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Why do you need fluid balance monitoring?
Last reviewed: 08.07.2025
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At rest, under optimal temperature conditions, the body's fluid balance is maintained at ±0.2% of total body weight. Daily fluid intake is closely balanced with the volume of fluid lost in urine, faeces, and sweat, through respiration, and through insensible water losses through the skin. This tight balance requires constant integration of input signals from hypothalamic osmoreceptors and vascular baroceptors to ensure that fluid intake accurately matches fluid losses.
Fluid balance is regulated by mechanisms that affect the excretion of water and sodium, as well as the sensation of thirst. Sweat losses are accompanied by a decrease in plasma volume and an increase in osmotic pressure (due to increased sodium and chloride concentrations). These changes are sensed by vascular receptors and osmoreceptors in the hypothalamus, causing an increase in the release of vasopressin (antidiuretic hormone) from the pituitary gland and renin from the kidneys. These hormones (including angiotensin II and aldosterone, which are formed as a result of increased plasma renin activity) stimulate the retention of water and sodium by the kidneys and provoke an increase in thirst. When fluid intake exceeds losses, the volume and osmolality of the plasma return to normal, and the water balance is restored by the kidneys (i.e., the excess fluid is excreted).
However, in physically active individuals, the body's fluid balance is often disrupted because the thirst-control mechanism is unable to accurately determine the body's fluid needs to ensure sufficient intake during exercise.