Glands that secrete substances onto a free surface (like sweat glands and oil glands) are considered glands.

Glands that secrete substances onto a free surface, such as sweat glands and oil glands, are classified as exocrine glands. Exocrine glands are characterized by their mechanism of secretion, which involves the release of substances through ducts that ultimately lead to the external environment or onto epithelial surfaces. This is distinct from endocrine glands, which release hormones directly into the bloodstream.

Exocrine glands play a vital role in maintaining various physiological processes. For instance, sweat glands help in thermoregulation by releasing sweat, which cools the body through evaporation. Similarly, sebaceous (oil) glands secrete oily substances that lubricate and protect the skin and hair. These glands can also be found in other locations, such as the salivary glands, which facilitate digestion, and the mammary glands, which produce milk. Understanding the functions and classifications of these glands is crucial in fields like biology, medicine, and physiology.