Hormones
<p >A <b>hormone</b> (from <a href="http://en.wikipedia.org/wiki/Greek_language" title="Greek language" >Greek</a> ὁρμή, "impetus") is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell <a href="http://en.wikipedia.org/wiki/Metabolism" title="Metabolism" >metabolism</a>. In essence, it is a chemical messenger that transports a signal from one cell to another.<sup id="cite_ref-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Hormones#cite_note-0" >[1]</a></sup> </p>
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All <a href="http://en.wikipedia.org/wiki/Multicellular_organism" title="Multicellular organism" >multicellular organisms</a> produce hormones;<a href="http://en.wikipedia.org/wiki/Plant" title="Plant" >plant</a> hormones are also called <a href="http://en.wikipedia.org/wiki/Phytohormone" title="Phytohormone" class="mw-redirect" >phytohormones</a>. Hormones in <a href="http://en.wikipedia.org/wiki/Animal" title="Animal" >animals</a> are often transported in the blood. Cells respond to a hormone when they <a href="http://en.wikipedia.org/wiki/Gene_expression" title="Gene expression" >express</a> a specific <a href="http://en.wikipedia.org/wiki/Receptor_(biochemistry)" title="Receptor (biochemistry)" >receptor</a> for that hormone. The hormone binds to the receptor <a href="http://en.wikipedia.org/wiki/Protein" title="Protein" >protein</a>, resulting in the activation of a <a href="http://en.wikipedia.org/wiki/Signal_transduction" title="Signal transduction" >signal transduction</a>mechanism that ultimately leads to cell type-specific responses.</p>
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<a href="http://en.wikipedia.org/wiki/Endocrine_system" title="Endocrine system" >Endocrine</a> hormone <a href="http://en.wikipedia.org/wiki/Molecule" title="Molecule" >molecules</a> are secreted (released) directly into the <a href="http://en.wikipedia.org/wiki/Bloodstream" title="Bloodstream" class="mw-redirect" >bloodstream</a>,
whereas <a href="http://en.wikipedia.org/wiki/Exocrine_gland" title="Exocrine gland" >exocrine hormones</a> (or ectohormones) are secreted directly into a duct, and, from the duct, they flow either into the bloodstream or from cell to cell by <a href="http://en.wikipedia.org/wiki/Diffusion" title="Diffusion" >diffusion</a> in a process known as <a href="http://en.wikipedia.org/wiki/Paracrine_signalling" title="Paracrine signalling" >paracrine signalling</a>.</p>
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Recently it has been found that a variety of <a href="http://en.wikipedia.org/wiki/Exogenous" title="Exogenous" class="mw-redirect" >exogenous</a> <a href="http://en.wikipedia.org/wiki/Xenoestrogen" title="Xenoestrogen" >modern chemical compounds</a> have hormone-like effects on both humans and wildlife. Their <a href="http://en.wikipedia.org/wiki/Endocrine_disruptor" title="Endocrine disruptor" >interference</a> with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body can change the <a href="http://en.wikipedia.org/wiki/Homeostasis" title="Homeostasis" >homeostasis</a>, reproduction, development, and/or behavior, just as <a href="http://en.wikipedia.org/wiki/Endogenous" title="Endogenous" class="mw-redirect" >endogenously</a> produced hormones do.<sup id="cite_ref-pmid9539004_1-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Hormones#cite_note-pmid9539004-1" >[2]</a></sup></p>