T3 L11: Lower urinary tract

• What is the urinary tract?

  Ureters

  Urinary bladder

  Urethra

  
• How does the final modification of tubular fluid occur?

  collecting duct under influence of ADH
• How is tubular fluid generated?

  Filtration → reabsorption → secretion
• Why is the renal papilla an important site?

  a site of drug-induced nephrotoxicity

  'renal papillary necrosis'

  
• What is the route of urine exit from the kidney?

  common collecting duct → inner medulla → renal papilla → minor calyx → major calyx → renal pelvis → stretch smooth muscle → distension → peristaltic contractions at hilus → fluid moves down ureter in pulses → bladder (storage & controlled release)

  
• What is the structure of ureters?

  Tubes approx. 30 cm

  Mucosal layer: transitional epithelium (impermeable to urine)

  Supported by layers of smooth muscle:

  • Inner: longitudinal (L)
  • Outer: circular/spiral muscle (C)
  • Extra: outer layer of longitudinal muscle (L)

  
• What is the function of the ureters?

  Dilation of renal pelvis → pacemaker cells in hilum → generate action potential → peristaltic waves
• How is the number of contractions by ureters modulated by the nervous system?

  Parasympathetic NS: enhanced

  Sympathetic NS: inhibited

  
• How are the peristaltic contractions in the ureters?

  longitudinal muscle contracts; circular muscle relaxes → longitudinal muscle relaxes (allowing bolus to form); circular muscle contracts (pushes against bolus)

  
• Why are ureteral openings slit-like rather than rounded?

  to prevent backflow of urine

  during contraction of bladder

  
• How are ureters attached to the bladder?

  attach to posterior wall of urinary bladder

  pass through bladder wall at oblique angle for 2-3 cm into bladder

  slit-like ureteral openings prevent backflow

  
• What is the structure of the urinary bladder?

  hollow muscular organ, consisting of fundus (body) and neck

  2 layers:

  Outer 'Detrusor' Muscle layer

  • consisting of longitudinal, circular/spiral muscles

  Inner mucosal layer:

  • transitional epithelium
  • folded into 'rugae' when bladder empty
  • highly elastic - expands as bladder fills
• What is 'The Trigone'?

  triangular area bounded by openings of ureters and entrance to urethra

  acts as a funnel to channel urine towards neck of bladder

  
• What is the function of the urinary bladder?

  temporary storage of urine

  up to 1L capacity

  stimulated to contract by parasympathetic NS
• What are the two sphincters of the urinary bladder?

  Internal urethral sphincter

  • loop of smooth muscle
  • convergence of detrusor muscle
  • INVOLUNTARY control
  • resting muscle tone keeps neck of bladder and urethra free of urine

  External urethral sphincter

  • circular band of skeletal muscle where urethra passes through urogenital diaphragm
  • acts as valve with resting muscle tone
  • VOLUNTARY control

  
• Why are females more susceptible to UTIs?

  shorter urethra
• What are the characteristics of the end of the urinary tract in females?

  opens via external urethral orifice (located between clitoris and vagina)

  shorter urethra

  external sphincter not as well developed
• Why can urinary incontinence happen in females following childbirth?

  due to injury

  external sphincter not as well developed

  
• What are the characteristics of the end of the urinary tract in males?

  • urethra → prostate gland → uro-genital diaphragm → penis
  • Longer urethra provides some protection against UTIs
  • Prostate gland enlarges in 50% males >60 yrs (along with detrusor muscle hypertrophy - may require surgical / hormonal treatment)
  • Prostate cancer: one of the most common cancers affecting older men

  
• What are the two stages of the micturition (urination) process?

  1. Bladder fills until pressure within bladder reaches a threshold level
  2. Micturition reflex - produces conscious desire to urinate (and eventual emptying of bladder)
• What is the Micturition reflex?

  autonomic reflex which is:

  • inhibited by higher centres in brain: contract external sphincter
  • facilitated by cortical centres in brain: initiate micturition reflex & relax external sphincter

  internal sphincter and external sphincter relaxing at the same time = urination
• How does the urinary bladder fill?

  at 1mL/min at normal levels

  bladder fills → intravesical pressure increases → micturition contractions / waves

  
• What are 'micturition contractions / waves'?

  pressure peaks

  periodic reflex contractions of short duration which occur above approx. 200mL urine

  • Partially full bladder: contractions relax spontaneously after a few seconds
  • Increasing bladder: contractions more frequent, intense, last longer

  
• When does the 1st desire to empty bladder occur?

  around 150mL

  discomfort around 300mL

  sense of urgency around 400mL

  
• What nerves inhibit and facilitate micturition?

  Inhibited by hypogastric and pudendal nerves

  facilitated by pelvic nerves
• What is the innervation of bladder and sphincters?

  Bladder: Hypogastric nerve

  • sympathetic (involuntary)
  • L2

  External sphincter:

  • Pelvic nerve
  • parasympathetic (involuntary)
  • S2 & S3
  • Pudendal nerve
  • somatic (voluntary)
  • S2 & S3
• How does the Guarding reflex promote coninence?

  progressive bladder distension causes:

  • stretch receptors in bladder wall & internal sphincter → stimulate pelvic nerve → stimulate hypogastric nerve:
  • relax bladder detrusor muscle (allow expansion)
  • contract internal sphincters
  • also, pudendal nerve → holds external sphincter closed

  
• What are the 3 nerves involved in Guarding reflex? (Prevent Pee Hypothesis)

  Pudendal Nerve (stimulates external sphincter)

  Pelvic Nerve (stimulates hypogastric nerve)

  Hypogastric nerve (relax bladder detrusor muscle & constrict internal sphincter)
• What is the mechanism of the Micturition Reflex?

  bladder fills with urine → stretch receptors → pelvic nerve:

  • contract detrusor muscle: periodic reflex micturition contractions that intensify)
  • relax internal sphincter

  → full bladder sensation → thalamus → cerebral cortex → desire to urinate → appropriate time: pudendal nerve → voluntarily relax external sphincter → micturition:

  • lower diaphragm
  • contract abdominal muscles
  • open internal sphincter
• What are neural disruptions of micturition?

  1. Paraplegia
  2. Partial spinal cord damage with loss of inhibitory descending signals
  3. Crush injury of dorsal roots

  
• How can Paraplegia disrupt micturition?

  'Automatic bladder'

  Paraplegia: complete severing of nerve inputs from cerebral cortex; inability to move lower parts of body

  • Micturition reflexes return, but without cortical control
  • Causes periodic but unannounced bladder emptying
• How can partial spinal cord damage cause disruption to micturition?

  "Uninhibited bladder"

  frequent urination as excitatory impulses from cerebral cortex remain unopposed
• How can crush injury of dorsal roots cause disruption to micturition?

  'Atonic bladder'

  afferent nerve destruction → micturition reflexes lost despite complete efferent system → bladder fills to capacity → overflows dropwise (overflow incontinence)
• Infants lack voluntary control over urination until corticospinal connections are established.
• What are some problems that can occur with the Micturition Reflex?

  1. Control of micturition can be lost due to: stroke injury, Alzheimer’s disease, problems affecting cerebral cortex or the hypothalamus (e.g. a brain tumour)
  2. Bladder sphincter muscles can lose tone (e.g. after pregnancy): leading to urinary incontinence
  3. Urinary retention may develop in males if enlarged prostate gland compresses the urethra and restricts urine flow
• What are the functional classifications of micturition reflex problems?

  Incontinence: failure to store urine

  Retention: failure to empty bladder

  • due to bladder dysfunction
  • due to problem with outlet of urine
• What are the three main types of urinary incontinence?

  Atonic bladder:

  • loss of sensory nerves - due to injury
  • bladder fills capacity → no signals from stretch receptors → overflow

  Urge incontinence:

  • involuntary bladder contractions - due to injury
  • increased frequency

  Sensitive bladder:

  • heightened urge incontinence
  • spicy food, xanthines (caffeine/chocolate), citrus fruits, carbonated beverages, excitement, laughter
• Pelvic nerve → release ACh → conract detrusor muscle, relax sphincters → incontinence
• What type of drugs are used to treat urinary incontinence?

  Anticholinergics (muscarinic ACh receptor antagonists):

  • Actions: inhibit bladder contractions, facilitate involuntary contractioin of internal bladder sphincter
  • Examples: OXYBUTININ
  • Unwanted effects: dry mouth, blurred vision, palpitations, drowsiness, facial flushing (Atropine-like)
• What anticholinergic is used to treat urinary incontinence?

  Oxybutinin