Hormone means to set in motion or to spur on. Hormones are chemical messengers secreted by endocrine glands and carried by blood and lymph to the target organ elsewhere in the body to stimulate a specific activity that may be a biochemical or physiological.
Thus, hormones are special chemicals that regulate the physiological or biochemical processes in our body. Bayliss and Starling, two British scientists, gave the term hormone in 1902 while working on hormonal secretions.
Endocrine and Exocrine Glands:
Hormones are secreted by endocrine glands (Greek: endo means within or inside; crin means secretion). These are also called ductless glands because they do not have their own ducts, and their secretions are directly released into the tissue space next to them.
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These secretions are carried by blood and/or lymph to the target organ. On the other hand an exocrine gland secretes material directly into the duct. They have their own ducts which carry the secretions directly to the target organ.
Characteristics of Hormones:
i. Hormones are regulatory chemicals which are secreted by endocrine glands directly into the blood.
ii. They are produced in very minute quantity
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iii. They are biologically very active and their action is very rapid.
iv. They act only on target organs or cells located usually away from their sources.
v. They regulate the physiological process of the body by bringing about chemical changes. Thus, they bring about metabolic regulation in the body.
vi. They are destroyed soon after their action and thus not stored in the body.
Two Main Categories of Hormones:
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Steroid Hormones:
They are steroid in nature. They diffuse through the cell membrane and bind to internal receptors. Examples: Oestrogen, progesterone, androgens, Cortisol, aldosterone, thyroid hormones, etc.
Peptide or Non-Steroidal Hormones:
These are amino acid derivatives, peptides or proteins. They bind to receptors located on the surface of target cell. Examples: Glucagons, ADH, Oxytocin, Insulin, Somatotropin, Prolactin, FSH, LH, TSH, etc.