Water Balance

Water levels in the blood have to be kept within a certain range, otherwise the body's cells become too shrunken (dehydrated) or too bloated (overhydrated) to work. For this reason, the kidneys, the endocrine system, and the circulatory system work together to maintain a healthy balance in our bloodstream.

Losing Balance

A number of commonly consumed substances upset our water balance. Alcohol, for example, blocks the pituitary gland from releasing ADH. This means that the kidneys, which are working hard to get rid of the alcohol in the bloodstream, send more water out into the urine. Drinking just one glass of wine can cause the body to get rid of the equivalent of four glasses of water. Substances that make us produce a lot of urine are called "diuretics". Caffeine is another diuretic.

Too little Water

We lose water constantly, but there are times when we lose a lot of water quickly - through sweating, vomiting or diarrhoea, for example. This results in both a decrease in blood volume and a rise in the level of salt relative to water in our blood. These act as triggers for balance to be restored. 

Low water alert

The hypothalamus receives signals that blood pressure is low and salt levels are high. It responds by increasing the production of ADH (antidiuretic hormone), which is carried to the pituitary gland, where it is released into the blood. 

Blood vessels contract

High levels of ADH instruct muscles in the walls of the blood vessels, which, given the current reduction in blood volume, restores blood pressure to normal. 

Water reabsorbtion

High ADH levels also signal the kidneys to reabsorb water and to retain the salts that are often lost through sweating or vomiting. 

Concentrated urine

With the body retaining as much water as possible, the bladder fills more slowly. This means that the urine is more concentrated, and darker in colour. 

Too much water

Far rarer than dehydration is overhydration, which can be caused by extreme water intake after exercise, drug abuse, or disease. This results in an increase in blood volume and a reduction in the level of salt relative to water in the blood. 

High water alert

The hypothalamus receives signals that blood pressure is high and that salt levels are low. It responds by producing less ADH. Since ADH instructs the kidneys to store water, a reduction in ADH means an increase in urination. 

Blood vessels dilate

Low levels of ADH instruct the blood vessel wall muscles to relaxe. This expands the blood vessels and eases the blood pressure caused by the excess of water. 

Water release

Low ADH levels also signal the kidneys to reduce the amount of water that they reabsorb, so that more water is added to the urine and passed out through the bladder. 

Diluted urine

With less water being reabsorbed by the body, the bladder fills quickly and more diluted urine is produced. The more diluted the urine, the lighter its colour.