The kidneys remove many waste products from the blood and form urine.
Most people have two kidneys, located toward the back of the abdominal cavity. Their microscopic functional units, called nephrons, filter blood plasma and then selectively reabsorb water and useful substances while secreting additional waste into the forming urine.
Kidneys help eliminate products such as urea, which results from protein metabolism. They also regulate water balance, electrolytes, blood acidity, and blood pressure. Urine travels from each kidney through a ureter to the urinary bladder before leaving the body through the urethra.
The liver also detoxifies chemicals and converts ammonia into urea, so it has an important relationship with kidney function. However, the kidneys are the organs that filter the blood and excrete the resulting soluble waste in urine. They also produce hormones and hormone-like signals involved in red-blood-cell production and bone metabolism.