Endocrine and exocrine refer to two different kinds of glands in the body, both of which secrete products in a watery solution. The terms indicate where and how these products are secreted. Exocrine glands release their products onto body surfaces, like the skin, or into cavities, such as those inside the digestive tract. Endocrine glands secrete substances directly into the bloodstream. Exocrine glands tend to be relatively simple and have local effects, while endocrine ones release hormones that travel throughout the body.
Most glands form in the embryo from epithelial tissue. Exocrine glands retain an opening in this tissue called a duct, also made of epithelial tissue, through which they secrete their products. Sweat glands are one example, as they are located beneath the surface of the skin but secrete sweat onto the surface. Endocrine glands do not form ducts and release their products directly into the blood. As a result, glands can be classified by whether or not they have a duct.
Another difference between endocrine and exocrine glands is that exocrine glands are usually much simpler and easier to categorize than endocrine ones. Although they are often made up of multiple cells, exocrine glands may be as small as a single cell. Goblet cells, for example, are single-celled glands in the respiratory tract that produce mucus.
The human body contains many types of exocrine glands that have diverse functions. They include the salivary glands of the mouth, the oil- and sweat-producing glands of the skin, and many others. Some that are often thought of as endocrine, such as the liver and pancreas, also have exocrine functions.
Since endocrine glands have no ducts and release their secretions directly, they are often known as ductless glands. Their secretions are called hormones, and send messages to other specific organs. Together, they form the endocrine system, which is responsible for regulating the functions of many organs of the body as well long-term processes like growth and digestion. Many of these glands play a role in other systems of the body, but some, such as the pituitary gland, belong to no other system.
Hormones affect other cells by targeting specific locations on them, causing changes in the function of the target organ. Important hormones include the sex hormones produced in the female ovaries and the masculine testes. Other endocrine glands include the adrenal glands, the pancreas, and the pineal glands.