Exocrine

Exocrine describes glands or portions of glands that discharge their products through ducts onto an epithelial surface or into a hollow organ, rather than releasing them into the bloodstream. This contrasts with endocrine glands, which secrete hormones directly into the circulatory system. Exocrine secretions typically act locally and include enzymes, mucus, bile, sweat, and other fluids.

Common exocrine glands include the salivary glands, which release saliva; lacrimal glands, which produce tears; and sweat glands, which secrete perspiration. The pancreas has an exocrine part that delivers digestive enzymes and bicarbonate to the duodenum via the pancreatic duct; the liver secretes bile into the biliary system for digestion. Many tissues contain unicellular secretory cells (such as goblet cells) or organized multicellular glands with ducts.

Anatomically, exocrine glands are often classified as simple or compound and as tubular, acinar (alveolar), or tubuloalveolar, depending on duct and secretory unit structure. Secretions can be mucous, serous, or mixed. The secretory mechanisms include merocrine secretion (exocytosis of vesicles), apocrine secretion (loss of the apical cytoplasm), and holocrine secretion (rupture of the entire cell).

Clinically, exocrine function can be impaired in disease. For example, cystic fibrosis affects exocrine glands by producing thick secretions that obstruct ducts in the lungs and pancreas, leading to respiratory and digestive problems. Exocrine insufficiency can cause malabsorption and digestion-related symptoms when pancreatic or biliary secretions are reduced.