Urine formed by the kidney is transported from the renal pelvis through the ureters and into the bladder. The first sensations of bladder filling ordinarily occur when about 100 to 150 milliliters of urine are present in the bladder. In most cases, there is a desire to void when the bladder contains approximately 200-300 milliliters. With 400-500 milliliters, a marked feeling of fullness is usually present.

With over-distention of the bladder, due to disease or injury, the elevated pressure in the bladder can be transmitted back through the ureters leading to ureteral distention and possible reflux of urine. This can lead to kidney infection (pyelonephritis) and damage from the elevated pressure (hydronephrosis). This can eventually result in renal failure.

Voiding of urine is prevented by contraction of the external urethral sphincter (muscle). This muscle is under voluntary control and is innervated by nerves from the sacral area of the spinal cord. Voluntary control is a learned behavior that is not present at birth. When there is a desire to void, the external urethral sphincter is relaxed and the detrusor muscle (smooth muscle of the bladder walls) contracts and expels the urine from the bladder through the urethra.

If the pelvic nerves to the bladder and sphincter are destroyed, voluntary control and reflex urination are destroyed, and the bladder becomes over-distended with urine. If the spinal pathways from the brain to the urinary system are destroyed (as in spinal cord transection), the reflex contraction of the bladder is maintained, but voluntary control over the process is lost. In both of these types of loss of bladder innervation, the muscle of the bladder can contract and expel urine, but the contractions are generally insufficient to empty the bladder completely, and residual urine is left behind, thus the need for catheterizations.

(KGH Learning Guide- Urethral Catheterization Adult 2003)