The endocrine system encompasses a group of tissues that release hormones into circulation. Hormones are signaling molecules that travel to targets distant from their site of production and interact with receptors to exert their effects. Some act only on a single tissue; others have effects on nearly all cells of the body. The effects of hormones on their targets vary—from enhancement of nutrient uptake to alteration of cell division and differentiation, among many others.
An endocrine tissue is typically a ductless gland (eg, pituitary, thyroid) that secretes hormones into capillaries that permeate the tissue. Endocrine glands are richly supplied with blood.
Nontypical endosecretory tissues also contribute important hormones to the circulation—eg, atrial natriuretic peptide from the heart, erythropoietin from the kidney, insulin-like growth factor from the liver, and leptin from fat. Some endocrine tissues are temporary (eg, placenta, corpora lutea).
The structure General Chemical Structure and Function of the Endocrine System of Animals The endocrine system consists of ductless vascular tissues that release a variety of hormones in regulated fashion. Endocrine hormones control or regulate a multitude of biological processes... read more , secretory patterns, and regulation Regulation of the Endocrine System in Animals Secretion of hormones is regulated by a system of sensing elements that can detect the need for both increased and decreased secretion. The particular sensing network, feedback elements, and... read more 
of endocrine hormones vary. New endocrine hormones continue to be discovered.
