The urinary system consists of the kidneys, ureters, bladder, and urethra. The key elements in the system are the kidneys, a pair of fist-sized organs located below the ribs toward the middle of the back. The kidneys remove excess liquid and wastes from the blood in the form of urine. They keep a stable balance of salts and other substances in the blood, and produce a hormone that helps to form red blood cells. Narrow tubes called ureters carry urine from the kidneys to the bladder - a sack-like organ in the lower abdomen.