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You have already learnt that the neural system provides a
point-to-point rapid coordination among organs. The neural
coordination is fast but short-lived. As the nerve fibres do not innervate
all cells of the body and the cellular functions need to be continuously
regulated; a special kind of coordination and integration has to be
provided. This function is carried out by hormones. The neural system
and the endocrine system jointly coordinate and regulate the
physiological functions in the body.
22.1 ENDOCRINE GLANDS AND HORMONES
Endocrine glands lack ducts and are hence, called ductless glands. Their
secretions are called hormones. The classical definition of hormone as a
chemical produced by endocrine glands and released into the blood and
transported to a distantly located target organ has current scientific
definition as follows: Hormones are non-nutrient chemicals which
act as intercellular messengers and are produced in trace amounts.
The new definition covers a number of new molecules in addition to the
hormones secreted by the organised endocrine glands. Invertebrates
possess very simple endocrine systems with few hormones whereas a large
number of chemicals act as hormones and provide coordination in the
vertebrates. The human endocrine system is described here.
C
HEMICAL
C
OORDINATION
AND
I
NTEGRATION
C
HAPTER
22
22.1 Endocrine
Glands and
Hormones
22.2 Human
Endocrine
System
22.3 Hormones of
Heart, Kidney
and
Gastrointestinal
Tract
22.4 Mechanism of
Hormone Action
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22.2 HUMAN ENDOCRINE SYSTEM
The endocrine glands and hormone
producing diffused tissues/cells located
in different parts of our body constitute
the endocrine system. Pituitary, pineal,
thyroid, adrenal, pancreas, parathyroid,
thymus and gonads (testis in males and
ovary in females) are the organised
endocrine bodies in our body
(Figure 22.1). In addition to these, some
other organs, e.g., gastrointestinal tract,
liver, kidney, heart also produce
hormones. A brief account of the
structure and functions of all major
endocrine glands and hypothalamus
of the human body is given in the
following sections.
22.2.1 The Hypothalamus
As you know, the hypothalamus is the
basal part of diencephalon, forebrain
(Figure 22.1) and it regulates a wide
spectrum of body functions. It contains
several groups of neurosecretory cells
called nuclei which produce hormones.
These hormones regulate the synthesis and secretion of pituitary
hormones. However, the hormones produced by hypothalamus ar
e of
two types, the releasing hormones (which stimulate secretion of pituitary
hormones) and the inhibiting hormones (which inhibit secretions of
pituitary hormones). For example a hypothalamic hormone called
Gonadotrophin releasing hormone (GnRH) stimulates the pituitary
synthesis and release of gonadotrophins. On the other hand, somatostatin
from the hypothalamus inhibits the release of growth hormone from the
pituitary. These hormones originating in the hypothalamic neurons, pass
through axons and are released from their nerve endings. These hormones
reach the pituitary gland through a portal circulatory system and regulate
the functions of the anterior pituitary. The posterior pituitary is under
the direct neural regulation of the hypothalamus (Figure 22.2).
Figure 22.1 Location of endocrine glands
Testis
(in male)
Ovary
(in female)
Adrenal
Pancreas
Thyroid and
Parathyroid
Thymus
Pineal
Pituitary
Hypothalamus
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22.2.2 The Pituitary Gland
The pituitary gland is located in a bony cavity
called sella tursica and is attached to
hypothalamus by a stalk (Figure 22.2). It is
divided anatomically into an adenohypophysis
and a neurohypophysis. Adenohypophysis
consists of two portions, pars distalis and pars
intermedia. The pars distalis region of pituitary,
commonly called anterior pituitary, produces
growth hormone (GH), prolactin (PRL), thyroid
stimulating hormone (TSH),
adrenocorticotrophic hormone (ACTH),
luteinizing hormone (LH) and follicle
stimulating hormone (FSH). Pars intermedia
secretes only one hormone called
melanocyte
stimulating hormone (MSH). However, in
humans, the pars intermedia is almost merged
with pars distalis. Neurohypophysis (pars
nervosa) also known as posterior pituitary, stores
and releases two hormones called oxytocin and
vasopressin, which are actually synthesised by
the hypothalamus and are transported axonally to neurohypophysis.
Over-secretion of GH stimulates abnormal growth of the body leading
to gigantism and low secretion of GH results in stunted growth resulting
in pituitary dwarfism. Excess secretion of growth hormone in adults
especially in middle age can result in severe disfigurement (especially of
the face) called Acromegaly, which may lead to serious complications,
and premature death if unchecked. The disease is hard to diagnose in
the early stages and often goes undetected for many years, until changes
in external features become noticeable. Prolactin regulates the growth of
the mammary glands and formation of milk in them. TSH stimulates the
synthesis and secretion of thyroid hormones from the thyroid gland. ACTH
stimulates the synthesis and secretion of steroid hormones called
glucocorticoids from the adrenal cortex. LH and FSH stimulate gonadal
activity and hence are called gonadotrophins. In males, LH stimulates
the synthesis and secretion of hormones called androgens from testis. In
males, FSH and androgens regulate spermatogenesis. In females, LH
induces ovulation of fully mature follicles (graafian follicles) and maintains
the corpus luteum, formed from the remnants of the graafian follicles
Posterior
pituitary
Anterior
pituitary
Hypothalamus
Hypothalamic
neurons
Portal circulation
Figure 22.2 Diagrammatic representation of
pituitary and its relationship with
hypothalamus
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after ovulation. FSH stimulates growth and
development of the ovarian follicles in females. MSH
acts on the melanocytes (melanin containing cells) and
regulates pigmentation of the skin. Oxytocin acts on
the smooth muscles of our body and stimulates their
contraction. In females, it stimulates a vigorous
contraction of uterus at the time of child birth, and milk
ejection from the mammary gland. Vasopressin acts
mainly at the kidney and stimulates resorption of water
and electrolytes by the distal tubules and thereby
reduces loss of water through urine (diuresis). Hence,
it is also called as anti-diuretic hormone (ADH).
An impairment affecting synthesis or release of ADH
results in a diminished ability of the kidney to conserve
water leading to water loss and dehydration. This
condition is known as
Diabetes Insipidus.
22.2.3 The Pineal Gland
The pineal gland is located on the dorsal side of
forebrain. Pineal secretes a hormone called melatonin.
Melatonin plays a very important role in the regulation
of a 24-hour (diurnal) rhythm of our body. For
example, it helps in maintaining the normal rhythms
of sleep-wake cycle, body temperature. In addition,
melatonin also influences metabolism, pigmentation,
the menstrual cycle as well as our defense capability.
22.2.4 Thyroid Gland
The thyroid gland is composed of two lobes which are
located on either side of the trachea (Figure 22.3). Both
the lobes are interconnected with a thin flap of connective
tissue called isthmus. The thyroid gland is composed of
follicles and stromal tissues. Each thyroid follicle is
composed of follicular cells, enclosing a cavity. These
follicular cells synthesise two hormones,
tetraiodothyronine or thyroxine (T
4
) and
triiodothyronine (T
3
). Iodine is essential for the normal
rate of hormone synthesis in the thyroid. Deficiency of
iodine in our diet results in hypothyroidism and
enlargement of the thyroid gland, commonly called
goitre. Hypothyroidism during pregnancy causes
defective development and maturation of the growing
Figure 22.3 Diagrammatic view of the
position of Thyroid and
Parathyroid
(a) Ventral side
(b) Dorsal side
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baby leading to stunted growth (cretinism), mental retardation, low
intelligence quotient, abnormal skin, deaf-mutism, etc. In adult women,
hypothyroidism may cause menstrual cycle to become irregular. Due to
cancer of the thyroid gland or due to development of nodules of the thyroid
glands, the rate of synthesis and secretion of the thyroid hormones is
increased to abnormal high levels leading to a condition called
hyperthyroidism which adversely affects the body physiology.
Exopthalmic goitre is a form of hyperthyroidism, characterised by
enlargement of the thyroid gland, protrusion of the eyeballs, increased
basal metabolic rate, and weight loss, also called Graves’ disease.
Thyroid hormones play an important role in the regulation of the basal
metabolic rate. These hormones also support the process of red blood cell
formation. Thyroid hormones control the metabolism of carbohydrates, proteins
and fats. Maintenance of water and electrolyte balance is also influenced by
thyroid hormones. Thyroid gland also secretes a protein hormone called
thyrocalcitonin (TCT) which regulates the blood calcium levels.
22.2.5 Parathyroid Gland
In humans, four parathyroid glands are present on the back side of the
thyroid gland, one pair each in the two lobes of the thyroid gland (Figure
22.3b). The parathyroid glands secrete a peptide hormone called
parathyroid hormone (PTH). The secretion of PTH is regulated by the
circulating levels of calcium ions.
Parathyroid hormone (PTH) increases the Ca
2+
levels in the blood. PTH
acts on bones and stimulates the process of bone resorption (dissolution/
demineralisation). PTH also stimulates reabsorption of Ca
2+
by the renal
tubules and increases Ca
2+
absorption from the digested food. It is, thus,
clear that PTH is a hypercalcemic hormone, i.e., it increases the blood
Ca
2+
levels. Along with TCT, it plays a significant role in calcium balance
in the body
.
22.2.6 Thymus
The thymus gland is a lobular structure located between lungs behind
sternum on the ventral side of aorta. The thymus plays a major role in
the development of the immune system. This gland secretes the peptide
hormones called thymosins. Thymosins play a major role in the
differentiation of T-lymphocytes, which provide cell-mediated
immunity. In addition, thymosins also promote production of antibodies
to provide humoral immunity. Thymus is degenerated in old individuals
resulting in a decreased production of thymosins. As a result, the immune
responses of old persons become weak.
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The adrenal medulla secretes two hormones called adrenaline or
epinephrine and noradrenaline or norepinephrine. These are
commonly called as catecholamines. Adrenaline and noradrenaline are
rapidly secreted in response to stress of any kind and during emergency
situations and are called emergency hormones or hormones of Fight
or Flight. These hormones increase alertness, pupilary dilation,
piloerection (raising of hairs), sweating etc. Both the hormones increase
the heart beat, the strength of heart contraction and the rate of respiration.
Catecholamines also stimulate the breakdown of glycogen resulting in
22.2.7 Adrenal Gland
Our body has one pair of adrenal glands, one at the anterior part of each
kidney (Figure 22.4 a). The gland is composed of two types of tissues.
The centrally located tissue is called the
adrenal medulla, and outside
this lies the adrenal cortex (Figure 22.4 b).
Underproduction of hormones by the adrenal cortex alters
carbohydrate metabolism causing acute weakness and fatigue leading
to a disease called Addison’s disease.
Figure 22.4 Diagrammatic representation of : (a) Adrenal gland above kidney
(b) Section showing two parts of adrenal gland
Adrenal gland
Adrenal cortex
Kidney
Adrenal medulla
(a) (b)
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an increased concentration of glucose in blood. In addition, they also
stimulate the breakdown of lipids and proteins.
The adrenal cortex can be divided into three layers, called zona
reticularis (inner layer), zona fasciculata (middle layer) and zona
glomerulosa (outer layer). The adrenal cortex secretes many hormones,
commonly called as corticoids. The corticoids, which are involved in
carbohydrate metabolism are called glucocorticoids. In our body, cortisol
is the main glucocorticoid. Corticoids, which regulate the balance of water
and electrolytes in our body are called mineralocorticoids. Aldosterone is
the main mineralocorticoid in our body.
Glucocorticoids stimulate gluconeogenesis, lipolysis and proteolysis;
and inhibit cellular uptake and utilisation of amino acids. Cortisol is also
involved in maintaining the cardio-vascular system as well as the kidney
functions. Glucocorticoids, particularly cortisol, produces anti-
inflammatory reactions and suppresses the immune response. Cortisol
stimulates the RBC production. Aldosterone acts mainly at the renal
tubules and stimulates the reabsorption of Na
+
and water and excretion
of K
+
and phosphate ions. Thus, aldosterone helps in the maintenance of
electrolytes, body fluid volume, osmotic pressure and blood pressure.
Small amounts of androgenic steroids are also secreted by the adrenal
cortex which play a role in the growth of axial hair, pubic hair and facial
hair during puberty.
22.2.8 Pancreas
Pancreas is a composite gland (Figure 22.1) which acts as both exocrine
and endocrine gland. The endocrine pancreas consists of ‘Islets of
Langerhans’. There are about 1 to 2 million Islets of Langerhans in a
normal human pancreas representing only 1 to 2 per cent of the pancreatic
tissue. The two main types of cells in the Islet of Langerhans are called
α
α
αα
α-cells and
ββ
β
β
β-cells. The α-cells secrete a hormone called glucagon, while
the
ββ
ββ
β-cells secrete insulin.
Glucagon is a peptide hormone, and plays an important role in
maintaining the normal blood glucose levels. Glucagon acts mainly on
the liver cells (hepatocytes) and stimulates glycogenolysis resulting in an
increased blood sugar (hyperglycemia). In addition, this hormone
stimulates the process of gluconeogenesis which also contributes to
hyperglycemia. Glucagon reduces the cellular glucose uptake and
utilisation. Thus, glucagon is a hyperglycemic hormone.
Insulin is a peptide hormone, which plays a major role in the
regulation of glucose homeostasis. Insulin acts mainly on hepatocytes
and adipocytes (cells of adipose tissue), and enhances cellular glucose
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uptake and utilisation. As a result, there is a rapid movement of glucose
from blood to hepatocytes and adipocytes resulting in decreased blood
glucose levels (hypoglycemia). Insulin also stimulates conversion of
glucose to glycogen (glycogenesis) in the target cells. The glucose
homeostasis in blood is thus maintained jointly by the two – insulin and
glucagons.
Prolonged hyperglycemia leads to a complex disorder called diabetes
mellitus which is associated with loss of glucose through urine and
formation of harmful compounds known as ketone bodies. Diabetic
patients are successfully treated with insulin therapy.
22.2.9
Testis
A pair of testis is present in the scrotal sac (outside abdomen) of male
individuals (Figure 22.1). Testis performs dual functions as a primary
sex organ as well as an endocrine gland. Testis is composed of
seminiferous tubules and stromal or interstitial tissue. The Leydig
cells or interstitial cells, which are present in the intertubular
spaces produce a group of hormones called androgens mainly
testosterone.
Androgens regulate the development, maturation and functions of
the male accessory sex organs like epididymis, vas deferens, seminal
vesicles, prostate gland, urethra etc. These hormones stimulate muscular
growth, growth of facial and axillary hair, aggressiveness, low pitch of
voice etc. Androgens play a major stimulatory role in the process of
spermatogenesis (formation of spermatozoa). Androgens act on the central
neural system and influence the male sexual behaviour (libido). These
hormones produce anabolic (synthetic) effects on protein and carbohydrate
metabolism.
22.2.10 Ovary
Females have a pair of ovaries located in the abdomen (Figure 22.1). Ovary
is the primary female sex organ which produces one ovum during each
menstrual cycle. In addition, ovary also produces two groups of steroid
hormones called estrogen and progesterone. Ovary is composed of
ovarian follicles and stromal tissues. The estrogen is synthesised and
secreted mainly by the growing ovarian follicles. After ovulation, the
ruptured follicle is converted to a structure called corpus luteum, which
secretes mainly progesterone.
Estrogens produce wide ranging actions such as stimulation of growth
and activities of female secondary sex organs, development of growing
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ovarian follicles, appearance of female secondary sex characters (e.g., high
pitch of voice, etc.), mammary gland development. Estrogens also regulate
female sexual behaviour.
Progesterone supports pregnancy. Progesterone also acts on the
mammary glands and stimulates the formation of alveoli (sac-like
structures which store milk) and milk secretion.
22.3 HORMONES OF HEART, KIDNEY AND GASTROINTESTINAL TRACT
Now you know about the endocrine glands and their hormones. However,
as mentioned earlier, hormones are also secreted by some tissues which
are not endocrine glands. For example, the atrial wall of our heart secretes
a very important peptide hormone called atrial natriuretic factor (ANF),
which decreases blood pressure. When blood pressure is increased, ANF
is secreted which causes dilation of the blood vessels. This reduces the
blood pressure.
The juxtaglomerular cells of kidney produce a peptide hormone called
erythropoietin which stimulates erythropoiesis (formation of RBC).
Endocrine cells present in different parts of the gastro-intestinal tract
secrete four major peptide hormones, namely gastrin, secretin,
cholecystokinin (CCK) and gastric inhibitory peptide
(GIP). Gastrin
acts on the gastric glands and stimulates the secretion of hydrochloric
acid and pepsinogen. Secretin acts on the exocrine pancreas and
stimulates secretion of water and bicarbonate ions. CCK acts on both
pancreas and gall bladder and stimulates the secretion of pancreatic
enzymes and bile juice, respectively. GIP inhibits gastric secretion and
motility. Several other non-endocrine tissues secrete hormones called
growth factors. These factors are essential for the normal growth of tissues
and their repairing/regeneration.
22.4 MECHANISM OF HORMONE ACTION
Hormones produce their effects on target tissues by binding to specific
proteins called hormone receptors located in the target tissues only.
Hormone receptors present on the cell membrane of the target cells are
called membrane-bound receptors and the receptors present inside the
target cell are called intracellular receptors, mostly nuclear receptors
(present in the nucleus). Binding of a hormone to its receptor leads to the
formation of a hormone-receptor complex (Figure 22.5 a, b). Each
receptor is specific to one hormone only and hence receptors are specific.
Hormone-Receptor complex formation leads to certain biochemical
changes in the target tissue. Target tissue metabolism and hence
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physiological functions are regulated by hormones. On the basis of their
chemical nature, hormones can be divided into groups :
(i) peptide, polypeptide, protein hormones (e.g., insulin, glucagon,
pituitary hormones, hypothalamic hormones, etc.)
(ii) steroids (e.g., cortisol, testosterone, estradiol and progesterone)
(iii) iodothyronines (thyroid hormones)
(iv) amino-acid derivatives (e.g., epinephrine).
Hormones which interact with membrane-bound receptors normally
do not enter the target cell, but generate second messengers (e.g., cyclic
AMP, IP
3
, Ca
++
etc) which in turn regulate cellular metabolism (Figure
22.5a). Hormones which interact with intracellular receptors (e.g., steroid
hormones, iodothyronines, etc.) mostly regulate gene expression or
chromosome function by the interaction of hormone-receptor complex
with the genome. Cumulative biochemical actions result in physiological
and developmental effects (Figure 22.5b).
(a)
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SUMMARY
There are special chemicals which act as hormones and provide chemical
coordination, integration and regulation in the human body. These hormones
regulate metabolism, growth and development of our organs, the endocrine glands
or certain cells. The endocrine system is composed of hypothalamus, pituitary
and pineal, thyroid, adrenal, pancreas, parathyroid, thymus and gonads (testis
and ovary). In addition to these, some other organs, e.g., gastrointestinal tract,
kidney, heart etc., also produce hormones. The pituitary gland is divided into
three major parts, which are called as pars distalis, pars intermedia and pars
nervosa. Pars distalis produces six trophic hormones. Pars intermedia secretes
Figure 22.5 Diagramatic representation of the mechanism of hormone action :
(a) Protein hormone (b) Steroid hormone
(b)
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only one hormone, while pars nervosa (neurohypophysis) secretes two hormones.
The pituitary hormones regulate the growth and development of somatic tissues
and activities of peripheral endocrine glands. Pineal gland secretes melatonin, which
plays a very important role in the regulation of 24-hour (diurnal) rhythms of our
body (e.g., rhythms of sleep and state of being awake, body temperature, etc.). The
thyroid gland hormones play an important role in the regulation of the basal
metabolic rate, development and maturation of the central neural system,
erythropoiesis, metabolism of carbohydrates, proteins and fats, menstrual cycle.
Another thyroid hormone, i.e., thyrocalcitonin regulates calcium levels in our blood
by decreasing it. The parathyroid glands secrete parathyroid hormone (PTH) which
increases the blood Ca
2+
levels and plays a major role in calcium homeostasis. The
thymus gland secretes thymosins which play a major role in the differentiation of
T-lymphocytes, which pr
ovide cell-mediated immunity. In addition, thymosins
also increase the production of antibodies to provide humoral immunity. The
adrenal gland is composed of the centrally located adrenal medulla and the outer
adrenal cortex. The adrenal medulla secretes epinephrine and norepinephrine.
These hormones increase alertness, pupilary dilation, piloerection, sweating, heart
beat, strength of heart contraction, rate of respiration, glycogenolysis, lipolysis,
proteolysis. The adrenal cortex secretes glucocorticoids and mineralocorticoids.
Glucocorticoids stimulate gluconeogenesis, lipolysis, proteolysis, erythropoiesis,
cardio-vascular system, blood pressure, and glomerular filtration rate and inhibit
inflammatory reactions by suppressing the immune response. Mineralocorticoids
regulate water and electrolyte contents of the body. The endocrine pancreas secretes
glucagon and insulin. Glucagon stimulates glycogenolysis and gluconeogenesis
resulting in hyperglycemia. Insulin stimulates cellular glucose uptake and
utilisation, and glycogenesis resulting in hypoglycemia. Insulin deficiency
and/or insulin resistance result in a disease called diabetes mellitus.
The testis secretes androgens, which stimulate the development, maturation
and functions of the male accessory sex organs, appearance of the male secondary
sex characters, spermatogenesis, male sexual behaviour, anabolic pathways and
erythropoiesis. The ovary secretes estrogen and progesterone. Estrogen stimulates
growth and development of female accessory sex organs and secondary sex
characters. Progesterone plays a major role in the maintenance of pregnancy as
well as in mammary gland development and lactation. The atrial wall of the heart
produces atrial natriuretic factor which decreases the blood pressure. Kidney
produces erythropoietin which stimulates erythropoiesis. The gastrointestinal tract
secretes gastrin, secretin, cholecystokinin and gastric inhibitory peptide. These
hormones regulate the secretion of digestive juices and help in digestion.
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EXERCISES
1. Define the following:
(a) Exocrine gland
(b) Endocrine gland
(c) Hormone
2. Diagrammatically indicate the location of the various endocrine glands in our
body.
3. List the hormones secreted by the following:
(a) Hypothalamus (b) Pituitary (c) Thyroid (d) Parathyroid
(e) Adrenal (f) Pancreas (g) Testis (h) Ovary
(i) Thymus (j) Atrium (k) Kidney (l) G-I Tract
4. Fill in the blanks:
Hormones Target gland
(a) Hypothalamic hormones __________________
(b) Thyrotrophin (TSH) __________________
(c) Corticotrophin (ACTH) __________________
(d) Gonadotrophins (LH, FSH) __________________
(e) Melanotrophin (MSH) __________________
5. Write short notes on the functions of the following hormones:
(a) Parathyroid hormone (PTH) (b) Thyroid hormones
(c) Thymosins (d) Androgens
(e) Estrogens (f) Insulin and Glucagon
6. Give example(s) of:
(a) Hyperglycemic hormone and hypoglycemic hormone
(b) Hypercalcemic hormone
(c) Gonadotrophic hormones
(d) Progestational hormone
(e) Blood pressure lowering hormone
(f) Androgens and estrogens
7. Which hormonal deficiency is responsible for the following:
(a) Diabetes mellitus (b) Goitre (c) Cretinism
8. Briefly mention the mechanism of action of FSH.
9. Match the following:
Column I Column II
(a) T
4
(i) Hypothalamus
(b) PTH (ii) Thyroid
(c) GnRH (iii) Pituitary
(d) LH (iv) Parathyroid
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