How does a normal bladder work?
The bladder is a hollow organ in the lower abdomen. It stores urine, the liquid waste produced by the kidneys. Urine passes from each kidney into the bladder through a tube called a ureter. Urine leaves the bladder and exits the body through another tube called the urethra.
As the bladder fills with urine, pressure is exerted on its walls, which you experience as the need to urinate. This triggers the brain to send a message to the layer of muscle that surrounds the inner lining of the bladder, forcing the muscle to contract (tighten) and the urine to flow out of the bladder.
At the same time, the sphincter muscle that surrounds the urethra relaxes, letting the urine flow out of the body. This process requires both nerves and muscles working together to prevent urine leakage. Damage, weakening or injury to either muscles or nerves can result in incontinence.