Part of the TeachMe Series

Voiding Phase of Micturition

star star star star star
based on 14 ratings

Original Author(s): Joe Reynolds
Last updated: 28th June 2021
Revisions: 19

Original Author(s): Joe Reynolds
Last updated: 28th June 2021
Revisions: 19

format_list_bulletedContents add remove

Urination is the process of excreting urine from the urinary bladder. This is also known as the voiding phase of micturition.

Most of the time, the bladder (detrusor muscle) is used to store urine. As it fills, the folded bladder walls (rugae) distend and a constant pressure in the bladder (intra-vesicular pressure) is maintained. This is known as the stress-relaxation phenomenon. The ability to voluntarily control micturition develops from 2 years as the CNS develops.

Micturition

Micturition is also known as the voiding phase of bladder control and it is typically a short-lasting event. Urinary flow rate in a full bladder is:

  • 20-25ml/s in men
  • 25-30ml/s in women

Whilst the capacity of the bladder varies from roughly 300-550ml, afferent nerves in the bladder wall signal the need to void the bladder at around 400ml of filling.

Voiding Phase of Micturition

Passing of urine is under parasympathetic control. Bladder afferents signals ascend through the spinal cord and then project to the pontine micturition centre and cerebrum. Upon the voluntary decision to urinate, neurons of the pontine micturition centre fire to excite the sacral preganglionic neurons.

There is subsequent parasympathetic stimulation to the pelvic nerve (nerve roots S2-4) causing a release of acetylcholine (ACh), which works on muscarinic ACh receptors (M3 receptors) on the detrusor muscle, causing it to contract and increase intra-vesicular pressure. The pontine micturition centre also inhibits Onuf’s nucleus, with a resultant reduction in sympathetic stimulation to the internal urethral sphincter causing relaxation.

Finally, a conscious reduction in voluntary contraction of the external urethral sphincter from the cerebral cortex allows for distention of the urethra and the passing of urine. In the female, urination is assisted by gravity, while in the male, bulbospongiosus muscle contractions along the length of the penis helps to expel all of the urine.

Clinical Relevance

Urinary Retention

Urinary retention is the inability to void the bladder i.e. being unable to urinate. It can be caused by a variety of conditions, such as:

  • Benign prostatic hyperplasia (BPH) – this is the most common cause of urinary incontinence. BPH causes
  • Nerve dysfunction
  • Infection – e.g. UTI
  • Constipation
  • Drugs – medications such as anticholinergics, antidepressants and opioids can cause incontinence.

Patients with urinary incontinence typically present with:

  • Intermittent flow
  • Straining
  • Vesical tenesmus (the feeling of incomplete emptying of the bladder following urination)
  • Hesitancy (a delay between trying to urinate and the urine stream beginning)

Complications include:

  • Urinary incontinence
  • Nocturia (the need to urinate at night)
  • Hydronephrosis – high pressure in bladder can push urine back up ureters into kidneys. This causes the renal pelvises to expand
  • Kidney failure
  • Sepsis
  • Bladder rupture – retention can lead to anuria (inability to pass urine). This can cause the bladder to stretch and possibly tear

In an acute setting, treatment includes urinary catheterisation, prostatic stenting or a suprapubic cystostomy. Long-term treatment is dependent on cause. BPH can be managed pharmacologically through drugs such as alpha-blockers e.g. tamsulosin or 5-alpha-reducatase inhibitors e.g. finasteride. Alternatively, if indicated, it can also be surgically treated through prostatectomy or transurethral prostatic resection.

TeachMePhysiology