The endocrine system is composed of ductless glands (glands that do not have tubes). Endocrine glands secrete hormones, which are chemical "messengers" that travel through the blood to regulate the activity of a target organ.

Glands that have ducts are called exocrine glands. The secretions of exocrine glands reach their target by traveling through a duct (tube). Exocrine glands are not part of the endocrine system. Some examples of exocrine glands are sweat glands and salivary glands.

Endocrine responses are slow because hormones must travel through the blood to reach their target organ(s). Also, the duration of the response is long because the hormone must be filtered out of the blood by the kidneys. Hormones often have multiple targets; as a result, their effects in the body are widespread. Check out this diagram of the endocrine system.