Image Caption : Female Endocrine System : 3D visualization reconstructed from scanned human data of the female endocrine system. The endocrine system is the regulator of the human body as it responsible for maintaining homeostasis by producing and directing chemical messengers called hormones. Hormones act on just about every cell to carry out a variety of functions related to the following: metabolism, water and mineral balance, sexual development, growth, and stress reactions. Most hormones travel throughout the body via the bloodstream to affect their target organs and tissues. Other hormones act locally and arrive at their site of action via microcirculation.
The endocrine system refers to the collection of glands of an organism that secrete hormones directly into the circulatory system to be carried towards distant target organs. The major endocrine glands include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus, gastrointestinal tract and adrenal glands. The endocrine system is in contrast to the exocrine system, which secretes its hormones using ducts. 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. The nervous system sends information very quickly, and responses are generally short lived. In vertebrates, the hypothalamus is the neural control center for all endocrine systems. The field of study dealing with the endocrine system and its disorders is endocrinology, a branch of internal medicine.
Special features of endocrine glands are, in general, their ductless nature, their vascularity, and commonly the presence of intracellular vacuoles or granules that store their hormones. In contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.
In addition to the specialised endocrine organs mentioned above, many other organs that are part of other body systems, such as bone, kidney, liver, heart and gonads, have secondary endocrine functions. For example the kidney secretes endocrine hormones such as erythropoietin and renin.
A number of glands that signal each other in sequence are usually referred to as an axis, for example, the hypothalamic-pituitary-adrenal axis.
As opposed to endocrine factors that travel considerably longer distances via the circulatory system, other signaling molecules, such as paracrine factors involved in paracrine signalling diffuse over a relatively short distance.
The word endocrine derives from the Greek words ἐνδο- endo- "inside, within," and κρίνειν krinein "to separate, distinguish".
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