CHARACTERISTICS OF HORMONES

CHARACTERISTICS OF HORMONES

Coordination in animals is brought about by the secretions of endocrine glands. Endocrine glands are ductless glands which secrete the chemical substances called hormones directly into the blood. Any chemical substance which is formed in the tissues of endocrine glands and are carried by the blood to other parts of the body for its specific actions is termed as hormone. An organ which responds to such a hormone is known as target organ.

• They are the secretions of endocrine glands.
• They are produced at a place and act on target organs which are mainly away from their source.
• They are poured directly into the blood stream.
• They are required in very small quantities.
• They are specific in function.
• Chemically they are mainly proteins. Some of them may be amino acids, steroids etc.
• They are harmful if present in less or excess amounts.
• Hormones are immediately destroyed after their action is over.

FEEDBACK MECHANISM:

Endocrine glands interact with each other, so that secretion of one gland may stimulate or depress the activity of another. The amount of hormone released by an endocrine gland is determined by the body's need at any given time e.g. The hypophysis produces a hormone that stimulates the thyroid to produce its hormone; in turn the thyroid secretion induces the hypophysis to produce less thyroid stimulating hormone. This is known as the `negative feedback'. This feed-back helps bring about a steady state in the body which is called as homeostasis.

VARIOUS ENDOCRINE GLANDS PRESENT IN THE HUMAN BODY ARE:

• Pituitary gland (or Hypophysis)
• Pineal gland
• Thyroid gland
• Parathyroid gland
• Thymus gland
• Pancreas
• Adrenal gland
• Ovaries
• Testes

Various endocrine glands of human being

PITUITARY GLAND (OR HYPOPHYSIS):

It is a small ovoid structure attached to the base of brain (hypothalamus) by a short stalk called infundibulum. Pituitary gland is also known as the master gland as it controls other endocrine glands. This gland consists of three lobes-anterior, middle and posterior.

Hormones of Anterior Pituitary:

• Growth hormone (GH): This hormone promotes and regulates the process of growth in the body. It's deficiency during childhood leads to dwarfism and over secretion leads to gigantism.
• Adrenocorticotrophic hormone (ACTH): This regulates the activity of adrenal cortex. It mainly stimulates the adrenal cortex to secrete corticosteroid hormone which defends the human body under stress conditions.
• Thyroid stimulating hormone (TSH): It stimulates thyroid gland to secrete thyroxine.
• Follicle stimulating hormone (FSH): It stimulates the production of gametes, and its equivalent in the males stimulates sperm production.
• Luteinising hormone (LTH): It causes ovulation and formation of corpus luteum.
• Luteotrophic hormone (LTH): This is also known as prolactin. This hormone stimulates growth of mammary glands during pregnancy and promotes lactation after delivery. Prolactin level rises during pregnancy and is very high during lactation.

Hormones of middle Pituitary:

• Melanocyte stimulating hormone (MSH): This is the only hormone secreted by middle pituitary which controls the growth and development of melanocytes responsible for skin colour.

Hormones of Posterior Pituitary:

• Vasopressin or Antidiuretic hormone (ADH): This causes the reabsorption of water into the blood from the collecting tubules of the kidneys, thereby concentrating the urine and reducing its volume.
• Oxytocin: This hormone stimulates uterus contractions at the time of child birth and causes release of milk from mammary glands. It is also known as birth hormone or milk ejecting hormone.

PINEAL GLAND:

It is a small gland reddish-grey in colour, about the size of a pea, attached to the roof of the third ventricle of the brain. It contributes in regulating gonadal development.

THYROID GLAND:

The thyroid gland consists of two lobes joined together by an isthmus. !t is situated in the lower part of the neck and when enlarged it forms goitre. Two hormones secreted by the thyroid gland are:

• Thyroxine: It is the principal hormone secreted by the thyroid gland and its main role is to increase the metabolic rate of the organs and tissues of the whole body. 60% of thyroxine consists of iodine an element which is essential to the gland to enable it to synthesize its hormone. The basal metabolic rate (B.M.R.) is increased in hyperthyroidism and reduced in hypothyroidism.

• Calcitonin: This hormone lowers the calcium level.

• Hypothyroidism: This results from lack or deficiency of thyroid hormone secretion. Cretinism affects children and is due to congenital defect of either absence or defect of the gland. In this disease growth is stunted, the features are coarse, frequently the child has a protruding tongue and an enlarged abdomen; the mentality of the child is low and retarded. Myxoedema is the condition caused by thyroid deficiency in adults. It affects women more frequently than men. It is characterized by puffy face, thick skin, dry cough, cold and loss of hair. BMR is lowered, Iodine deficiency causes simple goitre.
• Hyperthyroidism: This results from excessive secretion and over action of thyroid hormones. An excessive amount of thyroxine is poured into the blood and the metabolism of the body is speeded up. These toxic signs and symptoms are responsible for the condition being known as toxic goitre. Other names are thyrotoxicosis, exophthalmic goitre and grave's disease.

PARATHYROID GLAND:

These are small ovoid glands, smaller than the size of pea. They lie on the posterior surface to the thyroid gland. Usually there are two pairs of parathyroid glands, a superior pair and an inferior pair. The parathyroid secretion, parathormone has two main functions:

• It regulates the balance between the calcium in bones and in extracellular tissue fluid.
• It also controls the excretion of phosphates in the urine.

THYMUS GLAND:

This gland is situated in the thorax in midline under the sternum in front of trachea. It has 2 lobes which are further divided into many lobules. It secretes a hormone namely thymosin. It is one of the sites of lymphocyte formation in children. It helps in producing antibodies.

ADRENAL GLANDS:

These are two small semilunar structures lying one each on upper pole of the kidneys. That is why they are also known as supra renal glands. Each gland consists of two separate parts known as cortex and medulla. The cortex occupies outer peripheral portion which is yellowish in colour and medulla is inner brownish part.

Cortex secretes three different kinds of hormones known as corticosteroids. They are:

• Mineralocorticoids: These regulate sodium and potassium balance in the body.
• Glucocorticoids: They influence on carbohydrate metabolism.
• Sex hormones: Small quantities of sex hormones androgens, oestrogen are produced by adrenal glands.

Adrenal medulla is important in raising defence mechanisms and supplementing sympathetic actions in the body.

It secretes two hormones:

• Adrenaline: It is a stress hormone leading to increase in systolic blood pressure, dilation of coronary blood vessels, increased sweating and increase in metabolic rate.
• Noradrenaline: It is a general vasoconstrictor. Both systolic and diastolic pressures are increased. Both these hormones help to pass through emergency conditions. Thus are called as "fight or flight hormones".

PANCREAS:

Pancreas is the only heterocrine gland in the human body. It acts as exocrine as well as endocrine gland. It acts as exocrine as it secretes pancreatic juice. The endocrine tissue of the pancreas is in the form of clumps of secretory cells known as the islets of Langerhans. The islet cells are of three types-alpha, beta and delta.

• Insulin is secreted by the beta cells. Insulin is required to convert glucose to glycogen (glycogenesis) and store it in liver. Deficiency of insulin due to defect in Islets of Langerhans results in diabetes mellitus, a condition in which blood glucose is high and is passed in the urine.
• The alpha cells of pancreas secrete glucagon, the metabolic effects of which are opposite to those of insulin. It causes the breakdown of liver glycogen, thereby releasing glucose into the blood stream.
• The third hormone somatostatin is secreted by the delta cells of the Islets of Langerhans. It is able to inhibit the secretion of many hormones, as it inhibits the release of insulin and glucagon.

OVARIES:

Ovaries secrete three hormones namely:

• Oestrogen: FSH from the anterior pituitary controls the secretion of oestrogen. This hormone effects the development of female secondary sex characters.

• Progesterone: It is secreted by corpus luteum. This hormone is secreted only after ovulation. Progesterone prepares the uterus for receiving the embryo. It prepares inner lining of the uterus, endometrium to receive the implanting embryo for about a week. This hormone is essential for the maintenance of pregnancy and is therefore called pregnancy hormone.

• Relaxin: This hormone is secreted during later stages of pregnancy and leads to relaxation of muscles of the pelvic area so to enable easy child birth and no pressure on the foetus.

TESTES:

Testosterone is the main testicular hormone secreted by interstitial cells of the testis. It is mainly concerned with the development and maintenance of male sex characters and enhancing the process of spermatogenesis.