The urinary bladder is a hollow, muscular organ located in the pelvis that stores urine produced by the kidneys. It can hold about 400 to 600 milliliters of urine and expands as it fills. The bladder has a lining called the urothelium that allows it to stretch and contract. When the bladder is full, nerve signals prompt the urge to urinate. The process of releasing urine involves the relaxation of the external urethral sphincter and the contraction of the bladder muscles. This coordinated action allows urine to flow from the bladder through the urethra and out of the body.