The bladder in humans is located in the pelvis. Urine formed by the kidneys empties into the bladder by means of drainage tubes called ureters. Urine exits the bladder via a tube known as the urethra. A group of muscles surrounds the bottom of the bladder (urethral sphincter) forming a ring, that stops urine leaking from the bladder.
The human bladder has the capacity to hold at least 500cc of urine. When the bladder begins to fill, its muscles relax, allowing the lining to stretch and expand. The bladder then tells our brain what is happening via a complex system of reflexes; this results in our sensation of the "desire to urinate." In the next step, the brain coordinates a series of events that allow the urethral sphincter to relax and the muscles in the bladder wall to contract. This pushes urine out through the urethra.