Chapter 22 Chemical Coordination And Integration
ENDOCRINE GLANDS AND HORMONES
Endocrine glands lack ducts and are hence, called ductless glands. Their secretions are called hormones. Hormones are non-nutritional chemicals which act as intercellular messengers and are produced in trace amounts.
The endocrine system is composed of hypothalamus, pituitary and pineal, thyroid, adrenal, pancreas, parathyroid, thymus and gonads.
The pituitary gland is divided into three major parts:
The pineal gland secretes melatonin which plays a very important role in the regulation of 24 hour rhythms of our body (e.g. rhythms of sleep and state of being awake).
The thyroid gland hormones play an important role in the regulation of the basal metabolic rate, development and maturation of the central neural system, metabolism of carbohydrates, proteins and fats, menstrual cycle.
The thymus gland secretes thymosins which play a major role in the differentiation of T-lymphocytes, which provide cell immunity. Thymosins also increase the production of antibodies to provide humoral immunity.
The adrenal gland is composed of the centrally located adrenal medulla and the outer adrenal cortex. The adrenal medulla secretes epinephrine and norepinephrine. These hormones increase alertness, pupillary dilation, sweating, heartbeat, etc. The adrenal cortex secretes glucocorticoids and mineralocorticoids.
The endocrine pancreas secretes glucagon and insulin. Glucagon stimulates glycogenolysis and gluconeogenesis resulting in hyperglycemia. Insulin stimulates cellular glucose uptake and utilization and glycogenesis resulting in hypoglycemia. Insulin deficiency and/or insulin resistance result in a distance called diabetes mellitus.
The testis secretes androgens, which stimulates the development, maturation and functions of the male accessory sex organs, appearance of the male secondary sex characters, spermatogenesis, male sexual behaviour, etc.
The ovary secretes estrogen and progesterone. Estrogen stimulates growth and development of female accessory sex organs and secondary sex characters. Progesterone plays a major role in the maintenance of pregnancy as well as in mammary gland development and lactation.
The atrial wall of the heart produces atrial natriuretic factor which decreases the blood pressure.
Kidney produces erythropoietin which stimulates erythropoiesis.
The gastrointestinal tract secretes gastrin, secretin, cholecystokinin and gastric inhibitory peptide. These hormones regulate the secretion of digestive juices and help in digestion.