Hormones

A hormone is an organic substance (for example a peptide or a steroid) that is secreted by one tissue, enters the bloodstream, and induces a physiological response (for example growth or a change in metabolism) in other tissues; a hormone is, in short, an intercellular signalling molecule. Virtually every process in a complex organism is regulated by one or more hormones: the maintenance of blood pressure, blood volume and electrolyte balance, embryogenesis, hunger, eating behaviour and digestion are just a few examples. Hormones are made by endocrine glands, which are special groups of cells; the major endocrine glands are the pituitary, pineal, thymus, thyroid, adrenal glands and pancreas, and in addition men produce hormones in their testes and women produce them in their ovaries.

Chemically, hormones may be classified as either protein hormones (e.g. insulin, epinephrine) or steroid hormones (e.g. estrogen, androgen). Separately, hormones are also classified according to the distance over which they act, as endocrine, paracrine or autocrine hormones. Endocrine hormones act on cells that are distant from the site where they were released -- insulin and epinephrine, for example, are synthesised and released into the bloodstream by specialised ductless endocrine glands, and then travel through the blood to reach distant target cells. Paracrine hormones (also called local mediators) act only on cells close to the cell that released them; interleukin-1 (IL-1) is the example given. Autocrine hormones act on the very same cell that released them; the protein growth factor interleukin-2 (IL-2) is the example given. …