exocrine gland

If an endocrine gland is a factory pouring its product into the river that flows past, an exocrine gland is the factory with a proper delivery chute — a pipe that carries its product to a precise destination. That pipe is a duct, and it is what defines exocrine glands.

An exocrine gland secretes its products through ducts onto an epithelial surface — the skin, the lining of the gut, or the airways — rather than into the blood. Familiar examples are sweat glands, salivary glands, the tear-producing lacrimal glands, mammary glands, and the digestive-enzyme-producing part of the pancreas. Their products are not hormones in the strict endocrine sense.

Exocrine glands matter in endocrinology mainly by contrast: defining them sharpens the meaning of ductless, internal secretion. A caveat to remember: the same organ can be both exocrine and endocrine. The pancreas and the gonads, for example, run both kinds of business under one roof, secreting enzymes or fluids through ducts while also releasing hormones into the blood.