The body is constantly exposed to fluid loss, averaging about 2500 ml (2½ liters) per day. 100 ml is lost in the faeces, 200 ml through sweating, and 700 ml through the lungs and skin (excluding the amount lost in sweat). The remainder is lost through the renal excretion of urine.
This water loss must be compensated by fluid intake in order to maintain the fluid balance. This means that every day you must add on average about 2500 ml of fluid. Fluid intake comprises about 10% comes of the cell metabolism, e.g. the water formed when the cells produce ATP from glucose. 250 ml of water is produced from metabolism per day. Food ingestion constitutes about 30% of the daily fluid intake, while drinking accounts for the remaining 60% of the fluid consumed daily.
There are two main factors that control the water balance:
- water intake
- water loss
The main regulatory mechanism of water intake is the thirst mechanism. An increase in plasma concentration of 2-3% initiates the thirst mechanism in the hypothalamus. The experience of dry mouth will also trigger the sensation of thirst. A loss of blood volume and decreased blood pressure detected by baroreceptors, will trigger mechanisms to retain fluids but this will not occur until the volume is reduced by approximately 10-15%, and is therefore less important than the thirst mechanism.
In the pituitary gland within the thirst centre, osmoreceptors respond to the salt concentration in the blood and they are activated by increased plasma concentration of salts. They can also be stimulated by other factors, such as impulses from the baroreceptors and angiotensin II.
The most important mechanism for the regulation of water loss is by the antidiuretic hormone (ADH). ADH is secreted from the hypothalamus when there is an increase in plasma concentration due to loss of fluid and an increase in salt levels. ADH acts on the kidney to absorb more water and reduce the amount of water excreted in the urine. The urine also becomes more concentrated. The body absorbs the water from the kidney so that the plasma concentration decreases to normal levels (see Renal Physiology, chapter 2.2).