Secreting

Secreting is the biological process by which cells release substances produced within the organism to an external site or into the bloodstream. Secretion differs from excretion in that secreted products are typically used by other tissues or organisms rather than discarded as waste.

Secretions occur via secretory cells and glands and can be classified as exocrine, releasing through ducts to surfaces or cavities; or endocrine, releasing directly into the circulatory system.

Secretory mechanisms include merocrine secretion, in which vesicles fuse with the plasma membrane to release contents by exocytosis; apocrine secretion, where a portion of the cell apex is shed; and holocrine secretion, in which the entire cell disintegrates to release its contents.

Most secretions are produced in the endoplasmic reticulum and Golgi apparatus, packaged into secretory vesicles, and transported to the cell surface.

Examples of secretions include mucous from goblet cells that protects and lubricates luminal surfaces; digestive enzymes such as pancreatic amylase and lipase; saliva; sweat; milk; and hormones such as insulin.

Endocrine secretions act on distant targets through the bloodstream; exocrine secretions act locally on body surfaces or cavities.

Secretion is regulated by neural and hormonal signals, and feedback mechanisms modulate its rate.

Disorders of secretion can involve over- or under-secretion, affecting digestion, metabolism, or defense. In plants, secretions include nectar, resins, and essential oils used for attraction or protection.