Single-kidney anatomy. Artwork showing a posterior (from behind) view of human kidneys in relation to the bones of the skeleton. One kidney (brown, upper right) has a normal appearance. The other (blue) represents a missing or diseased kidney. The function of the kidneys is to filter waste products such as urea from the blood to form urine. The urine drains to the bladder through the ureters (pink tubes running downwards). Humans have two kidneys, but can function with only one. Some of the arteries (red) and veins (blue) are partially seen, obscured by the bones.