The main role of the bladder is to store the urine which continuously enters it, in order to achieve convenient intermittent voiding. Thus the bladder must act as an efficient low pressure continent reservoir. Urine from the kidneys enters the bladder via the ureters at a rate of 0.5-5 ml/min. Normally, the first sensation of bladder filling is noted at between 150 and 250 ml and there is a strong desire to void at approximately 400-600 ml (bladder capacity). During filling the bladder pressure should not normally rise by more than 10 cm of water to 300 ml, or 15 cm of water to 500 ml. In order to maintain continence the maximum urethral pressure must exceed the bladder pressure at all times except during micturition. Thus, for continence to exist it is not only essential that the intravesical pressure remains low but also that the urethral lumen should seal completely. Three essential components of urethral function are required to achieve hermetic closure: (1) urethral inner wall softness; (2) inner urethral compression; and (3) outer wall tension. These three functions are dependent on an intact urothelium, together with a major component from the submucosal vascular plexus as well as the collagen and elastic tissue within the urethra and the striated and smooth muscle.
During this time the urethra remains closed as previously described. Proprioceptive afferent impulses from the stretch receptors within the bladder wall pass via the pelvic nerves to the sacral roots S2-4. These impulses ascend the cord via the lateral spinothalamic tracts and the detrusor motor response is subconsciously inhibited by descending impulses from the basal ganglia. Gradually, as the bladder volume increases, further afferent impulses are sent to the cerebral cortex and the first sensation of desire to void is usually appreciated at about half the functional bladder capacity. Inhibition of detrusor contraction becomes cortically mediated. As the bladder fills further, these afferent impulses reinforce the desire to void and conscious inhibition of micturition occurs until a suitable time. When functional capacity is reached, voluntary pelvic floor contraction is initiated to aid urethral closure. This may result in marked variations in urethral pressure as the sensation of urgency develops.
At a suitable time and place, cortical inhibition is released and relaxation of the pelvic floor occurs, together with relaxation of the intrinsic striated muscle of the urethra. This results in a fall in urethral pressure which occurs a few seconds prior to the increase in bladder pressure. A few seconds later, a rapid discharge of efferent parasympa-thetic impulses via the pelvic nerve causes the detrusor to contract and also possibly to open the bladder neck and shorten the urethra. The detrusor pressure rises by a variable amount, normally less than 60 cm of water in women. However, it may not need to rise at all if the fall in urethral resistance is adequate for the urethral pressure to be lower than the intravesical pressure, so that urine is voided.
Once micturition has been initiated the intravesical pressure normally remains constant. The efficiency of detrusor contraction increases as the muscle fibres of the detrusor shorten, therefore decreasing the forces which are required to maintain micturition.
Interruption of micturition is usually achieved by contraction of the extrinsic striated muscle of the pelvic floor, associated with a rise in urethral pressure to exceed the intravesical pressure and thus stop the flow of urine. Since the detrusor is composed of smooth muscle, it is much slower to relax and therefore continues to contract against the closed sphincter; this causes an isometric detrusor contraction which will eventually die away to the premicturition detrusor pressure.
When the bladder empties at the end of micturition, the urinary flow stops, the pelvic floor and intrinsic striated muscle of the urethra contract and any urine which is left in the proximal urethra will be milked back into the bladder. As the urethra closes off, subconscious inhibition of the sacral micturition centre is reinstituted and the bladder storage phase begins again.
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