Endocrinology

Last Updated on Saturday, 11 December 2010 06:12 Written by Sandesh Monday, 5 April 2010 07:07

http://sandeshmalla.com.np/life-science/endocrinology

E-mail: info@sandeshmalla.com.np

Types of glands:

Group of cells specialized in structure and function to produce chemical substances needed in metabolic processes of the body are known as glands. All animals, invertebrates and vertebrates, have two kinds of glands in their body-exocrine and endocrine gland.

Exocrine glands: These glands discharge their products through duct, thus they are the glands of external secretion or exocrine glands. These glands may discharge their secretions into an internal organ. For example: ducts of salivary gland and liver carry their secretions to parts  of alimentary canal, sweat and mammary glands liberate their products to exterior through openings on the external surface.

Endocrine glands: Endocrine means internally secreting. The glands which do not have any duct are called glands of internal secretion and their secretions are known as internal secretions, endocrines, or more commonly the hormones. Hormones are released directly into the blood capillary and then transported by the blood stream to other parts of body where they coordinate their metabolic activities. The function of endocrine system is chemical coordination.

Some glands such as pancreas and gonads are both endocrine and exocrine in function. Exocrine part of pancreas secretes enzymes which are discharged through the pancreatic duct into duodenum, whereas the endocrine part (Islet of Langerhans), of pancreas secretes hormones which diffuse directly into the blood stream.

The study of endocrine glands and the hormones secreted by them is known as Endocrinology. It is concerned with the study of the biosynthesis, storage, chemistry, and physiological function of hormones and with the cells of the endocrine glands and tissues that secrete them. The study of hormones only is known as hormonology.

Hormones:

Definition: Secretion of endocrine glands is commonly known as hormones. The term hormone. (Gr., hormone= arouse) was coined by the British physiologist E.H. Starling in 1905. Hormones are molecules that act as signals from one type of cells to another. They regulate the function of a specific tissue or organ.

Endocrine Glands:

1. HYPOTHALAMUS:

2. PITUITARY GLAND:

3. THYROID GLAND:

Location: It is a bow tie shaped structure found in the neck.

Structure: It has two lobes on each side of the trachea and larynx connected by thin band of tissue.

Histology: It is made up of hundreds of thousands of tiny follicles having diameter of 0.1mm. each follicle is a hollow sphere whose wall  consists of a single layer of cuboidal epithelium. The epithelial cells secrete thyroxine. Thyroid hormone is present in two forms – tri-iodothyronine( T3) and tetra-iodothyronine(T4). T3 is active form of thyroid hormone present in less amount in blood while T4 is less active hormone present in large amount. Parafollicular cells or ‘C’ cells are present between follicles which secrete a hormone known as Calcitonin.

Functions of Thyroid hormones:

• T3 and T4 hormone controls basal metabolic rate (BMR). BMR is the rate at which oxygen and food are used to release energy and is directly related to rate of cell respiration.
• It promotes breakdown of glucose and fat to provide energy
• Helps in protein synthesis
• It also stimulates brain development
• It promotes growth and maturation.

Calcitonin: it regulates metabolism of calcium and phosphate.

Hupothyroidism:

1. Cretinism: it occurs in children. It leads to poor mental and physical growth in child resulting in physical and mantal retardation.
2. Myxoedema: Hyposecretion in adulthood. It decreases in metabolic rate, fatigue, lethargy, weight gain, cold intolerance, dry and flaky skin, constipation, slowed thinking etc.
3. Goitre: It is the enlargement of thyroid gland which causes swelling of neck. Deficiency of iodine causes the insufficient secretion of thyroxine hormone.

Hyperthyroidism:

It is due to excess secretion of thyroid hormones. It causes increased metabolic rate, heat intolerance, excessive sweating, weight loss, irregular heartbeat, exopthalmus or protruding eyeballs.

4. THYMUS GLAND: Thymus gland is located in upper chest. It secretes thymosin hormone. It helps in conversion of WBC, lymphocytes and T- cells. It plays important role in cell mediated immunity.

5. PINEAL GLAND: It is also called as pineal body. It is a small cone shaped structure located in brain. Extending posterior from 3^rd ventricles. it has specialized secretory cells called pinealocytes which synthesize hormone – melatonin. It is responsible for circadian rhythm – sleep-wake cycle and light – dark cycle.

6. PARATHYROID GLAND:

In human there are four small parathyroid glands embedded in the thyroid gland. They produce only one hormone called  Parathormone(PTH) which are peptide composed of 84 aminoacids. Parathormone and the thyroid hormone “calcitonin’ work antagonistically to regulate plasma, Ca ⁺² and PO₄ ion levels. Release of PTH is controlled by plasma Ca⁺² levels. When the Ca⁺² level  lowers  in plasma, PTH is released .

Functions: Release of parathormone increase the plasma calcium level to its normal level and decrease plasma phosphate level.

Hypo-parathyroidism: It causes tetany in which muscles remain contracted.

Hyper-parathyroidism: It causes bone deformation and pathological fractures.

7. ADRENAL GLAND:

Location: A pair of adrenal glands located one just above each kidney.

Structure: Each gland consists of two types of cells from different origins. Outer region is called cortex which forms 80% of the gland while inner region is called medulla.

A. Adrenal Cortex: It is absolutely essential for life. It is made up of three ill-defined zones-

1. outer zona  glomerulosa
2. Middle zona fasiculata
3. Inner zona reticularis

Adrenal cortex is under the control of ACTH. Hormone of cortex is responsible for the maintenance of constant body temperature, blood pressure and normal sexual behavior. 28 hormones have been isolated from adrenal cortex all of which are steroid hormones. The hormones fall into following three categories:

1. Mineralocorticoids( Eg. Aldosterone): It is secreted by zona glomerulosa of cortex. It controls water and salt content of the body by stimulating cation pumps in membranes to conserve Na+ and Cl- and to remove K+ ion.
2. Glucocorticoids( eg. Cortisol): It is secreted by zona fasiculata. It controls the metabolism of food and suppresses inflammation. It also helps in carbohydrate metabolism, protein metabolism. It causes decrease of antibody production. Therefore it is called as stress hormone.
3. Sexhormones( eg. Estrogen, Progesteron and several Androgens): It is responsible for normal sexual activity. It helps in development of secondary sexual characters.

Hyposecretion: It is due to hyposecretion of Glucocorticoids. It results in Addison’s disease in which patients suffer from vomiting, mental stress, low blood pressure, weakness, increase in excretion of Na+ and Cl- ions in urine.

Hypersecretion: In male child lead to precocious sexual maturity called as Cushing disease (due to hypersecretion of glucocorticoids). In female child masculization appears.

B. Adrenal Medulla: It secretes two hormones:

1. Adrenalin
2. Nor-adrenalin

1. Adrenalin: it is also referred to as emergency hormone.

Functions:

• It increases oxygen consumption, BMR and heat production in body.
• It converts liver glycogen into glucose and increase blood sugar level.
• It increases strength of muscle contraction and stimulates respiration.
• It dilates blood vessels, increase blood pressure and heart rate.
• It prepares body to face unexpected situations or emergency situations
• It is also known as three ‘F’ hormone as it takes care of emergency situation of body such as fear, fight and flight.

1. Nor-adrenalin: It is also called as Nor-epinephrine. It causes vasoconstriction of all blood vessels.

8.  ENDOCRINE PANCREAS:

It is the second largest gland in body. It is located in abdomen which has both endocrine and exocrine functions. Exocrine part secretes enzymes into pancreatic ducts. Endocrine part constitutes islets of Langerhans, discovered by Paul Langerhans in 1868 (professor of pathology). Islets of Langerhans has three types of cells – α-cells, β- cells and δ- cells. Α- cells secrete hormone glucagon, β- cells secrete hormone insulin and δ- cells secrete hormone somatostatin.

Insulin and glucagon have antagonistic effects.

i. Insulin:

• It induces protein synthesis.
• It induces synthesis of enzymes involved in the conversion of glucose into glycogen (process called glycogenesis).
• It increases the cell permeability to glucose.
• It inhibites the conversion of glycogen into glucose and thus regulates sugar level in blood.
• It stimulates the synthesis of fatty acids in the adipose tissue.

Hyposecretion:

• Deficiency of insulin leads to metabolic disease known as diabetes mellitus.
• Blood glucose level reaches such a level that it exceeds than which kidney can reabsorb, and so is excreted in the urine.
• It causes breakdown of muscles tissues.
• Loss of weight and tiredness.

Hypersecretion:

Hypersecretion of insulin leads to decrease in blood sugar level. This condition is known as hypoglycemia, characterized by blurring of vision.

ii. Glucagon:

• It increases blood sugar level. It stimulates conversion of glycogen into glucose (process called as glycogenolysis).
• It also stimulates breakdown of protein and fats to glucose.

iii. Somatostatin: It inhibits secretion of both insulin and glucagon.

9. SEX GLANDS:

1. Testis: Interstitial cells of testis secrete testosterone hormone in male. Testosterone causes development of secondary sexual characters in male.
2. Ovaries: Ovaries include two endocrine parts:
   1. Graafian follicles: It secretes oestrogen which develops secondary sexual characters in female.
   2. Corpus luteum: It secretes hormone progesterone.

10. OTHER ENDOCRINE GLANDS:

1. Stomach: stomach secretes hormone gastrin. Gastrin stimulates the production of HCl and enzyme pepsin for digestion of food.
2. Small intestine: Small intestine secretes:
   1. Cholecystokinin: It stimulates contraction of gall bladder which releases bile and also stimulates pancreas to secrete digestive enzymes.
   2. Secretin: It stimulates pancreas to release bicarbonate rich fluid to neutralize acidity of chyme.
   3. Heart: Heart secretes Atrial Natriuretic Peptide(ANP) which inhibits action of ADH and Aldosterone.
   4. Placenta: Placenta secretes human Chorionic Gonadotropin (B-HCG) which maintains corpus luteum.