The human urinary system

Excretion is the removal of waste made by the body's cells. The kidneys are two bean-shaped organs at the back of the abdomen. Blood carrying waste, such as urea from broken-down proteins, reaches each kidney through a renal artery from the aorta. Millions of tiny filtering units called nephrons remove the urea, extra salts and extra water, forming urine, and the cleaned blood returns through a renal vein to the vena cava. Urine flows down a tube called the ureter from each kidney to the bladder, which stores it until it leaves the body through the urethra.