is composed of an inner portion
called the medulla and an outer portion the cortex.
Adrenal Glands |
The hypothalamus exerts
control over the activity of both portions of the adrenal glands. The
hypothalamus, by means of ACTH (adrenocorticotrophic hormone) releasing hormone
controls the anterior pituitary's secretion of ACTH, which in turn, stimulates
the adrenal cortex.
Adrenal
Cortex
The two major types of
hormones produced by the adrenal cortex arc glucocorticoids, which help to
regulate the blood glucose level, and mineralocorticoids, which help to
regulate the level of minerals in the blood.
Adrenal
Medulla
Epinephrine
(adrenaline) and norepinephrine are produced by the adrenal medulla.
Epinephrine and norepinephrine are involved in the body's immediate response to
stress. They bring about all the bodily changes that occur when an individual
reacts to an emergency. Both the hormones contribute to the short-term stress
response. Both hormones stimulate liver cells to release glucose, thus making
more fuel for cellular energy. They also prepare the body for action by
increasing the blood pressure, the breathing rate, and the metabolic rate.
In addition,
epinephrine and norepinephrine change blood flow patterns making some organs
more active and other less so e.g. epinephrine dilates blood vessels in the
brain and skeletal muscles, thus increasing alertness and the muscles ability
to react to stress. At the same time, epinephrine and norepinephrine constrict
blood vessels elsewhere, thereby reducing activities that are not immediately
involved in the stress response, such as digestion. The short-term stress response
is rapid and usually subsides shortly after we first encounter stress.
Cortisol
It is the
glucocorticoid that promotes the hydrolysis of muscle protein to amino acid and
the liver converts these amino acids to glucose. Cortisol also favors
metabolism of fatty acids rather than carbohydrates, It also counteracts the
Inflammatory response that leads to the pain and the swelling of joints In
arthritis etc.
Corticosterone
It is both
glucocorticoid and a mineralocorticoid. It Increases blood glucose levels and
regulate mineral Ion balance.
Aldosterone
It is a
mineralocorticoid. It promotes renal absorption of sodium (Na+) and renal
excretion of potassium (K+).
When there is less
production of cortical hormone e.g. destruction of the adrenal cortex, such as
occurs in Addison's disease, will lead to general metabolic disturbance, in
particular, weakness of muscle action and loss of salts. Stress situation, such
as cold, which would normally be overcome, lead to collapse and death. When too
much cortical hormone is produced such as in Cushing's disease, symptoms are an
excessive protein breakdown resulting muscular and bone weakness. The high
blood sugar disturbs the metabolism as in diabetes.
Sex
Hormones
Adrenal cortex secretes
a small amount of male sex hormones (androgen) and a small amount of female sex
hormones in both male and female.
Gut
Many parts of the gut
function as endocrine tissue. The important hormones produced are:
(a)
Gastrin: It is produced by the mucosa of the pyloric region
of the stomach. It stimulates the production of gastric juice rich in
hydrochloric acid. It is produced under the influence of protein food in the
stomach after it is partially digested.
(b)
Secretin: It is produced by the mucosa of duodenum, when
acidic food touches its lining. It stimulates production of pancreatic juice by
the pancreas and also affects the rate of bile production in the liver, and
inhibits secretion of gastric juice.
(c)
Cholecytokinin (CCK): It is produced by mucosa of duodenum.
The stimulus for secretion is fatty food and protein in the duodenum. CCK
stimulates increased secretion of pancreatic juice rich in enzymes.
Gonads
The gonads are the
ovaries in the females and testes in the males.
Ovary:
Ovaries are located in the abdominal cavity. The ovaries secrete female sex
hormones (a) estrogen (oestrogen) and (b) progesterone.
Estrogen
This hormone is
secreted by ripening follicles whose development has been initiated by FSH from
the anterior pituitary gland. Estrogen performs the following functions
(1) Estrogen secreted
at the time of puberty is responsible for secondary sex characteristics in
females. The secondary sex characteristics include a more rounded appearance in
female than males because of a greater accumulation of fats beneath the skin.
Also the pelvic girdles extend more in females than the males, resulting in
females having a larger pelvic cavity and wider hips. Estrogen stimulates
enlargement of various accessory organs, including (internal vagina, uterus,
uterine tubes, and ovaries) and external reproductive structures.
(2) Estrogen is
necessary for egg maturation.
(3) At a point during
estrous or menstrual cycle exert a positive feedback, which results in a sharp
rise in LH output by the pituitary.
(4) Estrogen aids in
healing and repair of uterine wall after menstruation.
(5) Under influence of
estrogen, some of the cells of uterine wall become glandular and start
secreting proteinaceous secretions which are taken up by the embryo during
early stages of development.
Deficiency:
It leads in the young to failure to mature sexually and sterile in the adult.
Progesterone
This hormone is
produced by the ruptured follicles in response to LH (luteinizing hormone) from
the pituitary.
The functions performed
by progesterone are:
(1) It inhibits further
FSH secretion from the pituitary, thus preventing any more follicles from ripening.
(2) It causes further
thickening and vascularization of the uterus wall and other areas of the female
body preparing it for maintaining state of pregnancy.
(3) Progesterone
suppresses ovulation, so it is a major constituent of birth control pill.
(4) Progesterone
affects the mammary glands and help to regulate the secretion of gonadotropin
from the anterior pituitary gland.
Testes
The testes are located
in the scrotum. The testes consist of many coiled seminiferous tubules, where
sperms develop. The regions between the tubules consist of interstitial cells,
which produce gonad hormones called testosterone and 17-beta
hydroxytestosterone.
The male sex hormone
testosterone has many functions
(1) It is essential for
the normal development and functioning of the sex organs in males.
(2) It is necessary for
the maturation of sperms.
(3) In the fetus, it
initiates the development of the sex organs.
(4) Testosterone brings
about and maintains the secondary sex characteristics in males that develop at
the time of puberty. Testosterone causes growth of beard, axillary hair, and
pubic hair. It prompts the larynx and the vocal cords to enlarge, causing the
voice to change.
(5) Testosterone also
stimulates oil and sweat glands in the skin.
Thymus
The thymus is a lobular
gland that lies inside the thorax on the ventral side. This organ reaches its
largest size and is most active during childhood. Thymus produces various
hormones thymosin. Certain lymphocytes that originate in the bone marrow and then
pass through the thymus are transformed into T lymphocytes.
Pineal
Gland
It is located deep
between the cerebral hemisphere, and is attached to hypothalamus, produces
hormone called melatonin, primarily at night Melatonin is involved in a daily
cycle called a circadian rhythm.
Hypothalamus
and Pituitary Gland
The hypothalamus is a
portion of the brain that regulates the internal environment. For example, it
helps to control heart rate, body temperature, and water balance, as well as
the glandular secretion of the pituitary gland. The pituitary, a small gland
about 1 cm in diameter, is connected to the hypothalamus by a stalk like
structure. The pituitary has two portions the posterior pituitary and anterior
pituitary.
Posterior
Lobe
The axon endings in the
posterior pituitary store antidiuretic hormone (ADH, sometimes called
vasopressin) and oxytocin.
Vasopressin
ADH promotes the
re-absorption of water from the collecting ducts within the kidney. Increase
levels cause increase water re-absorption in distal part of kidney. Inability
to produce ADH causes Diabetes Insipidus (watery urine) in which a person
produces copious (plentiful, overflowing) amounts of urine with a resultant
loss of ions from the blood and great thirst.
Oxytocin is the other
hormone that is made in the hypothalamus and stored in the posterior pituitary.
Its release is stimulated by distention of cervix, decrease in progesterone
level in blood, and neural stimuli during parturition (childbirth) and
suckling. Oxytocin causes the smooth muscles of the uterus to contract and is
used to artificially induce labor. It also stimulates the release of milk from
the mammary glands when the baby is nursing.
Anterior
Pituitary - The Master Gland
It is called master
gland because it produces tropic hormones, which stimulate other endocrine glands
to release their hormone.
The anterior pituitary
produces at least six different types of hormones, each by a distinct cell
type.
Somatotropin
(STH)
It is also known as
growth hormone (GH). Somatotropin releasing factor (SRF) is secreted from
hypothalamus throughout the life. It promotes cell division, protein synthesis,
and bone growth. Evidence suggests that the effects on cartilage and bone may
actually be due to hormone called somatomedins, which are released by the liver
in response to GH, when growth has mostly ceased after adolescence, the hormone
continues to promote protein synthesis throughout the body.
During childhood if too
little GH is produced, the individual becomes a pituitary dwarf, and if too
much GH is secreted, a person can become a giant.
If there is
overproduction of growth hormone in adult, a condition called acromegaly
results. Some cartilage and bone becomes thickened. The feet, hands, and face
particularly the chin, nose, and eyebrow ridges become very large.
Thyroid
Stimulating Hormone (TSH)
The hormone produced by
thyroid is thyroxin. The levels of thyroxin in the blood control the release of
thyrotrophin releasing factor from hypothalamus.
If level of thyroxin is
low, there is more production of TSH and vice versa. TSH is secreted throughout
life, and reaches high levels during the period of rapid growth and
development, TSH acts directly on the cells of thyroid gland. It increases the
number of cells and secretory activity of the thyroid gland.
Adrenocorticotrophic
Hormone (ACTH)
It stimulates the
release of corticotrophin releasing factor from hypothalamus. This is
controlled by steroid level in the blood and by direct nervous stimulation of
the hypothalamus because of stress, e.g. cold, heat, pain, fright, infections.
Gonadotropin
Hormones
These are follicle
stimulating hormone (FSH), luteinizing hormone (LH, also called interstitial
cell stimulating hormone, ICSH in the male), protecting (sometimes
inappropriately called luteotrophic hormone, (LTH). FSH and LH/ICH share a
common hypothalamus releasing factor. The FSH acts upon an ovary to stimulate
maturation of a follicle and during its development, the follicular cells
produce increasing amounts of estrogen and some progesterone. In males, FSH
stimulates development of the germinal epithelium of the testes and sperm
production.
Prolactin
(PRL) is produced in quantity only after childbirth. It
causes mammary glands in the breasts to develop and produce milk. It also plays
a role in carbohydrate and fat metabolism. Prolactin is continuously produced
from the pituitary and is inhibited by protecting inhibiting factor (PIH) from
the hypothalamus.
LH works with FSH to
stimulate estrogen secretion and rupture of mature follicles to release ovum.
It also causes the luteinization (turning yellow) of the rupture follicles.
Under the influence of LH, the follicular cells enlarge to form a temporary
glandular structure called Corpus Luteum.
ICHS in the male
stimulates the interstitial cells of the testes (singular, testis) to secrete
testosterone.
Median
Lobe
It secretes Melanocyte
Stimulating Hormone (MSH). The concentration of this hormone in human is very
low. MSH causes inhibition of secretion controlled by hypothalamus. External
light governs its secretion. More secretion in pregnancy stimulates melanocytes
in skin to produce brown pigment, melanin, which darkens the skin. Excess MSH
is secreted in Addison's disease (It is the peculiar bronzing of the skin. It
is due to adrenal cortex hypo-secretion), and it’s one of the symptoms is
darkening of the skin.
Feedback
Mechanism
The general principle
is that the product of a series of reactions controls its own production by
turning off the pathway when it reaches a certain level. This is comparable
with a thermostat where the product, heat, switches off its own production when
a certain temperature is reached. If there is too little of the product, its
production is switched on again.
Commonly, the control
of hormonal secretions involves a negative feedback system. An endocrine gland
is sensitive either to the concentration of a substance it regulates or the
concentration of a product from a process it controls. Whenever this
concentration reaches a certain level, the endocrine gland is inhibited (a
negative effect) and its secretory activity decreases. Then, as the concentration
of the gland's hormone drops, the concentration of the regulated substance
drops also, and inhibition of the gland ceases. When the gland is no longer
inhibited, it begins to secrete its hormone again. Because of such negative
feedback system the concentration of some hormones remain relatively stable,
although they may fluctuate slightly within normal range. Examples of negative
feedback are secretion of thyroxin, cortisol, sex hormones etc.
Example: control of
thyroxin release.
1. Neurosecretory cells
of the hypothalamus are stimulated by stress or low body temperature and
release hormones (TSH).
2. TRH (Thyroid
releasing hormone) stimulates the anterior pituitary to release thyroid-stimulating
hormone (TSH)
a) The level of TSH
exerts feedback control over the hypothalamus to release thyroxin (thyroxin),
b) The level of
thyroxin exerts feedback control over the anterior pituitary.
c) The level of
thyroxin exerts feedback control over the hypothalamus.
Thyroxin causes
increase in the metabolic activity of the body, generating ATP and heat. The
higher body temperature and higher thyroxin levels in the blood inhibits
production of thyroxin. In this way, thyroxin controls its own secretion.
No comments:
Post a Comment