The urinary bladder is a hollow muscular organ that acts as a reservoir for urine. The adult bladder can hold a pint or more of fluid. It lies behind the pubic bone and is protected by the pelvis. The bladder walls are made up of muscle and an inner lining. At the back are two ureters, which carry the urine from the kidneys. The bladder collects and stores urine until it can be expelled from the body.