Hormones are specific chemical messengers that exert effects at points some distance removed from their sites of production. Hormones may of course diffuse from one place to another but as would be expected in animals with well-developed circulatory systems, most of their transport in mammals is by the blood. The tissues and organs that produce and release hormones are termed endocrine tissues and endocrine organs. The use of the word “endocrine” i.e. secreting internally is meant to convey that the hormones are secreted directly into the blood in the capillaries supplying the endocrine tissues and that no special ducts or tubes are involved. In fact, endocrine glands are frequently called the ductless glands.