Secretions

Secretions are substances produced by cells, tissues, or glands that are released for use outside the cell or into the body’s fluids. They can act locally on a surface, be carried to distant sites, or contribute to surface coatings. They differ from waste excretions in that secretions have a defined biological function.

Secretions are commonly classified as exocrine or endocrine. Exocrine secretions are discharged through ducts to external or mucosal surfaces, such as saliva from salivary glands, gastric juice, pancreatic juice, bile, sweat, tears, and mucus. Endocrine secretions are released into the bloodstream to act on distant targets, such as hormones. Some glands produce both exocrine and endocrine secretions.

The composition and functions of secretions vary widely. Examples include enzymes like amylase in saliva and pepsin in gastric juice; mucus and other lubricants; digestive fluids such as gastric juice and pancreatic juice; bile for fat emulsification; sweat for thermoregulation; tears for eye protection; and sebum for skin lubrication. Secretions may also carry signaling molecules and antimicrobial peptides.

Secretory activity is regulated by neural input, hormonal signals, and local factors, adapting to meals, irritation, or inflammation. Disorders of secretion can involve too little or too much production, affecting digestion, hydration, or signaling. Clinical examples include dry mouth (xerostomia), excessive sweating, and pancreatic insufficiency, each impacting normal physiology.