Functioning of the body's systems to filter wastes.

The kidneys filter waste materials and excess fluid from the blood and also produce hormones that are important for blood formation, blood pressure, and bone formation. Entering the kidneys through the renal artery, blood is processed in tiny tubes called nephrons and returned to circulation through the renal veins. The waste substances that are filtered out are turned into urine, which collects in the central part of the kidney (called the renal pelvis) and passes through the ureters to the bladder. When a half pint or more of urine has collected in the bladder, it is emptied from the body through the urethra.

The kidneys and ureters begin to form when an embryo is about four or five weeks old and are complete, in a rudimentary form, by the eighth week. However...