Kidney. Coloured computed tomography (CT) 3-D scan showing healthy kidneys (purple bean-shaped, top right and left), seen within the lower part of the rib cage. They are situated at the back of the abdominal cavity just above the waist on either side of the spinal column (purple at centre). The kidneys are responsible for filtering the blood and excreting waste products and excess water in the form of urine. Each kidney is about 10 to 12.5cm long and about 170g in weight. The kidneys also produce several hormones, including erythropoietin, which regulates the production and release of red blood cells from the bone marrow. Part of the pelvis is seen at lower centre.