exocrine gland

Meaning: An exocrine gland is a cluster of secretory epithelial cells whose product travels through a duct to a surface or lumen that is continuous with the outside of the body. Typical products are enzymes, mucus, sweat, saliva, bile, and milk. They act locally at the site of delivery and are not carried by the blood to distant target cells. Endocrine glands are the contrast: they have no ducts and release hormones into the bloodstream.

On the MCAT: Passages ask you to classify a structure as exocrine or endocrine from its description, usually by whether a duct is present and where the product goes. Salivary, sweat, sebaceous, and mammary glands are exocrine. The pancreas is the favorite example. Acinar cells secrete amylase, lipase, proteases (as zymogens such as trypsinogen), and bicarbonate through the pancreatic duct into the duodenum. The islet cells secrete insulin, glucagon, and somatostatin into the blood. The liver's bile, delivered by ducts to the small intestine, is also an exocrine secretion.

Common confusion: Students read "exocrine" as outside the body, or assume an organ must be one type or the other. The lumen of the digestive tract is topologically outside the body, so digestive secretions count as exocrine. The pancreas has both exocrine and endocrine tissue. The deciding question is the route: a duct to a surface or lumen means exocrine, and release into the blood means endocrine.