Secretion

Secretion is the process by which cells produce and release substances into an external environment or into a lumen or duct. It is distinct from excretion, which removes waste products. Secretion includes exocrine secretions into ducts that open onto surfaces, and endocrine secretions that enter the bloodstream to act at distant targets.

Exocrine secretions include digestive enzymes, mucus, and other substances released by salivary, gastric, pancreatic, biliary, lacrimal, and sweat glands. Endocrine secretions consist of hormones released into capillaries and carried by the circulatory system to regulate distant organs. Some tissues perform both exocrine and endocrine roles.

At the cellular level, secreted products originate in the secretory pathway: rough endoplasmic reticulum synthesizes proteins, they are processed in the Golgi apparatus, and packaged into secretory vesicles. Secretion can be constitutive, with vesicles fusing continuously with the plasma membrane, or regulated, triggered by stimuli such as hormones or neuronal signals. Exocrine secretions may follow merocrine (exocytosis), apocrine (apical portion of the cell is released with vesicles), or holocrine (whole cell disintegrates) modes, depending on tissue type.

Secretions are essential for digestion, lubrication, immune defense, reproduction, and signaling. Dysfunctions in secretion underlie various diseases, such as cystic fibrosis, where thickened secretions impair organ function, pancreatic insufficiency, and abnormal sweat or hormone release.