Video 3 - Urinary System: Diuresis, Anatomy, Physiology & Pathologies

Diuresis – Definition & Everyday Examples

• Diuresis = production of a large volume of urine.
  • Triggered when water re-absorption in nephrons falls.
  • Everyday diuretics (non-drug):
  • Plain water ("milk-jug of water" illustration → constant restroom trips).
  • Iced tea or other caffeine-containing drinks.
  • Metaphor: Think of the kidney as a faucet—diuretics open the valve wider so more water exits.

Pharmacological Diuretics – Sites & Mechanisms

• Goal: lower water re-absorption, ↑ urine output.
• Key drug classes (know site + mechanism):
  • Furosemide (Lasix)
  • Loop diuretic; blocks Na⁺ re-uptake in the ascending loop of Henle.
  • \text{Less }Na^+ \Rightarrow \text{Less }H_2O \text{ reabsorbed} → potent diuresis.
  • Thiazides
  • Act on the distal convoluted tubule (DCT).
  • Keep Na⁺ inside the tubule → water follows → ↑ excretion.
  • Spironolactone
  • Aldosterone antagonist (collecting duct / DCT).
  • Prevents aldosterone-mediated Na⁺ re-uptake and K⁺ secretion → mild, “K⁺-sparing” diuresis.

Urine Composition & Odor

• Varies with diet, hormones, and metabolism (e.g., asparagus).
• Contains scores of solutes; ammonia chiefly responsible for characteristic smell (odor molecules become airborne, detected by nasal chemoreceptors).

Ureters – Structure & Peristalsis

• Pair of muscular tubes from renal pelvis → bladder.
• Peristalsis moves urine:
  • Circular smooth muscle contracts sequentially, propagating waves (toothpaste-tube analogy).
• Histology:
  • Thick smooth-muscle coat.
  • Transitional epithelium lines lumen → stretches with volume.

Urinary Bladder – Anatomy & Histology

• Hollow, muscular reservoir; male model identified by prostate inferior to bladder.
• External supports:
  • Posterior inlets = ureters.
  • 3 round ligaments (left, midline, right) stabilize bladder.
• Wall layers:
  • Detrusor muscle (unique, powerful smooth muscle).
  • Mucosa with rugae (pleated folds) → accommodates expansion.

Urethra & Sphincters – Sex Differences & Control

• Two sphincters, both muscular rings:
  1. Internal urethral sphincter – involuntary (smooth muscle).
  2. External urethral sphincter – voluntary (skeletal muscle).
  • Learned during childhood “potty training.”
• Length disparity:
  • Female urethra = short → ↑ risk of UTIs.
  • Male urethra = long, traverses prostate & penis.

Micturition (Voiding) Reflex – Neural Sequence

• Filling phase
  • Stretch receptors in bladder wall → afferent signals.
  • Early signals elicit guarding reflex: detrusor relaxes while sphincters stay closed.
• Urgency phase
  • Receptors fire more frequently → message to micturition center (pons).
  • Brain decides: contract detrusor or keep guarding.
• Involuntary release if volume overwhelms external sphincter capacity → unavoidable urination.
• Infants lack corticospinal connections → cannot voluntarily inhibit external sphincter. (Stories of “potty-trained 6-week-olds” = parental cueing, not true voluntary control.)

Urinary Tract Infections (Acute Cystitis)

• Commonly caused by E. coli (fecal origin).
• More frequent in women until ~60 yrs.
• Symptoms: frequency, burning, incomplete emptying, possible hematuria & back pain.
• Cranberry juice
  • Contains compounds that inhibit bacterial adhesion to mucosa—useful as prevention, not cure.

Urinary Incontinence – Etiologies & Lifestyle Triggers

• Definition: inability to voluntarily control urination.
• Causes:
  • Pelvic/abdominal trauma.
  • Pregnancy & childbirth (bladder compressed → sphincter misalignment).
  • CNS/PNS disorders (e.g., multiple sclerosis).
  • Age-related muscle weakening.
• Exacerbating foods/drinks (“bladder irritants”):
  • Caffeine, carbonation (bubbles), artificial sweeteners, spicy foods.
• Management → limit irritants, pelvic-floor exercises (Kegels), medical/surgical interventions.

Nephrolithiasis (Kidney Stones)

• Composition: calcium oxalate, magnesium salts, or uric-acid crystals.
• Pathway & pain points:
  1. Form on kidney’s inner surface.
  2. Dislodge → renal pelvis (minimal pain).
  3. Enter ureter → intense pain (↑ capsular pressure, ureter spasms).
  4. Ureter → bladder → urethra: each narrow region can be excruciating.
• Treatment: hydration + analgesia; lithotripsy (ultrasonic shock waves) to fragment stones; surgery uncommon.

Renal Failure – Systemic Impact & Therapies

• Hypertension major precipitant (glomeruli already under high pressure; renin–angiotensin system worsens load).
• Consequences: disrupted H_2O & electrolyte balance, acid–base imbalance, neuromuscular & digestive dysfunction.
• Interventions:
  • Diet ↓ Na⁺, ↓ protein, fluid management.
  • Dialysis – extracorporeal blood filtration (artificial kidney). Mechanism: diffusion of urea, creatinine, ions across semipermeable membrane, then blood is returned.
  • Transplant – often definitive; success rates improving.

Glomerulonephritis – Post-Infectious Inflammation

• Trigger: large antigen–antibody complexes after infections (e.g., strep).
• Complexes clog filtration membrane → inflammation, ↓ GFR.
• Presents days–weeks after illness; monitor for hematuria, edema, hypertension.

Practical & Ethical / Developmental Connections

• Pediatric care: Recognize neuro-developmental limits on continence → avoid premature potty-training expectations.
• Geriatrics: Counsel on irritant avoidance & bladder training to maintain dignity and quality of life.
• Public health: High prevalence of hypertension & diabetes → rising end-stage renal disease; importance of screening & lifestyle education.
• Environmental health: Proper disposal of unused diuretic drugs to avoid water contamination.

Study Tips & Concept Links

• Link diuretic site of action to nephron anatomy from previous lectures.
• Remember: “Na⁺ pulls H₂O.” Any drug/pathology that leaves Na⁺ in tubule → more water lost.
• Compare peristalsis in ureters to GI tract (Chapter 22) for muscle-layer similarities.
• Recall transitional epithelium from histology lab—also lines bladder, not just ureters.
• Ethics: Incontinence products & coverage—access, dignity, and cost considerations.