The urinary system includes several structures, the kidneys, the ureters, the bladder and the urethra. These four structures occupy the posterior abdominal part and the pelvic cavity. The kidneys are partially protected by the lower part of the rib cage. The right kidney is slightly lower than the left kidney. Each ureter starts from the kidney, runs along the side of the lumbar spine vertically downward. They also pass back from the uterus into the pelvic cavity. They end in the posterior wall of the bladder with an orifice at each end of the bladder trigone. The urinary tract produces, stores and eliminates urine, wastes and toxins. It maintains homeostasis, that is to say inner balance.