The kidneys are bean-shaped glands about 12 cm long, 3 cm thick and 7 cm wide. They are located at the rear of the abdominal cavity, alongside the spine and just under cover of the lowest ribs.

Each kidney contains about a million functional units called nephrons which filter the blood and control its composition.

Blood enters each nephron through a tuft of capillaries called the glomeruli . Blood passing through the glomeruli is filtered into tiny tubules.

As the filtered fluid passes along the tubules, much of it is reabsorbed, thereby adjusting the composition of the remaining urine and preserving the body’s salt and water balance.