The bladder is an expandable, muscular sac. Urine accumulates in the bladder as it arrives from the ureters.
The bladder gradually increases in size to accommodate an increasing volume of urine. When the bladder is full, nerve signals are sent to the brain to convey the need to urinate. When a person urinates, the urinary sphincter, located at the bladder's outlet (where the bladder and urethra meet), opens to allow urine to flow out. Simultaneously, the bladder wall contracts automatically, creating pressure that forces the urine down the urethra. Voluntarily tightening the muscles of the abdominal wall assists by adding extra pressure. The ends of the ureters in the bladder wall remain tightly shut during contraction of the bladder to prevent urine from flowing back into the ureters toward the kidneys.
(See also Overview of the Urinary Tract.)