secretion, in biology, substance elaborated by the living material of an animal or plant. Secretions in humans can be produced by a single cell or by a group of cells commonly called a gland. Some secretions perform special functions in the body (true secretions); others are eliminated as waste products (excretions). Digestive secretions include saliva, gastric juice, intestinal juice, pancreatic juice, and bile. Certain secretions serve as lubricants, e.g., the synovial fluid in joints or the secretions from mucous membranes and from the lachrymal (tear) glands. The mammary glands secrete milk. The endocrine (ductless) glands secrete hormones that enter directly into the bloodstream (see gland). Among the excretions from the body are urine (from the kidneys), perspiration (from the sweat glands), and bile pigments (from the gall bladder). Plant secretions include nectar and various enzymes concerned with the digestion of nutrients within the plant cells.