The urinary bladder (or simply the bladder) is a hollow muscular organ with distensible walls. It acts as both a temporary reservoir and a pump for urine. The muscle cells (detrusor muscle) within the walls of the bladder contract during micturition, and in that way, the bladder assists in the expulsion of urine. The bladder is continuous with the ureters (Read more!) above and the urethra (Read more!) below it.