What is the filling and emptying cycle of the bladder?

The normal bladder fills and empties in cycles.
 Urine production by the kidneys is continuous – about 1ml per minute or 60ml an hour.

^{Figure 2.8: The bladder cycle of filling and emptying.}

Phase 1: Filling and storage

During the filling phase, the bladder volume increases with very little change to the internal pressure – this is termed compliance. Normal pressure rise on filling from 10ml to 400ml is only about 10cm H₂O.

As the bladder fills it becomes spherical and then pear-shaped as it rises up out of the pelvic cavity.

During filling, the detrusor is relaxed and the bladder neck and external sphincter are contracted maintaining continence.

The first desire to void is usually experience at 150-200ml capacity which can be suppressed by conscious inhibitory control.

As the bladder fills up, the desire becomes stronger and more difficult to suppress.
 When the bladder desire is strong the individual will display behaviours to help them hold on, for example becoming restless, hopping from one foot to another, fidgeting, wandering.


Phase 2: Voiding

Voiding is initiated voluntarily and can be delayed until a suitable time and place.

The external sphincter and bladder neck relax and simultaneously the detrusor muscle contracts. This is co-ordinated via the spinal-pontine-spinal reflex which involves the micturition control centre in the pons.

When the external sphincter relaxes the pressure in the urethra is lowered and the pelvic floor relaxes allowing the bladder neck to descend and open. This is known as funnelling.

The parasympathetic nerves stimulate the detrusor contraction, thus increasing the intravesical pressure and the urine is expelled under pressure which is about 100cm H₂O.

Phase 3: Termination of voiding

As emptying is completed the flow reduces and ends:

• External sphincter closes under voluntary control
• Urethra contracts forcing urine above the level of the external sphincter back up into the bladder
• Cortical micturition control centre takes control; this inhibition allows the filling cycle to start again