According to WebMD, the bladder is about the shape and size of a pear when it is empty. The bladder is a muscular sac that is located in the pelvis, just behind the pubic bone.
WebMD says that the normal capacity of an adult bladder is about 400 to 600 milliliters. Since the bladder is lined with layers of muscles, it is able to stretch to accommodate urine. The purpose of the bladder is to store urine so that urination is infrequent and voluntary. Urination is caused by the muscles of the bladder contracting to force urine out of the body.