Merck Manual

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Professional Version

The Endocrine System of Animals


Robert J. Kemppainen

, DVM, PhD, Department of Anatomy, Physiology and Pharmacology, College of Veterinary Medicine, Auburn University

Reviewed/Revised Jul 2023

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).

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