Produced from specialized group of cells called endocrine gland.
Exert effect on a distant organ /cell / part of body.
Acts upon specific target cell by regulating the rate of metabolic reactions.
Required in small amount (nanogram or pictogram).
Following body functions can be ascribed to hormones:
Metabolic action, such as the increase in the rate of oxidation thyroxine, insulin on carbohydrate metabolism, or parathyroid hormone on calcium metabolism.
equilibrium, e.g. anterior pituitary controls maintenance and development of thyroid, adrenal Regulation of growth of the whole body or of certain tissues and organs.
Endocrine cortex, and sexual glands, and reciprocally these glands regulate the activity of the anterior pituitary.
Sexual and reproductive i.e. stimulation or inhibition of functional and psychic sexual characters and the libido by gonadal hormones.
Mechanism of Hormone Action: Hormones are chemically protein, peptides, amino acid derivatives, cholesterol or fatty acid derivatives. They do not act as enzymes or biological catalysts rather they stimulate or retards ongoing metabolic reactions or physiological actions. For example insulin stimulates glucose utilization by cells, vasopressin increase water retension by kidneys, oxytocin stimulates smooth muscle contractions.
Protein Hormones: They have large molecular weight and so can not pass through plasma membrane. So they exert their action on cell membranes of target cells. Receptor sites binds with hormone on the membrane and converts ATP to cAMP. This cAMP (second messenger) affects protein synthesis and mitochondrion produces the hormone. ACTH, LH, FSH, TSH, produce their effect through this way e.g. insulin increases uptake of glucose by the cell.
Steroid Hormones: Steroid and thyroid hormone are smaller in size end hence can cross the cell membrane to enter cytoplasm. In cytoplasm, they binds with receptors (in lock & key fashion), the complex moves into nucleus on to chromosome. There the gene responds in a specific way by producing specific mRNA and that dictates synthesis of specific protein necessary for its function.
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