micturition (urination)

Physiology

• detrusor muscle
  • muscarinic cholinergic innervation, sacral plexus S2-S4
  • contracts bladder causing the bladder to empty
• bladder outlet
  • internal sphincter
    • alpha-adrenergic innervation, hypogastric plexus, T11-L2
    • contraction of the internal sphincter retains urine in bladder
  • external sphincter
    • somatic innnervation from pudendal nerve
• storage of urine
  • detrusor relaxation, closure of sphincters
  • normal bladder capacity is 300-600 mL
  • 1st urge to void is between 150-300 mL
• emptying
  • detrusor contraction & opening of the sphincters
  • detrusor pressure exceeds urethral resistance
• central nervous system
  • pontine micturition center facilitates urination
  • frontal cortex inhibits urination

Urinary changes with aging

• decrease in
  • bladder capacity & force of contractions
  • ability to postpone voiding (frequency)
  • urethral compliance
  • strength in pelvic support muscles
• increase in
  • post-void residual volume (> 100 mL is abnormal)
  • involuntary bladder contractions (urgency)
  • nocturia 1-2X/night

More general terms

• physiologic process

Additional terms

• age-associated changes in the urinary system
• micturition/postmicturition syncope
• oliguria
• polyuria
• urinary incontinence (UI)