Urinary bladder

The urinary bladder is a hollow, collapsible, muscular sac that stores urine temporarily. It is located in the pelvis behind the pelvic bones, and is held in place by ligaments. In women, the bladder is behind the uterus; in men, it is above the prostate gland. The size of the urinary bladder varies depending on the amount of urine it contains. When empty, it is normally no longer than 2 to 3 inches. During this state, its walls are thick and heavily folded. As it begins collecting urine, the urinary bladder's muscular walls stretch and expand, and it rises in the abdominal cavity. A urinary bladder that is moderately full measures about 5 inches in length and holds just over 1 pint of urine. When completely full, the urinary bladder can contain over 2 pints of urine. The muscular walls of the urinary bladder contract to expel urine out of the bladder into the urethra. A sphincter or ring of muscle surrounding the opening to the urethra, called the internal urethral sphincter, controls the flow of urine. This is an involuntary sphincter, meaning a person cannot consciously control its workings.