secretion
1. The manufacture and discharge of specific substances into the external medium by cells in living organisms. (The substance secreted is also called the secretion.) Secretory cells are often specialized and organized in groups to form glands. The substances produced may be released directly into the blood (endocrine secretion; see endocrine gland) or through a duct (exocrine secretion; see exocrine gland). Secretions can be classified according to the manner of their discharge. Merocrine (eccrine) secretion occurs without the secretory cells sustaining any permanent change; in apocrine secretion the cells release a secretory vesicle incorporating part of the secretory cell membrane; and holocrine secretion involves the disruption of the entire cell to release its accumulated secretory vesicles. Substances destined for secretion are prepared and packaged into membranous vesicles by the Golgi apparatus inside the cell.

2. The process by which a substance is pumped out of a cell against a concentration gradient. Secretion has an important role in adjusting the composition of urine as it passes through the nephrons of the kidney.

secretion describes the processes by which cells assemble materials and release them for action elsewhere. Commonly material is passed into a duct (exocrine secretion). For example, saliva, containing salts and enzymes, is assembled in the salivary gland cells, released into salivary ducts, and passed to the mouth to help with mastication and digestion. Other secretions are passed directly into the bloodstream (endocrine secretion). Generally the word is not used to describe how neurotransmitters or neurohormones are lost from cells; rather these are said to be released. Finally, secretion should not be confused with excretion, which refers to the loss of waste products from the body.

Alan W. Cuthbert

se·cre·tion / siˈkrēshən/ • n. a process by which substances are produced and discharged from a cell, gland, or organ for a particular function in the organism or for excretion. ∎  a substance discharged in such a way.

secretion Production and discharge of a substance, usually a fluid, by a cell or a gland. The substance so discharged is also known as a secretion. Secretions include enzymes, hormones, saliva, and sweat.

secretion (si-kree-shŏn) n.
1. the process by which a gland isolates constituents of the blood or tissue fluid and chemically alters them to produce a substance that it discharges for use by the body or excretes.

2. the substance that is produced by a gland.