The animal body has two levels of coordination: nervous coordination (see Chapter 29) and chemical coordination. Chemical coordination is centered in a system of glands known as endocrine glands. These glands are situated throughout the animal body and include such organs as the pancreas, thyroid gland, and adrenal gland. The glands secrete hormones, a series of chemical substances composed of protein or sterol lipids.
Hormones bring about changes that help coordinate body systems in a general way. For example, the pancreas secretes insulin, which facilitates the passage of glucose into all body cells for use in energy metabolism. Another example is thyroxine, a thyroid gland secretion that regulates overall body metabolism. In contrast to chemical coordination, the nervous system coordinates functions in the animal body on a more localized level as it delivers nerve impulses to contract body muscles or regulate gland activities.
The endocrine glands secrete their hormones into the bloodstream, where the blood carries the hormones to the target organs. Because the endocrine glands have no ducts, they are often called ductless glands. Other glands of the body (such as the enzyme-secreting salivary glands) deliver their enzymes via ducts and are referred to as exocrine glands.
The structure and physiology of hormones and endocrine glands are relatively similar in all animals; the emphasis in this chapter is on the human endocrine system.