Chemical messengers are involved in communication and maintaining consistency and equilibrium of the internal environment of the body of the animal and in the body’s response to various stimuli. One type of chemical messenger is known as hormone. Hormones act as chemical coordinator of the body. Only those cells that have specific receptors for a hormone can respond to that hormone. Hormones work along with the nerves to communicate and integrate activities with the body of the animals. But hormones are secreted under the control of the central nervous system. In other words, the endocrine and peripheral nervous system are the two parts of the same coordinating and integrating mechanism operating through central nervous system. The hormones are mainly excitatory in nature stimulating activity in other organs.

There are mainly two types of glands found in the body of the animals namely exocrine glands and endocrine glands. The glands having external secretion and provided with ducts to release their secretions are called exocrine glands e.g. salivary gland, sweat gland, mammary gland and lachrymal gland etc. Their secretions may be enzymes or some other substances, either released to body surface or into the alimentary canal or to some other parts. But the gland which have internal secretions and without any ducts but they release their secretions directly into the blood stream and transported to distant parts of the body to act upon targeted cells. These glands are called endocrine glands. The secretions from the endocrine glands are called hormones.

The endocrine system deals with a number of endocrine or ductless glands. The study of endocrine glands and their hormones is called Endocrinology. The secretions of these glands called hormones which are poured directly into the blood stream and circulated through the blood and affect the metabolic activity of a targeted cell or tissue in a specific way. The organ or tissue influenced by a particular hormone is called a target organ or tissue.

What is hormone?

It is a chemical agent which is released from one group of cells and travel via the blood stream to affect one or more different group of cells. Most of the hormones are proteins, derivation of amino acids or steroids and a few are fatty acid derivatives. Hormones secreted by the pancreas are proteins; those secreted by thyroid glands are amines, steroids are secreted by the ovaries, testes and adrenal glands. The effect of hormone is much slower. Many of the changes they control are rather long term in nature. Hormonal response is of limited duration and does persist indefinitely.

Hormones are effective in extremely small amounts. Only a few molecules of a hormone may be enough to produce a dramatic response in a target cell. In a target cell, hormones help to control biochemical reactions in three ways:

1. A hormone can increase the rate at which other substances enter or leave the cell.
2. It can stimulate a target cell to synthesize enzymes, proteins or other substances.
3. It can prompt a target cell to activate or suppress existing cellular enzymes. As in the case of enzymes, hormones are not changed by the reaction they regulate.

The secretory rate of the endocrine glands is influenced by the hormones produced by other members of the endocrine system and in certain cases by nervous impulses. Hormones may control the activities of another organ- the target organ. The production of hormones and their respective actions also depend upon the state of the organism and the environmental conditions. The endocrine organ not only affects the target organ but in turn it is also influenced by changes in the state of the latter. This phenomenon is described as feedback mechanism and is important in endocrine control of the body. There are certain hormones which have generalized or widespread effects and some other hormones are there which do not have any direct activity on the organ but simply permit the activity of another hormone. However the hormones control and maintain various metabolic activities and constitute a device for regulating such processes as growth, sex function, reproduction, metabolism and similar processes, thus coordinating activities in the animal.

Endocrine Glands

All endocrine glands no doubt, function independently but are independent also. Over activity or under activity of any of them affect all the systems. The endocrine glands are profusely supplied with blood vessels and their secretions control and coordinate the complex processes of metabolism, growth, reproduction and maintain homeostasis. In human beings, the endocrine system consists of endocrine glands, which have been grouped into four categories:

1. Pure endocrine glands are Pituitary, Thyroid, Parathyroid, Adrenal, Thymus and Pineal gland.
2. Exocrine glands containing endocrine tissues are a pair of ovaries, a pair of testis and pancreas.
3. Local endocrine tissues are many. Most of them are located in the gastro-intestinal tract and Gastric mucosa.
4. Temporary endocrine structures are Placenta, Graafian follicle and Corpus luteum.

Besides, a nervous structure called hypothalamus of the brain is integrated with the endocrine system and also secretes hormones.

The Pituitary gland

The human pituitary is a pea or nut shaped structure lies at the floor of the brain just behind the optic chiasma. It is attached to the hypothalamus part of the brain by a slender stalk. It is an outgrowth from the base of the forebrain and lodged in a depression. The pituitary gland is 0.5 gm in weight and 10 mm in diameter. It is popularly called as master gland of endocrine orchestra because it produces a number of different hormones, the majority of which regulates the activities of other endocrine glands in great extent and also regulates the metabolism and growth. The pituitary also influences the working of all other endocrine glands and in turn is influenced by them. The gland is also known as hypophysis.

The pituitary synthesizes six different hormones which are mentioned below:

1. Growth hormone (GH)
2. Adreno-corticotrophic hormone (ACTH)
3. Thyroid stimulating hormones (TSH)
4. Gonadotrophic hormones (GTH)
5. Lactogenic hormone
6. Melanocyte stimulating hormone (MSH)

All of these hormones are mostly polypeptides and all but two are true trophic hormones i.e. hormones whose primary target is another endocrine gland. The two non-trophic hormones are growth hormones and prolactin. The six hormones listed above are of two categories. The first category is concerned with growth and metabolism and the other category control the functions of thyroid, gonad and adrenals.

Growth hormone - Growth hormone does not influence a specific target tissue, rather it affects all parts of the body that are concerned with growth. It directly induces the cell division necessary for growth and protein synthesis. It also helps in promoting protein anabolism, conversion of glycogen to glucose and absorption of calcium from the intestine etc.

The over secretion of growth hormone causes gigantism during child hood because of excessive growth of the skeleton, boy of 18 years attaining the height of 8’3”  in the record. But in adult, this results a condition called acromegaly where bone increases in thickness, excessive growth of bones of face takes place. The hand and feet become large and spade like. There is also thickening of skin on face and hands.

Due to deficiency or under secretion of this hormone, there is stunted growth of the skeleton in the children; as a result they become dwarf. There are two types of dwarfs: (a) Lorain type where a dwarf individual is having well proportional body and physically and mentally normal. (b) Frohlick’s type where growth is stunted along with considerable obesity, mental deficiency and retarded sexual development. But in adult, the deficiency of this hormone causes early senility, the skin becomes dry and wrinkled, the hair becomes grey and sparse, degeneration of sexual organs with the stoppage of menstruation in female. This condition is known as Simmond’s disease.

Thyroid Stimulating hormone – This hormone controls the development, growth and activity of thyroid gland and stimulates the synthesis and secretion of thyroxine which is the main thyroid hormone.

Adreno-corticotrophic hormone – This hormone promotes and maintains normal growth and development of adrenal gland. It stimulates the adrenal gland to produce and secrete steroid hormone called glucocorticoids. Production of ACTH is important particularly during emotional and physical stress such as starvation, cold, burns and hemorrhage.

Gonadotrophic hormones – These hormones stimulate development and function of the gonads e.g. testes and ovaries. Two gonadotrophins are produced by the adenohypophysis namely: Follicle stimulating hormone (FSH) and Luteinizing hormone (LH).

Follicle stimulating hormone stimulates target organs like ovaries in female and testes in male. In the female, FSH stimulates the Graafian follicle cells in the ovaries to develop into mature eggs and also stimulates them to produce oestrogen. In the male FSH stimulates the development of seminiferous tubules and the cells of testes to produce sperms by the process of spermatogenesis.

Luteinizing hormone stimulates maturation of ovarian follicle, ovulation and formation of corpus luteum in the ovary. Corpus luteum secrets a hormone called progesterone. In the male, it stimulates interstitial cells of the testis to secrete the male sex hormone, testosterone into blood stream.

Lactogenic hormone – This hormone helps for the growth and development of breasts or mammary glands and milk production in females. It has direct effect on the breasts immediately after the delivery of a baby and expulsion of placenta. It stimulates milk secretion in the breasts. This hormone induces strong emotional attachment to the young. In male, it promotes protective attitude towards mate and offspring and expression of paternalism in general.

Melanocyte stimulating hormone – In man, it does not seem to play great role in the behaviour of melanocyte. The secretion causes darkening of skin. In human being, under certain conditions such as pregnancy, an increase in its secretion causes darkening of the skin. Normally large numbers of melanocytes are present in the human skin forming moles, freckles etc.

The Thyroid gland

The thyroid gland is located in the neck, and in the ventral and upper end of trachea. It is a bilobed gland, the two lobes of which are joined together by a narrow bridge called isthumus. This gives the gland more or less the shape of ‘H’. Thyroid gland I richly supplied with the blood capillaries. It secretes a protein hormone called Thyroxine which is rich in iodine. It also produces another hormone called calcitonin which regulates calcium level of the blood.

Function of Thyroxine – The thyroid hormone affects the general metabolism of the body such as:

-         Increase the oxygen utilization of most of the tissues and the rate of energy production, thus enhancing in the basal metabolic rate (BMR).

-         It is highly essential for normal growth and maturation.

-         It is essential for the normal activity of nerves and nervous system and also for maintaining the normal excitability of nerve fibers.

-         It helps in the maintenance of healthy skin and hair.

-         It plays important part in normal reproductive function.

-         It also plays important role in protein, fat and carbohydrate metabolism.

Abnormalities – Deficiency of thyroxine in children causes cretinism in which growth stops, as a result deformed bones, stunted body growth, mental deficiency, pot bellied abdomen are seen. Sexual organs and secondary sexual characters do not develop. Deficiency in adults causes myxoedema which is characterized by the lack of intelligence and alertness, slow heart rate, thick skin, fall of hairs, lower metabolism, increase in weight, anemia, muscular weakness, disturbances in sexual function, mental laziness etc. Under secretion also causes simple goiter, which results in swelling of the neck.

The Parathyroids

In man, there are four parathyroid glands which are slightly flattened oval bodies and are located on the dorsal side of the thyroid gland. The glands are reddish or yellowish brown in color. These glands are concerned with the secretion of hormone called parathormone or parathyroid hormone. The functions of these glands are:

-         Plays an important role in calcium and phosphate metabolism. It is done by its action on bone and kidney.

-         It increases the serum calcium level and it decreases the serum phosphorus level.

-         It increases the urinary excretion of calcium.

-         It also increases the rate of absorption of calcium from the intestine.

If this hormone is a deficient, the level of calcium in the blood is reduced. As a result, muscles go into a type a spasm known as tetany. Under hyper secretion there is an excessive destruction of bone tissue called osteoporosis and calcium is not properly utilized. This results in softening of bones and a high level of blood calcium.

The Adrenals

They are also called super renal glands as they are found attached to the kidneys interiorly and located in the abdomen. These are two elongated cap like structures. The functions of Aldosterone are as follows:

-         It promotes sodium reabsorption in the kidneys and thus water reabsorption. Hence it plays a major role in maintaining the homeostasis of extra cellular fluid.

-         It reduces the reabsorption of potassium in the renal tubules. This results into increased potassium excretion in urine leading to fall of serum potassium level.

-         It helps is maintaining concentration of solutes in the extra cellular fluid when either in food intake or metabolic activity change the amount of solutes entering the blood stream.

Normally the weak sex hormones secreted by the adrenal cortex have only a slight effect on the gonads of both the male and female.

Abnormalities - Chronic cortical hypo function in man causes Addison’s disease which results in muscular weakness, nervous weakness, lowering of blood pressure, bronzing of skin and ultimate death. The hyper adrenocorticism results in excessive fat deposition on the face, back of neck and abdomen, muscular weakness, development of female sex characteristics in male, tendency to develop male characteristics in female like excessive growth of hair on the chest and pubic region, menstrual flow stops and gradual atrophy of breasts.

Adrenal medulla – The adrenal medulla is under neural control and secretes two hormones- the adrenaline and non-adrenaline. Both hormones are biogenic amines and help to control blood circulation and carbohydrate metabolism. Adrenaline regulates blood pressure and sympathetic nervous system. It controls the contraction of involuntary muscles of lung, heart etc.

During the times of excitement, emergencies or stress, the adrenal medulla contributes to the overall mobilization of the body through sympathetic nervous system. In response to excess adrenaline and non- adrenaline, the heart rate increases, blood flow increases to many vital organs, the air ways in the lung dilate and more oxygen is delivered to all the cells of the body. This group of events is sometimes called the “fight or flight” response and permits the body to react strongly and quickly to emergencies. The adrenal gland has been named as emergency gland or glands of 3F concerned with Fright, Fight and Flight.

The Pancreas

The pancreas is an elongated and diffused gland located posterior to stomach. It is a mixed gland because it functions both as an exocrine gland to secrete digestive enzymes and as an endocrine gland. The endocrine portion of the pancreas makes up only about 1% of the gland. Two types of hormones are secreted namely insulin and glucagons.

Insulin: It is a blood sugar lowering hormone and the functions are:

-         It increases the permeability of glucose through body cell membranes.

-         It increases the conversion of glucose to glycogen both in liver and muscles.

-         It prevents production of glucose from proteins and fats.

-         It accelerates the oxidation of glucose in muscles and elsewhere.

-         It also stimulates protein synthesis and growth. The secretion of insulin is mainly controlled by blood sugar level.

If insulin secretion is low or deficient it causes the disease called diabetes mellitus. This is a disorder of carbohydrate metabolism and characterized by high blood sugar level, sugar in urine, weakness, recurrent infections etc. This also causes:

1. Hyperglycaemia – It is a condition in which blood sugar level increases above normal level (80 to 120 mg/100 ml blood).

2. Glycosuria – Presence of glucose in urine in large quantities. It occurs when blood sugar level exceeds the renal threshold which is 180 mg.

3. Polyuria - A disease in which the patient urinates large volumes of urine frequently. This is due to high glucose level in the nephric filtrate increasing osmotic pressure sharply reducing reabsorption of waterback to the blood.

Glucagon: It accelerates the process of gluconeogenesis. It also promotes the process of glycogenolysis by virtue of which glycogen is broke down to glucose in the liver. The secretion of glucagon from the pancreas is regulated by the blood sugar level. When blood sugar level is high, less glucagon is secreted. On the other hand, when blood sugar level is low, more glucagon is secreted.

The Gonads

The testes in male and ovaries in female, besides their primary function to produce sperms and ova respectively, also secrete steroid hormones which help to regulate reproductive functions.

Testis: In the male a pair of testes lies in the scrotum. They secrete male hormone called androgens. The principal hormone is testosterone. It functions in following ways:

-         It influences spermatogenesis within seminiferous tubules.

-         It is an important hormone for initiation and development of secondary sex characters such as development of hair on the face, upper lip, in the axillae and pubic region, deepening of voice, greater skeletal and muscular growth and masculine behaviour.

-         It has got a profound influence on growth and function of accessory organ of male reproductive system.

-         It exerts a feed back inhibitory effect on pituitary LH secretion.

-         This hormone promotes protein anabolism and therefore causes nitrogen retention and body growth.

Ovary: In the female a pair of ovaries is situated in the pelvic region of abdominal cavity. This gland has an endocrine function also. It secretes two main types of female hormones namely oestrogen and progesterone. The oestrogen is secreted by theca interna cells of graafian follicles and progesterone is secreted by corpus luteum which is formed in the ovary from the ruptured follicle after ovulation.

Function of oestrogen: This is released under the influence of FSH of anterior pituitary. With the onset of puberty in a girl, the secretion of this hormone increases. The main oestrogenic hormone in blood circulation is estradiol, secreted from theca interna cells of graafian follicile. Its functions are as follows:

-         It is feminizing hormone which initiates puberty changes and induces appearances of secondary sexual characters such as: development of breasts, deposition of fat in buttocks, enlargement of sex organs and growth of hairs around them.

-         It prepares the uterine mucosa for the later action of progesterone.

-         It thickens the vaginal mucous membrane and makes it cornified.

-         It causes deposition of increased amount of subcutaneous fat in particular regions to make a typical feminine body.

-         The genital organs like vagina, uterus, fallopian tubes etc. increase in size and volume.

-         It influences the behaviour and psychic patterns in females.

-         It suppresses the production of pituitary FSH.

Function of Progesterone: This hormone is secreted from corpus luteum which is a temporary endocrine structure.

-         It causes inner layer of uterus considerably thick and vascular for implementation of embryo.

-         It influences the alveolar growth of mammary glands.

-         This hormone inhibits oestrus, ovulation, and the production of pituitary LH.

-         Maintains the foetus by forming placenta and inhibit uterine muscles to contract during pregnancy.

The Thymus

The thymus gland is situated near the heart and in the upper part of thorax. It is in the form of a small lobes lying in either side of the lower end of trachea. It is large and prominent in young but diminishes in size throughout adulthood. It is partly an endocrine gland and partly a lymphoid structure. Its functions are:

-         It secretes a major hormonal product i.e. polypeptide hormone thymin which appears to be essential for the normal development of immune system.

-         It also depresses neuromuscular transmission.

-         In young children, it plays a major role forming a basis for the development of antibodies, thus increasing immunity for various diseases.

Gastro-Intestinal Hormones

The secretory and motor activities of gastro-intestinal tract are controlled by autonomic nervous system and hormones. These hormones are secreted by the local areas of alimentary tract and are called local hormones. This function of gut is under endocrine control. All gastro-intestinal hormones are polypeptides in nature. The release of hormones is regulated by the presence or absence of food in the lumen of gut.

Other Endocrine Activities

The hormone erythropoietin is formed by kidney which acts on hamemopoietic tissues stimulating the formation of blood cells. This hormone stimulates bone marrow to increase the production of red blood corpuscles.

The hormone atriopeptin is secreted by the heart which acts on blood vessels and kidneys. It lowers the blood pressure and maintains fluid balance.