Endocrine systems are systems which generally control long term activities of target organs as well as physiological processes such as digestion, metabolism, growth, development and reproduction, in contrast to more rapid activities under the control of the nervous system.
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These are effectors organs, in that they are under the control of the nervous system either directly -or indirectly.
Endocrine systems include certain glands called endocrine glands; these glands are found in different regions of the body of the animal.
Although, endocrine glands were first recognized only as ductless glands as they have no external ducts and which discharge their secretions directly into the blood stream. Particular emphasis was placed on the vascularity and specialized anatomy.
Now, however, endocrine glands are known to occur in animals with scanty circulating fluids, while very vascular organs such as the brain and the intestinal wall not only produce hormones but also, at the same time, perform quite different physiological activities.
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As a matter of fact, the first hormonal action to be successfully demonstrated was that of secretin produced by the intestinal wall and released into the intestinal lumen (Bayliss and Starling 1902; later Gabriel and Fogel, 1955), since 1902 a host of chemicals have been added to the list of hormones.
The activity of endocrine systems involves the release of special chemical substances called hormones which are usually (although not always) carried by the circulating blood to the sites where they may act.
Hormones are special chemical substances which are elaborated in restricted areas of the animal and influence the activity of various organs of the body and, thus, bring about a harmonious working of the body.
Hormonal actions are usually longer lasting than those produced by nerve impulses. Also hormonal effects are capable of being exerted throughout the body, whereas nervous actions are usually more localized.
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Hormonal actions take place wherever cells are found that possess the necessary reaction or combining sites with the chemical hormones.
Nearly all systems appear to be regulated by a combination of both hormonal and nervous systems. Hormones are sometimes called the “chemical messenger” of the body. They are effective in minute quantities.
Their regulatory actions are sometimes one of excitation and sometimes one of inhibition; consequently the word hormone, which comes from the Greek root hormao meaning “to excite” is really a misnomer.