The document discusses human physiology related to endocrine glands and hormones. It describes that endocrine glands secrete hormones which act as chemical messengers. The major endocrine glands discussed include the hypothalamus, pituitary gland, thyroid gland, parathyroid gland, adrenal gland, pancreas, gonads and pineal gland. It provides details on the hormones secreted by each gland and their functions in regulating other glands and target organs. Disorders related to some glands like pituitary are also mentioned.

1. Human Physiology –
Chemical Coordination and Integration
Prepared by –
Parna Dey
Biology Faculty
MSc Microbiology
Gate Qualified (Life Sciences)

2. Endocrine Glands are ductless glands whose secretions are called Hormones.
Hormones are non-nutrient chemicals which act as intercellular messengers and are produced in trace amounts.
They are produced by endocrine glands and released into the blood and transported to a distantly located target organ.
Holocrine Glands – Secretes only hormones (thyroid, parathyroid, adrenal ,pituitary)
Heterocrine Glands – Secretion of hormones as well as some other functions (pancreas, testes, ovaries)
Discovered by Bayliss and Starling
Term Hormone introduced by Starling
ENDOCRINE GLANDS AND HORMONES

3. • Transported by blood stream from endocrine gland to target cells or organs – endocrine nature
• They are chemical messengers
• Excess or deficiency leads to disorders –hyper and hypoactivity/conditions respectively
• Many hormones are produced as inactive ‘prohormones’
• Often used up in their regulatory action
Basic characteristics of Hormones

4. • Amino acid derivative hormone – Epinephrine , Norepinephrine (Tyrosine)
Melatonin, Seratonin (Tryptophan)
• Peptide and Protein Hormones – Antidiuretic Hormone , Oxytocin (Peptide)
Insulin, Glucagon, Growth Hormone, Thyroid Stimulating Hormone,
Thyrocalcitonin, Parathyroid (Protein)
• Iodothyronines - iodinated amino acids with a diphenyl ether ring system – Thyroid Hormones
• Steroid Hormones – Cortisol, Testosterone, Estrogens, Progesterone
Hormone Classification based on chemical nature

5. 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 organized endocrine bodies in our body.
In addition to these, some other organs, e.g., gastrointestinal
tract, liver, kidney, heart also produce hormones.
Human Endocrine System

6. Hypothalamus
Location and Structure : The hypothalamus is the basal
part of diencephalon, forebrain
It contains several groups of neurosecretory cells called
nuclei which produce hormones. These hormones regulate
the synthesis and secretion of pituitary hormones. The
hypothalamus is often called the control centre or
‘supreme commander’ of endocrine regulation.
Hormones : The hormones produced by hypothalamus are
of two types, the releasing hormones (which stimulate
secretion of pituitary hormones) and the inhibiting
hormones (which inhibit secretions of pituitary hormones).
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

7. Releasing Hormones
Gonadotrophin Releasing
Hormone (GnRH)
It stimulates the anterior lobe of the pituitary gland to release two gonadotrophins :
Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH)
Adrenocorticotrophic
Releasing Hormone (ARH)
It stimulates the anterior lobe of the pituitary gland to release Adrenocorticotrophic Hormone
(ACTH)
Thyrotrophin Releasing
Hormone (TRH)
It stimulates the anterior lobe of the pituitary gland to release Thyroid Stimulating Hormone
(TSH)
Growth Hormone
Releasing Hormone
(GHRH)
It stimulates the anterior lobe of the pituitary gland to release Growth Hormone
Prolactin Releasing
Hormone
It stimulates the anterior lobe of the pituitary gland to release Prolactin (PRL)
MSH releasing hormone
(MSHRH)
It stimulates the intermediate lobe of the pituitary gland to release Melanocyte Stimulating
Hormone (MSH)
Inhibiting Hormones
Somatostatin (SS) Also called Growth Hormone – Inhibitory Hormone (GHIH). It inhibits the release of growth
hormone from the pituitary
Prolactin Inhibitory
Hormone (PIH)
It inhibits the release of prolactin from the anterior lobe of the pituitary gland
MSH Inhibitory Hormone
(MSHIH)
It inhibits the release of Melanocyte Stimulating Hormone from the intermediate lobe of the
pituitary gland

8. Pituitary Gland
Location and Structure : The pituitary gland is the smallest
endocrine gland, located in a bony cavity called sella tursica
and is attached to hypothalamus by a stalk (infundibulum).
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 is commonly called anterior pituitary.
Neurohypophysis (pars nervosa) is known as posterior
pituitary.
It is called the ‘Master Endocrine gland’ as it secretes of
number of hormones which regulate the working of other
endocrine glands.

9. Hormones of the Anterior Pituitary
1. Growth Hormone (GH) – This hormone stimulates growth
2. Thyroid Stimulating Hormone (TSH) – This hormone controls the growth and activity of the thyroid gland
3. Adrenocorticotrophic Hormone (ACTH) – This hormone stimulates the cortex of the adrenal gland to produce its
glucocorticoid hormones
4. Prolactin Hormone (PRL) – ‘Hormone of Maternity’ – This hormone activates growth of breasts during pregnancy
and secretion of mammary glands after childbirth
5. Gonadotrophic Hormones –
i. Follicle Stimulating Hormone (FSH) – It stimulates growth of ovarian follicles(graafian follicles), development of
egg/secondary oocyte and their secretion of oestrogens in females. In males, FSH acts on Sertoli cells to secrete ABP
(Androgen Binding Protein), thus helps in Spermatogenesis.
ii. Luteinizing Hormone (LH)- In females, LH induces ovulation of fully mature follicles (graafian follicles) and maintains
the corpus luteum, formed from the remnants of the graafian follicles after ovulation . It stimulates the corpus luteum
of the ovary to secrete progesterone. In male it activates the Leydig’s cell (interstitial cells) of the testes to secrete
androgens , hence called Interstitial cells stimulating hormone (ICSH), thus helps in Spermatogenesis.

10. Hormone of the Pars Intermediate
Melanotrophin or Melanocyte Stimulating Hormone – Associated with the growth and development of melanocytes
which gives colour to the skin
Hormones of the Posterior Pituitary
Posterior Pituitary stores and releases two hormones called oxytocin and vasopressin, which are actually synthesised by
the hypothalamus and are transported axonally to neurohypophysis.
1. Oxytocin (OT) – Oxytocin promotes contraction of the uterine muscle and contraction of the myoepithelial cells of
the lactating breast , helping in milk ejection. Called ‘birth hormone’ and ‘ milk ejecting hormone’
2. Antidiuretic Hormone (ADH) or Vasopressin –
It increases the reabsorption of water from glomerular filtrate in the distal convoluted tubule and collecting ducts of
the nephrons of the kidneys (antidiuretic effect)
It stimulates contraction of involuntary muscles in the walls of the intestine, gall bladder, urinary bladder and blood
vessels raising blood pressure (pressor effect)

11. Pituitary Disorders
1. Pituitary Dwarfism – It is caused by the deficiency of growth hormones (GH) from childhood.
2. Gigantism – It is caused by the excess of growth hormone from early age.
3. Acromegaly – extremely large – It is caused by excess of growth hormone after adult size is reached. It is
characterized by disproportionate increase in size of bones of face , hands and feet. 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.
4. Diabetes Insipidus - 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.
Thus, it is caused by the deficiency of ADH , and is characterized by excessive dilute urine.

12. The Pineal Gland
Location and Structure : The pineal gland is located on the
dorsal side of forebrain. It is located in the epithalamus,
between the cerebral hemispheres. It is a small rounded
body which consists of pineal cells and supporting glial
cells.
It secretes Melatonin Hormone, also called the ‘sleep
hormone’, because it promotes sleep. Its secretion
increases in dim light and decreases in bright light.
Melatonin concentration in the blood appears to flow a diurnal (day-night) cycle as it rises in the evening and
through the night and drops to a low around noon. Because of this light mediated response , the pineal gland
may act as a kind of “biological clock” which may produce circadian rhythms (variations following a 24 hour
cycle). It also maintains body temperature, influences metabolism, pigmentation, the menstrual cycle as well
as our defense capability.

13. Thyroid Gland
Location and Structure : The thyroid gland is the largest
endocrine gland located anterior to the thyroid cartilage of
the larynx in the neck. The thyroid gland is composed of
two lobes which are located on either side of the trachea.
Both the lobes are interconnected with a thin flap of
connective tissue called isthmus. The thyroid gland is
composed of thyroid follicles and stromal tissues. Each
thyroid follicle is composed of follicular cells, enclosing a
cavity. The gland is well supplied with blood vessels. Beside
containing blood capillaries, the stroma contains small
clusters of specialized parafollicular cells or ‘C’ cells.
This gland is stimulated to secrete its hormones by Thyroid
Stimulating Hormone (Thyrotropin) by the anterior lobe of
the pituitary gland.

14. Hormones of Thyroid Gland
1. Thyroxine (Tetraiodo-thyronine or T4) and Tri-iodothyronine – secreted by thyroid follicular cells. T3 is more
active and potent than T4 although secreted in smaller amounts. T4 can be converted to T3 by removal of one
iodine in the liver, kidneys and some other tissues. Since, both have similar effects they are considered
together as Thyroid Hormone (TH). The Thyroid gland is the only gland that stores its hormones in large
quantity . Both are prepared by attaching Iodine to Tyrosine. Functions of T4 and T3 are:
i. They regulate the metabolic rate . Thus maintain BMR
ii. Stimulate protein synthesis, thus promote growth of body tissues
iii. Regulate the development of mental activities
iv. As they increase heat production, thus maintain body temperature
v. Help in metamorphosis of tadpole into adult frog. Larva  adult
vi. Increase action of neurotransmitters like adrenaline and noradrenaline
2. Thyrocalcitonin (TCT) /Calcitonin– secreted by the C-cells of the thyroid cells. It is secreted when Calcium level
is high in the blood. It then lowers the Calcium level by suppressing release of Calcium ions from the bones. Its
action is opposite to that of parathyroid hormone on calcium metabolism.

15. Thyroid Disorders
1. Hyperthyroidism
Exophthalmic goitre - Exophthalmic goitre is a form of hyperthyroidism, characterized by enlargement of the thyroid gland, protrusion
of the eyeballs, increased basal metabolic rate, and weight loss, also called Graves’ disease.
2. Hypothyroidism
a. Cretinism – This disorder is caused by deficiency of thyroid hormone in infants. A cretin has slow body growth leading to stunted
growth (cretinism) and mental development of reduced metabolic rate. This disease can be treated by an early administration of
thyroid hormones.
b. Simple Goitre – It is caused by the deficiency of iodine in diet because Iodine is needed for the synthesis of thyroid hormone. It
causes thyroid enlargement. It may lead to cretinism or myxoedema. Addition of Iodine to table salt prevents this disease.
c. Hashimoto’s Disease – In this disease all the aspects of thyroid function are impaired. It is an autoimmune disease in which the
thyroid gland is destroyed by autoimmunity.
d. Hypothyroidism during pregnancy causes defective development and maturation of the growing baby leading to stunted growth
(cretinism), mental retardation, low intelligence quotient, abnormal skin, deaf-mutism, etc
e. In adult women, hypothyroidism may cause menstrual cycle to become irregular.

16. 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. The
cells of parathyroid glands are of two types: chief cells or principal cells
and large oxyphil cells (or eosinophil cells). The chief cells of parathyroid
glands secrete a peptide hormone called parathyroid hormone (PTH),
also called Collip’s Hormone.
The secretion of PTH is regulated by the circulating levels of calcium
ions. This hormone regulates the calcium and phosphate balance
between blood and other tissues. PTH inhibits collagen synthesis by
osteoblasts. It mobilizes the release of calcium into the blood from the
bones. It increases Ca2+ absorption from the intestines. Parathyroid
hormone (PTH) increases the Ca2+ levels in the blood. PTH acts on
bones and stimulates the process of bone resorption (dissolution/
demineralization). PTH also stimulates reabsorption of Ca2+ by the
renal tubules and increases Ca2+ absorption from the digested food. It
is, thus, clear that PTH is a hypercalcemic hormone, i.e., it increases the
blood Ca2+ levels. Along with TCT, it plays a significant role in calcium
balance in the body.

17. Parathyroid Disorders
1. Hypoparathyroidism – It causes lowering of blood calcium levels. This increases the excitability of
nerves and muscles, causing cramps and convulsions. Sustained contractions of the muscles of
larynx, face, hands and feet are produced. This disorder is called parathyroid tetany.
2. Hyperparathyroidism – More Calcium drawn into bone. It causes demineralization, resulting in
softing and bending of the bones. Some of the bone substance is replaced by cavities that are filled
with fibrous tissues. This condition leads to osteoporosis. An excess of parathormone also causes
calcium deposition in kidneys.

18. Thymus Gland
The thymus gland is a lobular structure located between lungs behind
sternum on the ventral side of aorta. It is a prominent gland at the time
of birth, continues to grow till puberty and then undergoes gradual
atrophy in the adult. Disappearance of thymus causes ageing.
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.

19. Adrenal Gland
Theses are paired structures located on the top of the kidneys.
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. The cortex is surrounded by a fibrous capsule.

20. Adrenal Cortex
The Adrenal cortex has three layers : zona reticularis (inner), zona fasciculata (middle) and zona glomerulosa (outer)
The adrenal cortex secretes many hormones, commonly called as corticoids. – mineralocorticoids, glucocorticoids and
gonadocorticoids
a. Mineralocorticoids – These hormones regulates the balance of water and are responsible for mineral metabolism.
Aldosterone (salt-retaining hormone) is the principal mineralocorticoid, steroid in nature. It acts on kidneys to cause more
sodium to be returned to the blood and more potassium to be excreted. Thus, it increases both sodium and water in the blood.
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.
b. Glucocorticoids - The corticoids, which are involved in carbohydrate metabolism are called glucocorticoids. Glucocorticoids
stimulate gluconeogenesis, lipolysis and proteolysis; and inhibit cellular uptake and utilization 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 In our body, cortisol is
the main glucocorticoid. Cortisol has anti insulin effect. It has the capacity to cope with stress – stress hormone. This acts o the
cells of the liver
c. Gonadocorticoids – Sex hormones of adrenal glands. Male sex hormones – androgens – male foetus development. Androgens
stimulate the development of male secondary sexual characters like distribution of body hair. Female sex hormones – estrogens
– female secondary sexual characters

21. Adrenal Cortex Disorders
i. Addison’s Disease – deficiency of mineralocorticoids and glucocorticoids. Symptoms include low blood sugar, low
plasma Na+, high plasma K+, nausea, vomiting, diarrhoea
ii. Cushing’s syndrome – excess of cortisol, may be due to tumour of adrenal cortex. Symptoms include high blood
sugar, sugar in urine, rise in plasma Na+, fall in plasma K+, rise in blood volume, high blood pressure
iii. Conn’s syndrome – aldosteronism – excessive aldosterone . rise in plasma Na+, fall in plasma K+, rise in blood
volume, high blood pressure, polyurea
iv. Adrenal virilism – appearance of male characters in female due to excessive secretion of androgens (male
sexcorticoids) produces male secondary sexual characters like beard, moustache, hoarse, voice in woman
v. Gynaecomastia – development of enlarged mammary glands in the males due to excessive secretion of oestrogens
(female sex corticoids) in males. Decreased testosterone may also lead to gynaecomastia

22. Adrenal Medulla
The Adrenal medulla consists of rounded groups of relatively large and granular cells. These are modified postganglionic cells of
sympathetic nervous system which have lost normal processes and have acquired a glandular function. These cells are called
chromaffin cells. Adrenal medulla is an extension of the sympathetic nervous system. Stimulation of the sympathetic nerves to
adrenal medulla causes large quantities of epinephrine (adrenaline ) and norepinephrine(noradrenaline) to be released into
blood circulation. Both the hormones effect similarly – accelerates heartbeat, raises blood pressure , slow peristalsis, etc. These
are discussed together as sympatheticoadrenal system as sympathetic nervous system and adrenal medulla function as an
integrated system.
Hormones of Adrenal Medulla (glands of emergency):
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, pupillary
dilation, piloerection (raising of hairs), sweating etc. Both the hormones increase the heartbeat, the strength of heart contraction and
the rate of respiration. Catecholamines also stimulate the breakdown of glycogen resulting in an increased concentration of glucose in
blood. In addition, they also stimulate the breakdown of lipids and proteins.

23. Pancreas
Pancreas lies inferior to the stomach in a bend of the duodenum.
It is a composite gland which acts as both exocrine and endocrine
gland. A large pancreatic duct carries enzymes and other exocrine
digestive secretions from the pancreatic acinar cells to the small
intestine. The tissue of pancreas in addition to acinar cells has a
group of cells called islets of Langerhans. These produces
endocrine secretions. The two main types of cells in the Islet of
Langerhans are called α-cells and β-cells. The α-cells secrete a
hormone called glucagon, while the β-cells secrete insulin.

24. Hormones of Pancreas
1. Glucagon – Stimulates liver to convert stored glycogen into glucose. It is also called anti-insulin hormone. It
acts on the cells of the liver and adipose tissue. 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 utilization. Thus, glucagon is a hyperglycemic hormone.
2. Insulin – It is antagonistic to Glucagon. It converts Glucose into Glycogen (Glycogenesis) in the liver and
muscles. 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.
uptake and utilization. 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.
Insulin also reduces catabolism of protein and promotes protein synthesis, increases synthesis of fats and
reduces breakdown and oxidation of fat – anabolic hormone
The glucose homeostasis in blood is thus maintained jointly by the two – insulin and glucagons.

25. Disorders of Pancreas
1. Diabetes Mellitus (Hyperglycemia) - The body breaks down carbohydrates and sugar present in food and then
converts them into a special sugar called glucose. This glucose is used to fuel up the body. Excess glucose
present in the blood is converted into glycogen as an energy reserve. This process requires insulin. Diabetes
mellitus is a condition during which the body does not produce sufficient insulin, which leads to high blood
glucose levels. This condition is usually related to the pancreas, an organ which is involved in producing
insulin.
i. Type 1 Diabetes Mellitus – Insulin Dependent Diabetes Mellitus (IDDM) – Juvenile Diabetes
Caused by failure of beta cells to produce adequate insulin
ii. Type 2 Diabetes Mellitus – Also called insulin-independent diabetes (NIDDM)
Involves failure of insulin to facilitate movement of glucose into cells
In both the conditions , blood glucose is elevated – hyperglycemia , causing excessive urination with glucose in
urine, dehydration of body, excessive thirst (polydipsia). Cell is unable to use glucose for energy production.
Instead use proteins and fats producing harmful ketone bodies (ketosis).
Diabetic patients are successfully treated with insulin therapy.
2. Hypoglycemia – Occurs when blood glucose levels fall below normal. May be caused due to excess of insulin,
deficiency of glucagon or failure of secretion of the two hormones to regulate blood sugar. Symptoms include
weakness, profuse sweating, irritability, confusion, convulsions. It needs urgent intake of sugar or glucose.

26. Testes
A pair of testis is present in the scrotal sac (outside abdomen) of male individuals. Testis is composed of
seminiferous tubules and stromal or interstitial tissue. The connective tissue present between the seminiferous
tubules in a testes contains small clusters of endocrine cells called interstitial cells or Leydig’s cells. These cells
secrete various male sex hormones called androgens . Testosterone is the principal androgen.
Functions of Androgens:
i. Androgens regulate the development, maturation and functions of
the male accessory sex organs like epididymis, vas deferens, seminal
vesicles, prostate gland, urethra etc.
ii. These hormones stimulate muscular growth, growth of facial and
axillary hair, aggressiveness, low pitch of voice etc.
iii. Androgens play a major stimulatory role in the process of
spermatogenesis (formation of spermatozoa).
iv. Androgens act on the central neural system and influence the male
sexual behavior (libido).
v. These hormones produce anabolic (synthetic) effects on protein and
carbohydrate metabolism.
Inhibin/Actin is secreted by Sertoli cells of seminiferous tubules of testes.
Inhibin inhibits and actin activates FSH production from pituitary.

27. Ovary
Females have a pair of ovaries located in the abdomen. They are located in the pelvic cavity . 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
1. Estrogens produce wide ranging actions such as stimulation of growth
and activities of female secondary sex organs, development of growing
ovarian follicles, appearance of female secondary sex characters (e.g.,
high pitch of voice, etc.), mammary gland development. Estrogens also
regulate female sexual behavior. Estradiol is principal feminizing estrogen.
2. 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. Both estradiol and
progesterone are required for ovulation.
Progesterone has a negative feedback effect on GnRH which in turn limits
FSH, LH and Progesterone secretion. Inhibin is also secreted which has a
negative effect on GnRH and FSH secretion.

28. HORMONES OF HEART, KIDNEY AND GASTROINTESTINAL TRACT
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.

29. 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. 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 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, IP3 , Ca++ etc) which in turn regulate cellular metabolism. 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.

30. Mode of Protein Hormone Action through Extracellular receptors

31. Mode of Steroid Hormone action through Intracellular receptors