Importance of water balance in the body

Water content in the blood

Osmoregulation is the control of water levels and mineral salts in the blood.

Water levels and mineral salts in the blood are controlled to protect animal cells by stopping too much water from entering or leaving them. The concentration of water and salts is the same inside and outside of the cells. If body cells lose or gain too much water by osmosis, they do not function efficiently.

Red blood cells in different solutions

If the concentration of water is the same inside and out the cells, then there is no net movement of water into or out of the cell. The cells remain in their normal state.

If the water concentration is too high outside of the cell then water enters the cell by osmosis. If too much water enters, the cell will burst. If the water concentration is too low outside compared to the inside of the cells, water leaves the cells by osmosis. This causes cells to shrivel.

Diagram of a normal cell, then a cell that is surrounded by too much water
A wilted houseplant
Wilted plant due to plant cells losing water

Water can move in and out of plant cells by osmosis in the same way that it can in animal cells. The plant cells are turgid or firm, when the cytoplasm is full of water. If plant cells lose water, they become flaccid and the cytoplasm shrinks away from the cell wall. This causes the plant to wilt.

Human excretory organs

The organs of excretion in humans include the skin, lungs and kidneys.

Water is lost from the body as:

  • urine from the kidneys
  • sweat from the skin
  • water vapour, from the lungs when we exhale


Sweat glands in the skin produce sweat. Water, ions and urea are lost from the skin as they are contained in sweat.

Cross section of human skin
Cross section of human skin


Water and excess carbon dioxide leaves the body via the lungs when we exhale.

We cannot control the level of water, ions or urea loss by the lungs or skin. For example, in a hot climate, your body sweats to help keep you cool. In the same way, when we breathe out we lose water vapour, and we cannot alter the amount we lose.


The kidneys are organs of the urinary system. The urinary system removes excess water, salts and urea.

Move on to Video