The endocrine system encompasses a group of tissues that release hormones into circulation for travel to and action on distant targets. An endocrine tissue is typically a ductless gland (eg, pituitary, thyroid) that releases its hormones into capillaries that permeate the tissue. These glands are richly supplied with blood. However, nontypical endocrine tissues also contribute important hormones to circulation, eg, secretion of atrial natriuretic peptide from the heart, erythropoietin from the kidney, insulin-like growth factor from the liver, and leptin from fat. New hormones continue to be discovered. Some act only on a single tissue, whereas others have effects on virtually all cells of the body. The effects of hormones on their targets are varied—from enhancement of nutrient uptake to altering cell division and differentiation, among many others.
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