The endocrine system consists of a collection of glands that are scattered throughout the body and secrete chemical substances called hormones. A hormone may be defined as a chemical messenger produced and secreted by a specific endocrine cell and circulates in trace amounts in the blood and acts on specific target cells, usually remote from the endocrine cell. The hormone binds to receptors in the target cell, initiating specific intracellular pathways, and this results in alterations
of cellular actions such as uptake or output of mediators involved in the continuous, long-term regulation of physiological processes (e.g., fluid and electrolyte balance, energy metabolism, growth and development, digestion, reproduction and adaptation to physiological stresses). An important feature is signal amplification due to the self-multiplication of intracellular pathways.