Endocrine System
<p>(An example of an endocrine gland is the <a href="http://en.wikipedia.org/wiki/Adrenal_gland" title="Adrenal gland" >adrenal gland</a>, which is found on top of the kidneys and secretes the hormone <a href="http://en.wikipedia.org/wiki/Adrenaline" title="Adrenaline" class="mw-redirect" >adrenaline</a>, among others).</p>
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The <b>endocrine system</b> is the system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the <a href="http://en.wikipedia.org/wiki/Exocrine_gland" title="Exocrine gland" >exocrine system</a>, which secretes its chemicals using <a href="http://en.wikipedia.org/wiki/Duct_(anatomy)" title="Duct (anatomy)" >ducts</a>. </p>
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It derives from the Greek words "endo" meaning inside, within, and "crinis" for secrete. </p>
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The endocrine system is an information signal system like the nervous system, yet its effects and mechanism are classifiably different. The endocrine system's effects are slow to initiate, and prolonged in their response, lasting from a few hours up to weeks. </p>
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The nervous system sends information very quickly, and responses are generally short lived. Hormones are substances (chemical mediators) released from endocrine tissue into the bloodstream where they travel to target tissue and generate a response. Hormones regulate various human functions,</p>
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including <a href="http://en.wikipedia.org/wiki/Metabolism" title="Metabolism" >metabolism</a>, <a href="http://en.wikipedia.org/wiki/Human_development(biology)" title="Human development (biology)" >growth and development</a>, <a href="http://en.wikipedia.org/wiki/Tissue(biology)" title="Tissue (biology)" >tissue</a> function, and <a href="http://en.wikipedia.org/wiki/Mood_(psychology)" title="Mood (psychology)" >mood</a>. The field of study dealing with the endocrine system and its disorders is <a href="http://en.wikipedia.org/wiki/Endocrinology" title="Endocrinology" >endocrinology</a>, a branch of <a href="http://en.wikipedia.org/wiki/Internal_medicine" title="Internal medicine" >internal medicine</a>.</p>
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Features of endocrine glands are, in general, their ductless nature, their vascularity, and usually the presence of intracellular vacuoles or granules storing their hormones.</p>
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In contrast, <a href="http://en.wikipedia.org/wiki/Exocrine_gland" title="Exocrine gland" >exocrine glands</a>, such as <a href="http://en.wikipedia.org/wiki/Salivary_gland" title="Salivary gland" >salivary glands</a>, <a href="http://en.wikipedia.org/wiki/Sweat_gland" title="Sweat gland" >sweat glands</a>, and <a href="http://en.wikipedia.org/wiki/Glands" title="Glands" class="mw-redirect" >glands</a> within the <a href="http://en.wikipedia.org/wiki/Gastrointestinal_tract" title="Gastrointestinal tract" class="mw-redirect" >gastrointestinal tract</a>, tend to be much less vascular and have ducts or a hollow <a href="http://en.wikipedia.org/wiki/Lumen_(anatomy)" title="Lumen (anatomy)" >lumen</a>.</p>
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In addition to the specialised endocrine organs mentioned above, many other organs that are part of other body systems, such as the <a href="http://en.wikipedia.org/wiki/Kidney" title="Kidney" >kidney</a>, <a href="http://en.wikipedia.org/wiki/Liver" title="Liver" >liver</a>,<a href="http://en.wikipedia.org/wiki/Heart" title="Heart" >heart</a> and <a href="http://en.wikipedia.org/wiki/Gonad" title="Gonad" class="mw-redirect" >gonads</a>, have secondary endocrine functions. </p>
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For example the kidney secretes endocrine hormones such as <a href="http://en.wikipedia.org/wiki/Erythropoietin" title="Erythropoietin" >erythropoietin</a> and <a href="http://en.wikipedia.org/wiki/Renin" title="Renin" >renin</a>.</p>
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The endocrine system is made of a series of glands that produce chemicals called <a href="http://en.wikipedia.org/wiki/Hormones" title="Hormones" class="mw-redirect" >hormones</a>. A number of glands that signal each other in sequence are usually referred to as an axis, for example, the <a href="http://en.wikipedia.org/wiki/Hypothalamic-pituitary-adrenal_axis" title="Hypothalamic-pituitary-adrenal axis" class="mw-redirect" >hypothalamic-pituitary-adrenal axis</a>.</p>