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 organ of excretion of 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 cleanched fist. Kidneys clean 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 urinate.
We lose around 5 pints (3 ltr) of water a day through our skin as sweat, and in our breath and urine. We also get rid of extra salt in sweat, and expel waste carbon dioxide gas when we breathe out.