Exocrine glands include sweat glands, mammary glands, salivary glands and glands of the liver and pancreas. Exocrine glands vary in location within the body but essentially perform the same tasks. Several methods and techniques are used to classify exocrine glands, such as structure, method of secretion and product secreted.
Exocrine glands primarily work by secreting products (with the exception of hormones and other chemical messengers) from ducts and duct glands that lead directly onto surfaces such as skin. These glands are the counterparts to endocrine glands, which work by secreting products or hormones directly into the bloodstream through ductless glands. Alternatively, they release special hormones called paracrines into nearby target cells, which only enter the adjacent release site. Endocrine glands include adrenal glands, which are situated on top of the kidneys and secrete the hormone adrenaline along with several other hormones.
Exocrine glands have distinct structures characterized by glandular portions and duct components. Duct portions may be branched or unbranched; these structures are called compound and simple respectively. The glandular portion of glands may be classified as tubular or acinar or be a combination of the two types, referred to as tubuloacinar. Lastly, exocrine glands are termed apocrine, holocrine or merocrine depending on how they secrete products.