Working alone or in concert as a single neuroendocrine system, they perform the same general functions. The endocrine glands secrete very minute quantities of chemical messengers called hormones (Gk. hormoein : to excite).
The hormones are released directly into the bloodstream. There are no ducts in endocrine glands, hence, called ductless glands.
Most their widespread despite distribution by the blood, are highly specific in their action. Cells that respond to a particular hormone are called target organ cells.
The science concerned with the structure and functions of endocrine glands, diagnosis and treatment of disorders of endocrine glands is known as endocrinology.
CHEMICAL NATURE OF HORMONES
Hormones are organic compounds of diverse chemical nature. Chemically, hormones may be proteinaceous (FSH, LH, TSH, etc.), peptides (ACTH, oxytocin, vasopressin, etc), steroids (sex hormones) or amino acid derivatives (thyroxine, epinephrine, etc.).
CHARACTERISTICS OF HORMONES
These are listed below:
- Hormones are the secretions of endocrine glands. They are transported by way of blood.
- They always act away from the site of their secretion. Their action is always at distant place called target organ.
- They are secreted and required in very small quantities.
- They are organic compounds of diverse nature and are soluble in water.
- They may accelerate or inhibit specific physiological processes.
- They are usually used up in their regulatory actions.
- Specific action : Hormones are very specific in their action and they do not initiate chemical reactions.
- Slow in action : The hormones are generally slow in action.
- Feedback inhibition : Hormones usually work on the principle of feedback inhibition i.e. the secretion of a hormone is stimulated or inhibited by the end-product of its activity.
- Prepare the body to cope with emergency: Hormones prepare the body to cope with the emergency conditions, such as, infections, trauma, fear, emotional stress, sexual desire, etc.
- They are harmful both when secreted in excess (hypersecretion) or deficiency (hyposecretion)
- The hormones are not stored in the body. They are destroyed and excreted immediately after their action.
Functions of hormones
- Control and regulate metabolic activities.
- Maintain homoeostasis.
- Control and regulate morphogenic activities, e.g., growth, development, differentiation.
- Regulate mental activities.
- Control growth, maturation and regeneration.
- Produce secondary sexual characters and regulate reproductive activities.
- Control the activities of other endocrine glands.
- Cause adaptations to external stimuli.
ENDOCRINE GLANDS IN HUMANS
In human, several endocrine glands are present. Some of them are purely endocrine in function and others are partly endocrine glands.
The purely endocrine glands (Holocrine) are pituitary, thyroid, parathyroid, adrenal, pineal and thymus. Their sole function is to produce hormones.
Whereas partly endocrine glands (Heterocrine) in addition to secretion of hormones also perform some other functions. These include pancreas, gonads (testes and ovaries), lining of alimentary canal, kidneys, placenta, etc.
Various endocrine glands and their location in man. These endocrine glands are given below. The syllabus includes only the first four ones (given in bold). Hence, the other four are not discussed.
PITUITARY GLAND [Associated with Brain]
The pituitary is a small, red-grey, pea- shaped gland. It hangs from the base of the midbrain below hypothalamus. The pituitary consists of 3 parts :
(a) Anterior pituitary
(b) Intermediate lobe
(c) Posterior pituitary
(a) Anterior pituitary
It constitutes about 75% of the total weight of the gland. Anterior pituitary secretes 6 hormones- GH, LTH, TSH, ACTH, FSH, LH or ICSH, out of which except GH all others are tropic hormones. Tropic hormones are those which stimulate certain other endocrine glands to secrete their secretions.
- Growth hormone or Somatotrophic hormone (GH or STH) : It stimulates the growth of bones and soft tissues of the body.
- Prolactin (LTH) or Luteotrophic hormone : It stimulates the mammary glands to secrete milk immediately after the birth of a baby. Moreover, it also helps to maintain corpus luteum of ovary after ovulation, hence, called luteotrophic hormone.
- Thyroid Stimulating hormone (TSH) : It stimulates thyroid gland to secrete hormone thyroxine.
- Adreno Cortico Trophic hormone (ACTH) : It promotes and maintains growth and development of adrenal cortex and stimulates it to secrete its hormones.
- Follicle Stimulating hormone (FSH) : It stimulates ovarian follicles to start growth and to continue till the point It also stimulates the of ovulation. follicle cells to secrete hormone estrogen. In males FSH helps in the development of seminiferous tubules and spermatogenesis.
- Luteinizing hormone (LH) : LH helps in the process of ovulation from the Graafian follicle and formation of corpus luteum in the ruptured follicle. It also stimulates corpus luteum to secrete hormone progesterone and estrogen. In males, it is called ICSH (interstitial cell stimulating hormone). It stimulates interstitial cells in the testes to develop and secrete testosterone (male hormone).
(b) Intermediate lobe
It is a small middle part of pituitary gland. It secretes hormone MSH (melanocyte stimulating hormone). This hormone controls the distribution of pigments in the skin.
(c) Posterior pituitary
Posterior lobe of pituitary serves as a storage area for the two hormones – ADH or antidiuretic hormone or vasopressin and oxytocin. These two hormones are secreted by the hypothalamus but released by the posterior pituitary.
Antidiuretic hormone (ADH) : It prevents the formation of large amount of urine and thus decreases the water loss from the body (conservation of body water). It can also raise the blood pressure by bringing about constriction of the blood vessels (vasoconstriction) in several tissues.
Deficiency of ADH causes diabetes insipidus (water diabetes). In this urination is frequent, resulting in loss of water from the body making a person become thirsty.
Oxytocin : It has two actions. It stimulates powerful contractions by the pregnant uterus during childbirth. The term oxytocin itself means swift childbirth. (Greek oxys-swift, tokos-childbirth).
It also causes milk ejection from lactating breast. Under its influence, alveoli release the milk into the ducts of breast.
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THYROID GLAND [Associated with Pharynx (neck)]
It is the largest endocrine gland (5 x 3 cm) weighing about 25 g. It lies on the ventral and lateral sides of trachea just below the larynx in the neck.
It is brownish-red, butterfly shaped, bilobed gland, one lobe present on either side of the trachea. The two lobes are interconnected by a transverse glandular band called isthmus.
Histologically, it consists of about 3 million small oval or rounded thyroid follicles.
ADRENAL GLANDS [Associated with Kidneys]
Adrenal glands are located above the kidneys, fitting like a cap over these organs. The outer portion of the gland is called cortex, the inner substance, the medulla. These two parts function as separate endocrine glands and secrete different hormones.
Hormones of adrenal cortex
Adrenal cortex secretes many hormones, all of them are steroid in nature. They can be placed into 3 broad classes as follows :
(a) Mineralocorticoids : As the name indicates, these hormones regulate mineral metabolism; mainly sodium and potassium. Aldosterone is the hormone of this group and it increases reabsorption of sodium in the renal tubules. It helps in raising the sodium chloride content of blood.
(b) Glucocorticoids : They are concerned with carbohydrate, protein and fat metabolism. They are under the control of ACTH from adrenohypophysis. The main hormones of this group are cortisol and corticosterone. They have following biological action:
- Effect on carbohydrate metabolism: Glucocorticoids raise the blood sugar level. The liver cells form glucose from non-carbohydrate source i.e. proteins and fats. This process is known as gluconeogenesis.
- Effect on protein and fat metabolism: Glucocorticoids decrease the rate of protein synthesis. They also stimulate the formation of fat droplets from fats.
- Effect on lymph and blood : Glucocorticoids cause the decrease in the number of eosinophils and lymphocytes, thereby reducing the antibody formation.
- They prepare the body to cope with stress situation, thus they are anti-stress hormones.
(c) Sex corticoids : They stimulate the development of external sex characters in males. Adrenal androgen is the hormone of this group.
Hormones of adrenal medulla
The adrenal medulla secretes catechola- mines or emergency hormones. Adrenalin (epinephrine) and noradrenalin (norepine- phrine) are the two hormones of this group. Both adrenalin and noradrenalin act on organs and tissues supplied by sympathetic nerve fibres of the autonomic nervous system.
They produce most of the effects like those of sympathetic stimulation. They mainly control the contraction of involuntary muscles including the muscles of heart and arteries.
In man, adrenalin is secreted much more than noradrenalin. At rest adrenal medulla secretes a very small amount of hormones. But, a sharp increase in the rate of release of these hormones occurs during emergencies (fight or flight reactions).
For example, if there is a danger for life of an organism, it can react to this by running away (flight) for safety or by giving a tough fight to the enemy (fight reaction).
In general adrenal medulla and sympathetic nervous system function as closely integrated system called sympatheticoadrenal system. It shows a close co-ordination between nerves and hormones.
Adrenalin is also known as emergency hormone. It is secreted in large amount at the time of emergency. It helps to face physical stress like fall in blood pressure or blood sugar, muscular exertions (like dance and running), pain, cold, injury and emotional stress like anger, fear and grief.
PANCREAS [Associated with Liver]
Pancreas is both an exocrine as well as an endocrine gland. It is located just below the stomach in the loop of duodenum of small intestine. It is a flattened, almost triangular structure.
It contains two types of secretory structures i.e. acini (exocrine part) and islet of Langerhans (endocrine part). The islet of Langerhans are present in the connective tissue of the pancreas. Three types of cells are differentiated in the islet of Langerhans :
(a) Alpha cells (a-cells) : These cells secrete hormone glucagon. Glucagon converts glycogen to glucose whenever glucose level of the blood decreases. Low glucose level in the blood, exercise and protein-rich diet stimulates its formation.
(b) Beta cells (B-cells) : These cells secrete hormone insulin that converts glucose to glycogen (glycogenesis) in the liver and muscle cells. It helps to maintain the blood glucose level constant.
(c) Delta cells (8-cells) : These secrete hormone somatostatin which is a growth hormone-inhibiting hormone (GH-IH). It inhibits the secretion of both glucagon and insulin.