- Endocrine system is formed
of endocrine glands which are also called ductless glands.
- The branch of biology which
deals with the study of endocrine glands and functions of their hormones
is called endocrinology. Thomas Addison is called the father of
- The collective study of
nervous system and endocrine system of body is called neuro-endocrine
- Heterocrine glands are partly endocrine and
partly exocrine. Exocrine part releases the secretions through duct
while endocrine part releases hormones directly in the blood due to lack
- The hormones are carried
from the glands to the organs through blood. These organs on which the
hormones act are called target organs.
- The word hormone was coined
by Starling in 1905.
- Two English physiologists
William M. Bayliss and Earnst H. Starling discovered the first hormone
secreted by the mucosa layer of duodenum. This was called secretin and
its action is to stimulate the secretion of pancreatic juices. But the
first isolated hormone was insulin which was isolated from
pancreas of dogs by Banting and McLeod.
- Hormones may be amines,
steroids or proteins.
- Examples of amine
hormones are thyroxine from thyroid gland, epinephrine and
nor-epinephrine from adrenal gland.
- Examples of steroid
hormones are cortisol, corticosterone, etc. from adrenal gland,
testosterone from testes, progesterone from
- Examples of proteinous
hormones are TSH, FSH, LH, ACTH, etc. from pituitary gland; insulin and
glucagon from pancreas; parathormone from parathyroid gland.
- Characteristic Features of
They are released in blood and transported through blood.
They are produced in very low concentration.
They have low molecular weight, thus can easily pass through
They are inactivated or destroyed when their function is over.
- The term 'hormone' was
coined from the Greek word 'Hormaein' which means to excite. Starling in
1905 coined this word.
- Thyroid gland is the largest
endocrine gland (5 × 3 cm) in size and 25 gm in weight. Thyroid hormones
regulate BMR, i.e., basal metabolic rate of body. They control rate of
cell respiration as well as energy production in mitochondria. Thus,
these hormones control mental, physical as well as sexual growth of body
by Calorigenic effect. Thyroidless tadpole larva of frog does not
- Exophthalmic goitre produced
due to hyperthyroidism is also called Grave's disease.
- Hashimoto's Disease: When thyroxine secretion
becomes very less due to age factor or injury to thyroid gland, the
antibodies are formed which destroy thyroid gland. This is called
- Parathyroid glands are four
in number. Two glands are present in each lobe of thyroid gland. These
glands secrete parathormone (PTH) which increases calcium level in
blood. Parathormone is also called Collip's hormone because it
was reported by Phillip Collip in 1925.
- Thyrocalcitonin and
Parathormone are antagonistic to each other.
- Adrenal gland shows two
distinct regions, namely, adrenal cortex and adrenal medulla. Adrenal
cortex secretes 40 different steroid hormones called corticoids.
- Adrenal glands are also
called '4-S glands' because they control sugar metabolism, salt retention,
source of energy and sex hormone secretion.
- Deficiency of
mineralocorticoids causes Addison's disease during which sugar
level in blood decreases, nervous depression, low blood pressure and
bronze like skin colour appears.
- Oversecretion of cortisol
(glucocorticoids) causes Cushing's syndrome during which sugar
level in blood rises, loss of weight, rise in blood pressure and swollen
face, hands, etc. appears.
- Sex corticoids stimulate
secondary sexual characters of male type. Over secretion of these in
female causes, virilism, in which male type secondary sexual
characters such as hoarse voice, facial hair appear in the female.
- Adrenalin (epinephrine) is
secreted by adrenal medulla. This hormone has same biological effect as
of sympathetic nervous system. Adrenalin is called emergency hormone
because it prepares the body to face certain emergency conditions like
fall in blood pressure, cold, pain, anger, injury, fear, etc. The
adrenal glands are thus 'glands of emergency'.
- Hypothalamus is the base of
diencephalon. It shows neurosecretory cells which secrete oxytocin and
vasopressin. These hormones are stored in the axons of neurosecretory
cells which extend in the posterior lobe of pituitary gland.
- Pituitary gland is very
small (15 × 3 mm). It is called 'Master gland' or 'Master of endocrine
orchestra' or 'Executive officer of hypothalamus'.
- Melanocyte stimulating
hormone secreted by pars intermedia controls synthesis and dispersal of
melanin in the skin of fish, amphibians and causes darkening of skin.
- Adenohypophysis of pituitary
is a pharyngeal outgrowth, and is thus called Rathke's pouch.
- Pineal body is a very small
knob-like structure present in the mid brain. It secretes melatonin
hormone, which controls development and concentration of melanin of skin
of frog-making the skin lighter in colour.
- Pancreas is a heterocrine
gland because it is partly endocrine and partly exocrine in function.
Islets of Langerhans form the endocrine part while the acini form the
exocrine part of gland. Islet of Langerhans show three types of cells,
namely, alpha cells secreting glucagon, beta cells secreting insulin
and gamma cells which are precursors of alpha and beta cells.
- Glucagon and insulin are
- Ovaries secrete three female
hormones, namely, estrogens, progesterone and relaxin.
- Progesterone is secreted by
corpus luteum while relaxin is secreted by corpus albicans. Estrogen is
secreted by graafian follicles.
- Leydig's cells of testes
produce testosterone which controls secondary sexual characters and
growth of secondary sex organs.
- Contraceptive pills contain
progesterone and estrogens which check ovulation and pregnancy in