The urinary bladder is a hollow muscular organ that stores urine. It is situated in the pelvis and is approximately the size and shape of a pear when empty, notes WebMD. The bladder expands to varying sizes depending on the volume of urine it contains, according to Inner Body.
Urine is produced in the kidneys and then flows to the bladder via two ureters where it is stored until voluntary urination. The bladder normally holds 400 to 600 milliliters of urine, WebMD explains. Tiny wrinkles called rugae, as well as several layers of muscle inside the bladder, have elastic properties that allow the bladder to stretch to accommodate increased capacities of fluid, explains Inner Body. During urination, the muscles contract, and two sphincters open to allow urine to flow out through the urethra. Urination is designed to be an infrequent event throughout the day due to the storage capacity of the bladder.