In physiology, the endocrine system is a system of glands, each of which secretes a type of hormone into the bloodstream to regulate the body. It derives from the Greek words endo (Greek ένδο) meaning inside, within, and crinis (Greek κρινής) for secrete. The endocrine system is an information signal system like the nervous system. Hormones are substances (chemical mediators) released from endocrine tissue into the bloodstream that attach to target tissue and allow communication among cells. Hormones regulate many functions of an organism, including mood, growth and development, tissue function, and metabolism. The field of study that deals with disorders of endocrine glands is endocrinology, a branch of internal medicine.
The endocrine system is made up of a series of ductless glands that produce chemicals called hormones. A number of glands that signal each other in sequence is usually referred to as an axis, for example, the hypothalamic-pituitary-adrenal axis. Typical endocrine glands are the pituitary, thyroid, and adrenal glands. Features of endocrine glands are, in general, their ductless nature, their vascularity, and usually the presence of intracellular vacuoles or granules storing their hormones. In contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.
In addition to the specialised endocrine organs mentioned above, many other organs that are part of other body systems, such as the kidney, liver, heart and gonads, have secondary endocrine functions. For example the kidney secretes endocrine hormones such as erythropoietin and renin.