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The term water retention (also known as fluid retention) signifies an abnormal accumulation of fluid in the circulatory system or within the tissues or cavities of the body. Water is found both inside and outside the body’s cells. It forms part of the blood, helping to carry the blood cells around the body and keeping oxygen and important nutrients in solution so that they can be taken up by tissues such as glands, bone and muscle. Even the organs and muscles are mostly water. The body uses a complex system of hormones and hormone-like substances called prostaglandins to keep its volume of fluid at a constant level. If one were to intake an excessive amount of fluids in one day, the amount of fluid would not be affected in the long-term. This is because the kidneys quickly excrete the excess in the form of urine. Likewise, if one did not get enough to drink, the body would hold on to its fluids and urinate less than usual. == Where water retention occurs == Fluid rich with oxygen, vitamins and other nutrients passes all the time from the capillaries (the smallest blood vessels) into the surrounding tissues, where it is known as tissue fluid or interstitial fluid. This fluid nourishes the cells and eventually should return to the capillaries. Water retention is said to occur as a result of changes in the pressure inside the capillaries, or changes that make the capillary walls too leaky (see edema and vascular permeability). If the pressure is wrong, or the capillaries are too leaky, then too much fluid will be released into the tissue spaces between the cells. Sometimes so much fluid is released that it cannot all return to the capillaries and remains in the tissues, where it causes the swelling and waterlogging which is experienced as water retention.〔Essentials of pathophysiology: concepts of altered health states. Porth C, Gaspard J, Matfin G. Lippincott Williams & Wilkins, 2006, p. 109〕 Another set of vessels known as the lymphatic system acts like an "overflow" and can return a lot of excess fluid back to the bloodstream. But even the lymphatic system can be overwhelmed, and if there is simply too much fluid, or if the lymphatic system is congested, then the fluid will remain in the tissues, causing swellings in legs, ankles, feet, abdomen or any other part of the body.〔Essentials of Rubin's pathology. Rubin E. Lippincott Williams & Wilkins, 2008, p. 124〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Water retention (medicine)」の詳細全文を読む スポンサード リンク
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