The sphincter muscle sits right below the bladder. If you are not ready to empty your bladder, then the sphincter muscle tightens up, like a knot at the bottom of a water-filled balloon. When you decide that it is time to empty your bladder, the sphincter muscle opens up, like untying the knot. The process of urination then begins as the bladder pushes the urine out down a tube called the urethra. The urethra opens to the outside of your body.
Urination takes place only when the muscle and nerves are working correctly.
As streets allow cars to carry people from one place to another, so nerves carry messages from the brain to the bladder and sphincter muscles. The urodynamics test helps to determine how well these nerves and muscles are working together. The urodynamics tests takes approximately two hours. This time is spent registering, asking questions you may have, changing clothes and having small catheters put into you bladder for the actual test. You may eat and drink anything you want before the test, unless given other instructions by your doctor.