Storage of urine and micturition
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)