The urinary system, or the genitourinary system, is the organ system that excretes, stores, and eliminates urine. The urinary system consists two kidneys, two ureters, the bladder, the urethra, and the penis in males. The nephron, which is an evolutionary modification of the nephridium, is the kidney's functional unit. Waste is filtered from the blood and collected as urine in each kidney. Urine leaves the kidneys by ureters, and collects in the bladder. The bladder can distend to store urine that eventually leaves through the urethra.