· There are two integrating systems in the body, nervous and endocrine. Both are interdependent.
· The word 'endocrine' derives from a Greek word meaning 'I separate within'.
· The endocrine system achieves coordination and integration by transmitting information through 'chemical messengers' called the hormones.
· Huxley called hormones as 'chemical messengers'.
· Hormones are also known as autocoids' or informational molecules.
· The word 'hormone' was introduced by Starling in 1905.
· The word 'hormone' is derived from a Greek word meaning 'I excite or arouse'.
· The word 'hormone' was first used in reference to secretin and gastrin.
· First hormone discovered was secretin by Bayliss and Starling in 1903.
· Hormones are secreted by ductless glands, released directly into the blood.
· Hormones regulate metabolism, growth, secretion, digestion, excretion, reproduction, etc.
· The study of endocrine glands and hormones is 'Endocrinology'.
· The father of endocrinology is Thomas Addison. The first endocrine disease reported was Addison's disease (1855) caused by the destruction of the adrenal cortex.
· The gland with duct is called the exocrine gland. The ductless gland is termed an endocrine gland.
· Pancreas is a mixed gland (heterocrine), with exocrine and endocrine parts

· Hormones have many similarities with enzymes. They are:
i) Not obtained from food, but synthesized by the body.
ii) Easily soluble and diffusible.
iii) Require and secrete in minute quantities.
iv) Not stored (except thyroxine) and have high specificity.

· Hormones differ from enzymes in following:
i) All enzymes are proteins, but hormones are chemically different.
ii) Enzymes are mostly used locally, hormones are circulated in the blood.
iii) Enzymes catalyse metabolic reactions and are reusable, but hormones are completely used up in metabolism.
· The tissue on which the hormone acts is known as 'target tissue'.

Chemical Nature of Hormones
1. Amino Acid Derivatives (Biogenic Amines)
  1. Derivatives of Tyrosine
  2. Epinephrine and Norepinephrine (Adrenal medulla)
  3. Dopamine (Hypothalamus)
  4. Thyroxine (Thyroid gland)
  5. Derivatives of Tryptophan,
  6. Melatonin (Pineal gland)
2. Peptide Hormones
  1. Short peptides and small proteins
  2. ADH, oxytocin and regulatory hormones (Hypothalamus)
  3. ACTH, GH, NISH and Prolactin (Pituitary)
  4. Insulin and glucagon (Pancreas)
  5. Parathyroid hormone (Parathyroid gland)
  6. Calcitonin (C cells of thyroid)
  7. Atrial natriuretic peptide (Heart)
  8. Gastrointestinal hormones (GI tract)
  9. Glycoproteins
  10. TSH, LH and FSH (Pituitary)
  11. Erythropoietin (Kidney)
  12. Inhibin (Reproductive organs)
3. Lipid Derivatives
  1. Steroid hormones
  2. Androgens, estrogens and progestins (Gonads)
  3. Corticoids (Adrenal cortex)
  4. Calcitriol (Kidney)
  5. Eicosanoids (Lipid derivatives of arachidonic acid)
  6. Prostaglandins

 When some hormones work together to control a process, this is called synergism, e.g., FSH and LH.
· When two hormones work against each other to control a process, this is called antagonism, e.g., Insulin and glucagon, Calcitonin and parathormone.
· There are some hormone‑like substances, but not the products of endocrine glands. They are parahormones, e.g., Prostaglandins and pheromones.

Properties of Hormones
· Hormones are specific in origin and in their activity.
· in very low concentration and produce the measurable change in organism.
· Hormones are not Hormones act ordinarily stored or they have no cumulative effect because they are quickly destroyed and excreted as soon as their actions are over.
· Hormones can modify metabolic process by altering the rate of reactions.
· Specific secretary stimuli are necessary for their secretion.
· Hormones are soluble in water and blood. They can easily diffuse through the wall of cell membrane.


It is secreted by endocrine glandsIt is secreted by exocrine glands.
Hormones are proteinous in nature.Enzymes are proteinous, lipoproteins, glycoproteins, steroids in nature.
In the case of hormone, site of origin and site of action or target organ are always located far from each otherIn case of enzyme, site or origin and site of action or target organ are always located near to each other.
Hormones are destroyed quickly.Enzymes are not destroyed quickly in comparison to hormone.
Small amount of hormones can produce remarkable effects.Small amount of enzymes cannot produce remarkable effects.

Similarities between hormones and enzymes:
· They are easily soluble and diffusible.
· They are not directly obtained frpm food but sythesize in the body.
· They are required and secreted in minute quantity.
· They cannot be stored except thyroxin.

· The most important endocrine glands in man include:
(i) Pituitary (Hypophysis)
(ii) Thyroid
(iii) Parathyroids
(iv) Adrenals (Suprarenals)
(v) Pineal gland
(vi) Thymus gland
(vii) Pancreas
(viii) Kidneys
(ix) Gonads
(x) Placenta

Origin of Endocrine Glands
PituitaryAdrenal cortexThyroid
Pineal glandGonadsParathyroids
Adrenal medulla

Also, Read our Other Notes related to Endocrinology of Mammals:

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