The parts of the urinary tract are illustrated above.

 The Kidneys are two fist-sized organs located in the back of the abdominal cavity. The kidneys perform complex functions that are vital to life. They constantly receive blood from throughout the body and filter it to remove toxic wastes from the blood and regulate the water volume and chemical concentrations of the body. In this blood filtering process, the kidneys use fluid and waste products to form urine.

 The Ureters are tubes through which urine leaves the kidneys and travels to the bladder. The ureters enter the bladder through the ureterovesical junction, the valve in the muscle wall between the ureter and the bladder. The function of this one-way valve is to prevent urine from flowing backward to the kidneys from the bladder (reflux). Under normal circumstances, this valve prevents kidney damage by preventing reflux.

 The Bladder is located in the pelvis and composed of intertwining layers of muscles. It serves as a reservoir for urine.

The Bladder neck is the outlet of the bladder and is formed by layers of bladder muscle. When the bladder contracts, these layers of muscle pull the bladder neck open into a funnel-shape so the urine can flow out easily.

 The Urethra is the tube through which urine leaves the bladder and is discharged outside the body.

The External sphincter is part of the urethra, located below the bladder neck and surrounded by a circular muscle. The external sphincter muscle can tighten to prevent the passage of urine, or it can relax and open to allow urine to flow out of the bladder.

     Normal urination, or voiding, depends on an intact nerve supply. Prior to brain injury when the bladder was filled with urine, the nerves in the bladder were stimulated and a message was sent along the nerves, through the spinal cord, to the brain that told you your bladder was "full." When the message was received by your brain, you felt the sensation of fullness and realized you needed to urinate.

     If you were in a situation where you could not urinate, the brain sent a message down the spinal cord that told the external sphincter muscle to tighten and the bladder to remain relaxed. When this message was received by the external sphincter muscle and the bladder, you could voluntary stop the bladder from contracting and hold back the flow of urine into the urethra.  When you reached a bathroom and could urinate, the brain sent a message down the spinal cord telling the external sphincter muscle to relax and open and the bladder to contract.

     Although this may sound like a fairly simple process, in fact, normal urination is a very complex process that requires an intact nerve supply, coordination of the voluntary and involuntary nerves, and the ability to recognize and respond appropriately to bodily changes.