These pages have some information and links for patients.
Understanding how the bladder works
Urine Storage
The kidneys make urine all the time. A trickle of urine is constantly passing to the bladder down the ureters (the tubes from the kidneys to the bladder). You make different amounts of urine depending on how much you drink, eat and sweat.
The bladder is made of muscle and stores the urine. It expands like a balloon as it fills with urine. The outlet for urine (the urethra) is normally kept closed. This is helped by the muscles beneath the bladder that sweep around the urethra (the pelvic floor muscles).
When a certain amount of urine is in the bladder, you become aware that the bladder is getting full. When you go to the toilet to pass urine, the bladder muscle contracts (squeezes), and the urethra and pelvic floor muscles relax.
Complex nerve messages are sent between the brain, the bladder, and the pelvic floor muscles. These tell you how full your bladder is, and tell the right muscles to contract or relax at the right time.
Micturition (process of urine excretion)
The normal micturition cycle provides for storage and emptying phases. During the storage phase, the bladder fills without pressure increase and the urethral sphincter and pelvic floor contract. A first sensation to void occurs at about 150 cc of filling. Urine starts to accumulate during bladder filling, causing distension of the bladder which generates signals that are transmitted to the brain. If the brain considers that micturition is inappropriate, it sends an inhibitory message downward.
When the normal desire to void occurs (at about 400-500 cc of filling) and a woman wishes to empty her bladder, the bladder contracts, causing an increase of pressure inside the bladder; the urethra and pelvic floor relax and micturition occurs. At the completion of voiding, the cycle returns to the storage phase.