Chapter 71 – Micturition


The bladder is a hollow, muscular organ situated in the pelvis. Its role is the storage and voiding of urine. Urine is produced in the kidneys, enters the bladder through the ureters and exits via the urethra.

Chapter 71 Micturition

How is urine stored and excreted from the body?

The bladder is a hollow, muscular organ situated in the pelvis. Its role is the storage and voiding of urine. Urine is produced in the kidneys, enters the bladder through the ureters and exits via the urethra.

Important aspects of lower urinary tract anatomy are:

  • Urothelium lines the inner wall of the ureters, bladder and urethra, providing a highly impermeable barrier to ion, solute and water flux. This is the same type of epithelium that lines the renal collecting duct and renal pelvis (see Chapter 67).

  • The detrusor muscle is the smooth muscle of the bladder wall and is arranged in spiral, longitudinal and circular bundles. The detrusor muscle exhibits an unusually high compliance – as the bladder fills, the detrusor muscle relaxes and stretches, resulting in only a small increase in intravesical pressure. This is known as viscoelasticity. The detrusor is innervated by both the sympathetic (via the hypogastric nerve) and parasympathetic (via the pelvic splanchnic nerves) nervous systems and is therefore under involuntary control.

  • The internal urethral sphincter (IUS) surrounds the urethra at its junction with the bladder. The IUS is an extension of the detrusor smooth muscle and is under control of the sympathetic nervous system.

  • The external urethral sphincter (EUS) is a portion of striated muscle that surrounds the urethra distal to the internal sphincter (females) or distal to the prostate (males). Because the EUS consists of striated muscle, it is under voluntary control via the pudendal nerve.

What is urinary continence? How is it achieved?

Urinary continence is the ability to store urine without leakage until the bladder can be emptied at an appropriate time. Biomechanically, urine will not leak from the bladder so long as the outlet resistance exceeds intravesical pressure. Overall control of urinary continence is directed by two pontine centres: the storage centre and the micturition centre.

During the storage phase of micturition, the pontine storage centre increases sympathetic nervous system outflow, causing:

  • Relaxation of the detrusor muscle through stimulation of β3-adrenoreceptors in the fundus and body of the bladder;

  • Tonic contraction of the IUS through stimulation of α1-adrenergic receptors at the bladder neck.

The coordinated relaxation of the bladder and contraction of the IUS allows the bladder to fill without leakage of urine.

What happens to intravesical pressure as the bladder fills?

As the bladder fills and its radius increases, bladder wall tension increases due to the law of LaPlace (see Chapter 20). Bladder wall compliance is relatively high at low urine volumes; above 400 mL capacity, the compliance of the bladder wall decreases substantially. When intravesical pressure rises above 10 cmH2O (around 200 mL stored urine):

  • Stretch receptors in the bladder wall are activated. Afferent signals are relayed to the lumbar spinal cord via the hypogastric nerve, where two reflexes are triggered:

    1. The micturition reflex triggers firing of parasympathetic cholinergic neurons (originating from the S2–S4 segments of the spinal cord), resulting in a transient contraction of the detrusor muscle. The micturition reflex is self-regenerative; that is, detrusor contraction results in further bladder wall stretch receptor activation, which triggers further detrusor contraction.

    2. The guarding reflex is a reflex contraction of the EUS during these transient detrusor contractions that prevents urinary leakage.

  • Sensation of bladder fullness: the afferent signals produced by the stretch receptors are interpreted by higher brain centres as a sensation of bladder fullness. As the bladder fills, the micturition reflex occurs more frequently and the detrusor contracts more powerfully:

    1. The first desire to void occurs when the bladder contains around 200 mL of urine.

    2. The bladder feels ‘uncomfortably full’ at around 350–400 mL.

    3. When the bladder reaches 700 mL capacity, pain ensues.

  • Involuntary micturition: once the micturition reflex is powerful enough, it causes reflex opening of the EUS (unless higher centres inhibit this) and micturition (voiding) occurs. This is the situation for babies and infants, before full maturation of the pontine micturition centre has developed.

  • Voluntary micturition:

    1. If voiding is not desired or is inconvenient, the frontal cortex instructs the pontine control centres to increase sympathetic outflow and decrease parasympathetic outflow, which relaxes the detrusor and contracts the IUS, preventing leakage of urine. As the micturition reflex becomes more powerful, the pontine control centres also initiate tonic contraction of the EUS to counteract the increasing intravesical pressure. Higher centres can also prevent voiding when the micturition reflex is becoming more powerful by voluntary tonic contraction of the EUS.

    2. When it is convenient to urinate, abdominal muscles contract to increase the pressure within the bladder, thus stretching the bladder walls. The pontine micturition centre simultaneously triggers a micturition reflex and inhibits the EUS so that urination can occur.

Sep 27, 2020 | Posted by in ANESTHESIA | Comments Off on Chapter 71 – Micturition
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