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Chemical Coordination & Integration 

Endocrine system formed of all endocrine glands of body" Though different endocrine glands are different in embryonic origin and are isolated from one another but these interact with one another so collectively form an endocrine system
Glands of body: Animals have three types of glands:

(i) Exocrine gland (Gr., ex = out + krinein = to secrete) :
These glands have ducts for discharging their secretions. Therefore, they are called as duct glands. ex - Liver, Sweat gland, Sebaceous gland, Gastric glands and some intestinal glands.

(ii) Endocrine glands (Gr., endo = within + krinein = to secrete): These glands lack ducts and pass secretions into the surrounding blood directly. Therefore they are called ductless glands. ex - Thyroid, parathyroid, adrenal, pituitary, pineal body and thymus.

(iii) Heterocrine glands: These glands consist of both exocrine and endocrine tissue. The exocrine discharge its secretion by a duct and the endocrine tissue discharges its secretion into the blood. Pancreas and gonads are heterocrine glands. These are also called mixed glands. 

Location of Many Endocrine Glands




Hormone is a chemical produced by endocrine glands and released into the blood and transported to a distantly located target organ has current scientific definition: Hormones are non-nutrient chemicals Which act as intercellular messengers and ate produced in trace amounts.

First hormone discovered was 'secretin. It was discovered by two English physiologists: William M Bayliss and Ernest H. Starling in 1903.

Term hormone was coined by starling (1905) from Greek word Homone means to excite. It is a misnomer because a number of hormones are known to have inhibitory effect (e.g., Somatostatin). Properties of hormones:

(1) These are Secreted by endocrine gland (biogenic in origin).

(2) Their secretions is released directly into blood (except local hormones e.g., gastrin).

(3) These are carried to distantly located specific organs, called target organ.

(4) These have specific physiological action (excitatory or inhibitory). These co-ordinate, different physical, mental and, metabolic activities and maintain homeostasis.

(5) The hormones have low molecular weight e.g., ADH has a molecular weight of 600-2000 daltons.

(6) These act in very low concentration e.g., around 10-10 molar;

(7) Hormones are non antigenic.

(8) These are mostly short-lived. So have a no cumulative effect,

(9) Some hormones are 'quick acting e.g., adrenalin, while some acting slowly e.g., oestrogen of ovary.

(10)  Some hormones secreted in inactive form called prohormone e.g., Pro-insulin.

(11)  Hormones after their action destroyed in liver and kidney.

Classification of hormones

(1) On the basis of chemical nature: On the 'basis of chemical composition hormones are, classified into three categories.

(i) Amine hormones: These are derived form tyrosine amino add and have amino group e.g., Thyroxine!' Epinephrine, Nor-epinephrine.

(ii) Steroids: These are fat soluble and have sterol 'group. These are derived from cholesterol' e.g., hormones of adrenal cortex (cortisol, cortisone, corticosterone, aldoster-one) testes (testosterone) and ovaries (estrone, estradiol, progesterone etc.)

(iii) Proteinaceous and peptide hormones: These are formed of, 3-200aminoacids interlinked by peptide bonds and are water soluble e:g.,

(a) Proteinaceous hormones like STH, TSH, FSH, LH etc. Out of these FSH and LH are glycoproteins.

(b) Long peptide hormones, like insulin and glucagon, ACTH, Paratharmone,

(c) Short peptide hormones like oxytocin, ADH, MDH. These hormones are formed of a few amino acids.

(2) On the basis of mode of action:

(i) Quick acting hormones: These hormones initiate immediate response from their target cells. There receptor is always located on the outer surface of plasma membrane of target cell because these are large sized. Hormone receptor complex activates a membrane enzyme adenyl cyclase which hydrolyse ATP into cyclic AMP, which acts as secondary messenger, c-AMP activates an inactive enzyme system by cascade effect. So their mode of action is called second messenger hypothesis. e.g., These includes proteinous, peptide and amine hormones.

(ii)  Delayed acting hormones: These hormones initiate response after some time. These are small sized- so are diffusable through the plasma membrane of their target cell. These bind their proteinaceous receptor present in the cytosol. These always operate through de-novo synthesis of m­RNA by activation of certain genes. So their mechanism of action is called m-RNA hypothesis. e.g.. These include steroid hormones of testes, ovary and adrenal cortex. 

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