Water retention (medicine)

The term water retention (also known as fluid retention) or hydrops, hydropsy signifies an abnormal accumulation of clear, watery fluid in 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 fluid 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. Imbalances in this system can lead to water retention, which can range from mild and unnoticeable to symptomatic with swelling.

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.

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Wikipedia