Why do you need a fluid balance control?

, medical expert
Last reviewed: 15.05.2018

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At rest under optimal temperature conditions, the body's fluid balance is maintained at ± 0.2% of the total body weight. Daily fluid intake is closely balanced with the volume of fluid that is excreted in the urine, feces and sweat, through breathing, and through insensible loss of water through the skin. Such close balance requires constant integration of input signals from the osmoreceptors of the hypothalamus and vascular baroceptors, so that the fluid intake exactly corresponds to its losses.

The liquid balance is regulated by mechanisms that affect the release of water and sodium, as well as a thirst. Losses with sweat are accompanied by a decrease in the volume of plasma and an increase in the osmotic pressure (due to an increase in the concentrations of sodium and chlorine). These changes are perceived by receptors of blood vessels and osmoreceptors of the hypothalamus, which causes an increase in the release of vasopressin (antidiuretic hormone) from the pituitary and renin from the kidneys. These hormones (including angiotensin II and aldosterone, which are formed as a result of increased plasma renin activity) stimulate retention of water and sodium by the kidneys and provoke an increase in thirst. When the fluid intake exceeds its losses, the volume and osmotic capacity of the plasma return to the normal level, and the water balance is restored by the kidneys (that is, the excess fluid is released).

However, in physically active individuals, the body's fluid balance is often disrupted, since the thirst control mechanism is not able to accurately determine the body's fluid needs in order to provide sufficient intake during exercise.

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