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Water content

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

If the concentration of water is the same inside and out the cells, they remain in their normal state. If the water concentration is too high outside, water enters the cell by osmosis and they may burst.

On the other hand, if the water concentration is too low outside compared to the inside of the cells, water will leave by osmosis and the cells may shrivel. If body cells lose or gain too much water by osmosis they do not function efficiently.

Plants also undergo the process of osmosis, in the same way that animals cells do. Cell walls are turgid or firm, when they are full of water. If cell walls lose water, they become flaccid and the cytoplasm shrinks away from the cell wall.

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

Skin

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

Lungs

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

We cannot control the level of water, ion 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.

Kidneys

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