Water balance and the effect of osmosis on cellsSpec 4.5.3.3
In short
Water balance means controlling how much water the body loses. Water leaves via the lungs when we breathe out, and water, ions and urea are lost in sweat, but only the kidneys can adjust the amounts lost, removing excess water, ions and urea in urine. Cells that gain or lose too much water by osmosis do not function efficiently.
The body loses water in several ways.
- Water leaves the body via the lungs during exhalation.
- Water, ions and urea are lost from the skin in sweat.
- Excess water, ions and urea are removed via the kidneys in the urine.
There is no control over water, ion or urea loss by the lungs or skin. Only the kidneys can adjust the amounts lost, which is why they are central to controlling the water level.
Effect of osmotic changes on cells
Water moves into and out of cells by osmosis. If body cells lose or gain too much water by osmosis they do not function efficiently. If the fluid around the cells is too dilute, water moves into the cells; if it is too concentrated, water moves out of the cells.
Name osmosis and say which way the water moves, for example: ‘water moves out of the cells by osmosis, so they shrink and cannot work properly’.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026