Exocrine and Endocrine glands : These glands are secretory sites of the body. They are of two kinds—the exocrine and the endocrine glands. The exocrine glands convey their secretion to the place of their action via ducts whereas the endocrine glands secrete the active chemical messengers, the hormones and pour their secretion into the blood stream which takes these substances to their place of action. Pancreas and Gonads glands are of mixed nature.
Hormones : Hormones are very important and informational chemicals. These are inhibitory or stimulatory in nature. These hormones coordinate the activities of different organs and tissues. They are usually made up of proteins, amino acids, peptides, catecholamines or steroids.
Hypothalamus has certain cells which secrete neurohormones. These neurohormones (such as thyrotropin releasing hormone, gonadotropin releasing hormone somatostatin and corticotropin releasing hormone) are carried to the anterior pituitary or adenohypophysis by portal veins. They regulate secretion of its hormones-thyrotropin, gonadotropin, STH, corticotropin respectively.
Certain neurons are modified into glandular cells. These cells situated on hypothalamus are known as neurosecretory cells and their secretion is called as neurosecretory material (NSM). They are carried to the posterior pituitary or neurohypophysis. This part of pituitary stores the hormones vasopressin and oxytocin.
Pituitary gland
Pituitary gland is the master gland. This gland is divided on the basis of its histology and nature into adenohypophysis and neurohypophysis.
Adenohypophysis : The adenohy-pophysis secretes its own synthesised hormone. It includes anterior and middle pituitary.
The hormones secreted by anterior pituitary are :
(1) Growth hormone (GH) or somatotropic hormone (STH) : It regulates the body growth.
(2) Thyroid stimulating hormone (TSH) : It stimulates the thyroid for hormonal secretion.
(3) Adreno-corticotropic hormone (ACTH) : It stimulates the adrenal cortex to secrete glucocorticoids.
(4) Follicle stimulating hormone (FSH)
(5) Luteinizing hormone (LH or ICSH) : It controls the male and female gonadal development and their hormone secretion.
(6) Prolactin hormone (PRL or MTH or LTH) : It controls milk secretion in females.
The secretion of pituitary is controlled by the age, external stimuli and feedback system of hormone.
Middle pituitary : It secretes the melanocyte stimulating hormone (MSH) or intermedin. It controls the pigmentation of skin.
Neurohypophysis : The neurohypop-hysis stores the hormones secreted by the neurosecretory cells of hypothalamus and includes posterior pituitary only.
The hormones of neurohypophysis or posterior pituitary are :
(1) Oxytocin or pitocin : It contracts the smooth muscle of uterus and mammary glands.
(2) Antidiuretic hormone (ADH) or vasopressin or pitressin : It regulates the reabsorption of water by kidneys.
Thyroid
Thyroid gland secretes the thyroxin and tri-iodothyroxin which control the basal metabolic rate, body growth, tissue differentiation and metamorphosis of tadpoles.
Parathyroid
Parathyroid secretes the parathormone and calcitonin which increase the blood calcium level by increasing the bone calcium and renal reabsorption of calcium from the urine.
Adrenal
Adrenal cortex : It secretes the following steriod hormones.
(1) Adrenalin or epinephrin : Its function is the vasodilation of arterioles of skeletal muscles and vasoconstriction of arterioles of skin, mucous membrane and viscera. At the time of fear, it increases the heartbeat rate and blood pressure.
(2) Nor-adrenalin or norepinephrin: Its function is antagonistic to the above hormone.
Adernal medulla and sympathetic nervous system together act in the integrated and co-ordinated way and form a sympathetico-adrenal system.
Pancreas
Pancreas work as mixed gland both exocrine and endocrine gland. Its islets of Langerhans tissue has two types of cells—alpha and beta cells.
(1) Alpha cells : These cells secrete the glucagon hormone. This converts the liver glycogen into glucose. Thus it works opposite to that of insulin.
(2) Beta cells : These cells secrete the insulin (hypoglycemic antidiabetic factor). It stimulates the glycogenesis and opposes the gluconeoenesis.
Gonads
Interstitial cells of the testis secrete the testosterone. This hormone stimulates the growth and functions of secondary sexual characters.
Graafian follicle and corpus luteum: This secretes the estrogen and progesterone respectively. The former stimulates the growth of female secondary sexual characters; where as the pregnancy changes are controlled by the progesterone.
Testosterone is excreted in the urine in the form of ketosteroid, androsterone and eitrocholanole. The estrogen is excreted in the form of estron or estradiol; whereas the pregnanediol is the main excretory product of progesterone.
Pineal body : This is situated on the dorsal surface of the diencephalon and it secretes melatonin and seratonin hormone. These hormones regulate hormonal monthly cycle, reproductive cycle and related physiology.
Mucous membrane of stomach and duodenum : It secrete the gastrin, secretin, pancreozymin, cholecystokinin, enterokinin and enterogastrin.
Gastrin : It stimulates the gastric glands to secrete the gastric juice. It is secreted by the gastric mocosa by the mechanical stimulation.
Secretin : It stimulates the pancreas to produce pancreatic juice. It is secreted by the duodenal mucosa.
Pancreozymin : It regulates the composition or pancreatic juice.
Cholecystokinin : It contracts the gall bladder.
Enterokinin : It stimulates the intestinal mucosa for succus entericus secretion.
Enterogastrin : It controls the gastric secretion while stomach is empty.
Adrenal Cortex : It produces two groups of steroid hormones;
(i) Glucocorticoids e.g. cortisone, cortisol, corticosterone. They regulate the metabolism of carbohydrates, proteins and fats. They also increase the blood glucose level.
(ii) Mineralocorticoids e.g. aldosterone. They reduce the sodium loss from the body in urine, sweat, saliva etc.
Adrenal Medulla : It produces two hormones :
(i) Adrenaline or epinephrine
(ii) Non-adrenaline or non-epinephrine.
Both of these hormones together regulate blood pressure.
Insulin : It is a protein hormone produced by the b-cells of islets of langerhans (Endocrine part of pancreas forms the islets of langerhans). It regulates the amount of glucose or sugar in the blood.
Diabetes mellitus : If sufficient quantity of insulin is not produced, this prevents the liver and muscle to convert the excess glucose into glycogen (glucogenesis). This causes more of sugar to enter the blood stream which is eliminated along with urine. This disease is called diabetes mellitus.
Glycogenolysis : The process of converting glycogen into glucose is known as glycogenolysis.
Glucagon : It is a protein hormone which regulates the amount of glucose in blood. This is produced by cells of islest of langerhans.
Testosterone : This steroid hormone is produced by testis. This hormone at puberty helps to develop male sexual characteristics i.e. enlargement of scrotum and penis, growth of pubic hair, broadening of shoulder, deepening of voice etc.
Androsterone : It is a hormone released by testis and it works similar to those of testosterone.
Androgens : Two harmones testosterone and androsterone together are called androgens.
Estradiol or estrogen : This hormone is secreted by ovary. It controls the growth of secondary sexual characters in females.
Progesterone : This hormone is also secreted by ovary. It is responsible for maintenance of pregnancy, development of mammary glands and the regulation of menstrual cycle.
Relaxin : It is a hormone secreted by ovary. It softens the pelvic ligaments at the time of childbirth.
Castration : It refers to the removal of testis.
Addison’s syndrome : It is a disease caused due to the deficiency of glucocorticoids and mineralcorticoids. Symptoms of this disease are low blood pressure, low plasma Na+, nausea, vomitting and diarrhoea.
Cushing’s syndrome : It is caused by excessive production of hormones by cells of tumor. A tumor is produced due to the excessive rate of divison of the cells of adrenal cortex. This diesease is marked by high blood pressure, presence of sugar in urine, obestity, increase in plasma, rise in blood volume etc.
Aldosteronism : It is a disease caused by an abnormal secretion of aldosterone from adrenal cortical tumor. In this disease one has high plasma Na+, low plasma K+, rise in blood volume and high blood pressure.
Adrenal virilism : It is an abnormal development of females by an excessive secretion of sex corticoids. It results in the formation of external characters such as beards and moustaches and male like voice in women.
Simple Goitre : Due to deficiency of iodine the thyroid gland grows larger to produce more thyroxin.
The enlargement of thyroid gland is known as simple goitre.
Dwarfism : It results due to hypoactivity of pituitary gland (i.e. insufficiency of growth homone).
Gignatism : It results due to hyperactivity of the pituitary gland (i.e., pituitary gland secretes more growth hormone).
Melatonin : Melatonin is secreted by pineal glands. It is an antigonadotrophic because it reduces the quantity of follicle-stimulating harmone (FSH) and luteinising hormone (LH).
Serotonin : Serotonin is also secreted by pineal glands. It helps to decrease the diameter of blood vessel i.e. vascoconstrictor.
Prolactin : This hormone is responsible for regulating the secretion of milk after delivery.
The growth Hormone (GH) : This hormone acts on the tissues of the body and thus promotes general growth.
Renin : Renin is produced by certain cells present near the glomerular region of kidney under the influecne of nervous stimuli.
Pheromones : These are the chemicals that are released by some exocrine glands. They are secreted to the outside and function as chemical messengers. They help in identifying the members of the same species particularly of opposite sex. They help in locating the males in animals.
Some of the pheromones indentified are bombykol (from silk moth) gyptol (from gypsy moth).
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