M

Urine Transport, Storage, and Elimination

Ureters

  • Paired epithelial-lined fibromuscular tubes conducting urine kidney → bladder
  • Exit kidney at hilum; enter posterolateral bladder wall at oblique angle → compression during filling prevents reflux
  • Histology
    • Mucosa: transitional epithelium + lamina propria; folds permit distension
    • Muscularis: inner longitudinal & outer circular smooth muscle; peristaltic waves propel urine
  • Dual autonomic innervation (sympathetic & parasympathetic)

Urinary Bladder

  • Distensible muscular sac posterior to pubic symphysis; capacity ≈ (700\text{–}800\ \text{mL})
  • Wall
    • Mucosa: transitional epithelium + rugae (expansion)
    • Muscularis (detrusor): inner longitudinal, middle circular, outer longitudinal layers; autonomic control
  • Trigone: immobile, funnel-shaped region between ureteric orifices → directs urine to urethra
  • Sphincters
    • Internal urethral sphincter: smooth muscle, involuntary (ANS)
    • External urethral sphincter: skeletal muscle, voluntary (somatic pudendal nerve)

Micturition (Urination) Reflex

  • Stimulus: bladder volume (200\text{–}400\ \text{mL}) → wall stretch → baroreceptors
  • Afferent path: stretch signals to sacral micturition center (S2–S4) & pontine micturition center (PMC)
  • Reflex (parasympathetic pelvic splanchnic nerves)
    1. Detrusor contracts
    2. Internal sphincter relaxes
    3. Inhibition of pudendal nerve → external sphincter relaxes → urine expelled
  • Voluntary control: cerebral cortex can inhibit PMC → detrusor relaxes & both sphincters contract (delay voiding)
  • Loss of control
    • Infants (cortex undeveloped)
    • Spinal cord lesion above S2 or pontine damage → reflex incontinence

Urethra

  • Epithelial-lined fibromuscular tube: bladder (internal orifice) → exterior (external orifice, stratified squamous)
  • Contains same two sphincters as listed above
  • Sex differences
    • Female: short; only urinary
    • Male: longer; conveys urine & semen (urinary + reproductive)

Key Homeostatic Roles of Urinary System

  • Works with cardiovascular system to regulate blood volume & pressure
  • Cooperates with respiratory system to maintain extracellular \text{pH}