Our bodies’ cells ‘burn’ nutrients and oxygen to make energy to live and grow, in much the same way that wood and other materials are burnt to produce another sort of energy – heat. But just as fires produce waste gases and ash, so the ‘burning’ that takes place in our body cells also creates waste products. These must be removed, or they would poison us.
The removal of these waste materials is known as excretion, and the body’s main organs of excretion are the kidneys. You have two kidneys positioned in the small of your back, one on either side of your backbone. They look like large reddish – brown beans and each one is about the size of a clenched fist. Kidneys clear the blood by filtering out waste materials and straining off any water the body doesn’t need. This liquid waste is called urine. It is stored in your bladder and leaves your body when you go to the toilet.