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Function (Physiology) of the Bladder Filling and Micturition

References: (Benninghoff, 1993).

Biomechanical Basics of the Urinary Bladder

Laplace's Law:

The wall tension of a spherical body (bladder) is dependent on its internal pressure (P_ves), from the wall thickness (d) and radius (r) of the body:

wall tension = (P_ves × r) / (2d)

The wall tension increases during urinary retention, although the bladder pressure remains relatively constant (overflow incontinence). The wall tension increases because of the enlarged radius and the decrease in wall thickness. This leads sometimes to an enormously dilatated bladder

Compliance of the bladder:

The compliance (elasticity) of the bladder depends on both neuromuscular factors and the quantity of connective tissue in the bladder wall. The bladder compliance is calculated by the increase in bladder volume as a function of intravesical pressure (normal 20–60 ml/cm H₂O):

bladder compliance = (change in volume) / (change in bladder pressure)

Detrusor Pressure:

The detrusor pressure can be calculated by the difference of the intravesical pressure and intra-abdominal pressure. Both pressures can be easily measured with the help of Catheter in bladder and rectum:

Detrusor pressure = (intravesical detrusor) − (abdominal pressure)

Urethral Resistance:

The urethral resistance can be calculated using the maximum urinary flow (Q_max) and the detrusor pressure at maximum urine flow (P_det at Q_max) using the formula below:

Urethral resistance = (P_det at Q_max)/Q_max²

Normal values for the urethral resistance using the formula above are for men <0.6 and women <0.2. The formula should only be used with voiding volumes between 200–400 ml. The low urethral resistance explains the very low and sometimes not measurable detrusor pressures in women during normal micturition.

Neural Control of the Bladder

Two functional states of the urinary bladder must be distinguished: the storage phase and emptying phase (micturition, voiding).

Storage Phase of the Bladder:

The filling of the bladder is enabled by:

• contraction of the striated sphincter (somatic innervation)
• contraction of smooth muscle sphincter (sympathetic innervation)
• inhibition of detrusor activity (sympathetic innervation)

Emptying Phase of the Bladder:

Voiding (micturition) is enabled by:

• relaxation of the striated sphincter (somatic innervation)
• relaxation of the smooth muscle sphincter and opening of the bladder neck (sympathetic innervation)
• detrusor contraction (parasympathetic innervation)

Neural Reflex Arcs to Control the Bladder Function:

The afferent signals from stretch and volume receptors transmit information about bladder filling to centers of the spinal cord and CNS. Depending on the storage phase, several reflex arcs are activated:

Spinal pathways:

Spinal reflexes inhibit micturition during the filling phase and activate the striated sphincter (pudendal nerve), inhibit the detrusor muscle and activate the smooth muscle sphincter via activation of the sympathetic nervous system. Afferent signals for the activation of the spinal pathway derive from the bladder (increasing bladder filling), from the pelvic floor muscles, the penis, vagina and rectum. These mechanisms explain the risk of urinary retention after operations in the regions mentioned above. The spinal reflexes are also the basis for the sacral neurostimulation in therapy of the overactive bladder.

Pontine Micturition Center:

An increasing bladder filling increases the afferent neuronal activity of the bladder, this activates the pontine micturition center in the brainstem (pons). The pontine micturition center (Barrington's nucleus) inhibits the spinal reflexes: this causes the activation of the detrusor muscle and inhibition the urinary sphincter.

Central pathways:

Central pathways inhibit the micturition reflex. At a certain bladder filling, the filling is consciously perceived and afferent signals thus passed on cortical centers. The initiation of micturition is voluntary controlled, the central pathways can inhibit the pontine micturition center over a certain range of the bladder filling. Lesions of the cortical micturition center leads to the elimination of the inhibitory pathways and to an overactive bladder.

Like any striated muscle the external sphincter of the urinary bladder is voluntarily controlled via the pyramidal tract and the extrapyramidal system.

Urethra to bladder reflexes:

Urine flow or mechanical stretching of the urethra cause a stimulation of bladder contractions. This reflex has an important function in complete bladder emptying. The reflex serves as an explanation for the combined urge and stress incontinence in women: stress-induced urine leakage into the urethra leads to a detrusor contraction.

References

  Deutsche Version: Physiologie der Harnblase