What Do Kidneys Do?

Each body has two kidneys, either side of the backbone.  Each kidney receives blood through the renal artery.  The kidneys then filter and clean the blood, the return it via the renal vein.   Each kidney has about a million or so filters which are known as nephrons.  
Kidneys also control the amount of water, minerals and salts in the blood.  If a person drinks a lot of water, then this needs to be removed from the blood, or else it would be diluted.  The kidneys remove the water and pass it out as urine.  If not enough water is taken in, the kidneys conserve water in the body by removing less of it from the blood and so less urine is produced.
Many people have only one kidney which works and this has no effect on their day to day life, since one kidney can cope with the renal function.