Endocrinology
<p ><b>Endocrinology</b> (from <a href="http://en.wikipedia.org/wiki/Ancient_Greek" title="Ancient Greek" >Greek</a> <a href="http://en.wiktionary.org/wiki/%E1%BC%94%CE%BD%CE%B4%CE%BF%CE%BD" class="extiw" title="wiktionary:ἔνδον" >ἔνδον</a>, <i>endon</i>, "within"; <a href="http://en.wiktionary.org/wiki/%CE%BA%CF%81%CE%AF%CE%BD%CF%89#Ancient_Greek" class="extiw" title="wiktionary:κρίνω" >κρίνω</a>, <i>krīnō</i>, "to separate"; and <a href="http://en.wiktionary.org/wiki/-%CE%BB%CE%BF%CE%B3%CE%AF%CE%B1" class="extiw" title="wiktionary:-λογία" >-λογία</a>, <i><a href="http://en.wiktionary.org/wiki/-logia" class="extiw" title="wiktionary:-logia" >-logia</a></i>) is a branch of biology and medicine dealing with the <a href="http://en.wikipedia.org/wiki/Endocrine_system" title="Endocrine system" >endocrine system</a>, its diseases, and its specific secretions called <a href="http://en.wikipedia.org/wiki/Hormone" title="Hormone" >hormones</a>, as well as the integration of developmental events proliferation, growth, and differentiation (including <a href="http://en.wikipedia.org/wiki/Histogenesis" title="Histogenesis" >histogenesis</a> and <a href="http://en.wikipedia.org/wiki/Organogenesis" title="Organogenesis" >organogenesis</a>), and also the psychological or behavioral activities of <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, <a href="http://en.wikipedia.org/wiki/Sleep" title="Sleep" >sleep</a>, <a href="http://en.wikipedia.org/wiki/Digestion" title="Digestion" >digestion</a>, <a href="http://en.wikipedia.org/wiki/Respiration(physiology)" title="Respiration (physiology)" >respiration</a>, <a href="http://en.wikipedia.org/wiki/Excretion" title="Excretion" >excretion</a>, <a href="http://en.wikipedia.org/wiki/Mood(psychology)" title="Mood (psychology)" >mood</a>, <a href="http://en.wikipedia.org/wiki/Stress_(physiology)" title="Stress (physiology)" class="mw-redirect" >stress</a>, <a href="http://en.wikipedia.org/wiki/Lactation" title="Lactation" >lactation</a>, <a href="http://en.wikipedia.org/wiki/Motor_coordination" title="Motor coordination" >movement</a>, <a href="http://en.wikipedia.org/wiki/Reproduction" title="Reproduction" >reproduction</a>, and <a href="http://en.wikipedia.org/wiki/Sensory_perception" title="Sensory perception" class="mw-redirect" >sensory perception</a> as caused by hormones.
Endocrinology is concerned with study of the biosynthesis, storage, chemistry, biochemical and physiological function of hormones and with the cells of the endocrine glands and tissues that secrete them. Various specializations exist, including behavioral endocrinology<sup id="cite_ref-1" class="reference" ><a href="http://en.wikipedia.org/wiki/Endocrinology#cite_note-1" >[1]</a></sup><sup id="cite_ref-2" class="reference" ><a href="http://en.wikipedia.org/wiki/Endocrinology#cite_note-2" >[2]</a></sup><sup id="cite_ref-3" class="reference" ><a href="http://en.wikipedia.org/wiki/Endocrinology#cite_note-3" >[3]</a></sup> and <a href="http://en.wikipedia.org/wiki/Comparative_endocrinology" title="Comparative endocrinology" >comparative endocrinology</a>.
The endocrine system consists of several glands, all in different parts of the body, that secrete hormones directly into the blood rather than into a duct system. Hormones have many different functions and modes of action; one hormone may have several effects on different target organs, and, conversely, one target organ may be affected by more than one hormone.
In the original 1902 definition by Bayliss and Starling (see below), they specified that, to be classified as a hormone, a chemical must be produced by an organ, be released (in small amounts) into the blood, and be transported by the blood to a distant organ to exert its specific function. This definition holds for most "classical" hormones, but there are also <a href="http://en.wikipedia.org/wiki/Paracrine" title="Paracrine" class="mw-redirect" >paracrine</a> mechanisms (chemical communication between cells within a tissue or organ), <a href="http://en.wikipedia.org/wiki/Autocrine_signaling" title="Autocrine signaling" class="mw-redirect" >autocrine</a> signals (a chemical that acts on the same cell), and <a href="http://en.wikipedia.org/wiki/Intracrine" title="Intracrine" >intracrine</a> signals (a chemical that acts within the same cell).<sup id="cite_ref-4" class="reference" ><a href="http://en.wikipedia.org/wiki/Endocrinology#cite_note-4" >[4]</a></sup> A<a href="http://en.wikipedia.org/wiki/Neuroendocrine" title="Neuroendocrine" class="mw-redirect" >neuroendocrine</a> signal is a "classical" hormone that is released into the blood by a neurosecretory neuron (see article on <a href="http://en.wikipedia.org/wiki/Neuroendocrinology" title="Neuroendocrinology" >neuroendocrinology</a>).
Hormones act by binding to specific <a href="http://en.wikipedia.org/wiki/Receptor_(biochemistry)" title="Receptor (biochemistry)" >receptors</a> in the target organ. As <a href="http://en.wikipedia.org/wiki/%C3%89tienne-%C3%89mile_Baulieu" title="Étienne-Émile Baulieu" >Baulieu</a> notes, a receptor has at least two basic constituents:</p>
<ul >
<li >a recognition site, to which the hormone binds; and</li>
<li >an effector site, which precipitates the modification of cellular function.<sup id="cite_ref-5" class="reference" ><a href="http://en.wikipedia.org/wiki/Endocrinology#cite_note-5" >[5]</a></sup></li>
</ul>
<p >Between these is a "transduction mechanism" in which hormone binding induces allosteric modification that, in turn, produces the appropriate response.</p>