The lower urinary tract Flashcards
Passage of tubular fluid out of the kidneys and body via the urinary tract
Ureters
Urinary bladder
Urethra
Exit of urine out of the kidneys 1: out of the end of the collecting duct
Tubular fluid generated within the nephron by filtration, reabsorption, secretion
Final modification in collecting duct under influence of ADH
Fluid travels through common collecting duct deep into inner medulla
Fluid exits collecting duct at tip of renal pyramid (renal papilla)
Exit of urine out of the kidneys 2: into the renal pelvis and ureter
Minor and major calyces lead to renal pelvis
Fluid deposition into renal pelvis stretches smooth muscle
Distension triggers peristaltic contraction at hilus
Fluid moves down ureter in pulses towards bladder for storage and controlled release
Structure of ureter
Tubes approximately 30cm long
Mucosal layer: transitional epithelium
- 3-8 cells thick, impermeable to urine
Supported by laters of smooth muscle
- inner: longitudinal
- outer: circular/ spiral
- extra outer layer of longitudinal
Function of ureters
Dilation of renal pelvis generates action potential from pacemaker cells in hilum
Peristaltic waves generated between 1 to 6 per minute
Number of contractions modulated by nervous system
Ureters and peristalsis
Consist of successive waves of contractions and relaxation of longitudinal and circular
L contracts first followed by C relaxation
L starts to relax allowing bolus to form followed by C contraction which pushes against the bolus
Pattern repeated resulting in slow progressive movement of a pulse of urine along the ureter
The urinary bladder: entrance of urine
Ureters attach to posterior wall of urinary bladder
Pass through bladder wall at oblique angle for 2-3cm into bladder
Ureter opening are slit like
This helps prevent backflow of urine up ureters during contraction of bladder
Structure of the bladder
A hollow muscular organ, consisting of fundus and neck
Outer ‘detrusor’ muscle layer
- consists of longitudinal and circular/ spinal muscle
Inner mucosal layer
- transitional epithelium
- folded into ‘rugae’ when bladder empty
- highly elastic- expands as bladder fills
The trigone
Triangular area bounded by openings of ureters and entrance to urethra
Acts as funnel to channel urine towards neck of bladder
Function of bladder
Temporary storage of urine
Up to 1L capacity
Stimulated to contract by parasympathetic nervous system
Internal urethral sphincter
Loop of smooth muscle
Convergence of detrusor muscle
Under involuntary control
Normal tone keeps neck of bladder and urethra free of urine
External urethral sphincter
Circular band of skeletal muscle where urethra passes through urogenic diaphragm
Acts as a valve with resting muscle tone
Under voluntary control
Voluntary relaxation permits micturition
Elimination of urine: females
Opens via external urethral orifice located between clitoris and vagina
Shorter urethra in females (more susceptible to UTIs)
External sphincter not as well developed (incontinence following childbirth due to injury)
Elimination of urine: males
Urethra passes through prostate gland and through uro-genital diaphragm and penis
Longer than females provides some protection to UTIs
Prostate gland enlarges in 50% males >60 (may require surgical or hormone treatment)
Prostate cancer - one of the most common in older men
Micturition
Two stages
- bladder progressively fills until pressure within bladder reaches a threshold level
- This elicits the micturition reflex which produces conscious desire to urinate
Inhibition of micturition by higher centres in the brain
Higher centres keep the micturition reflex under inhibition
Prevents micturition by stimulating continual tonic contraction of external sphincter
Facilitation of micturition by cortical centres in the brain
Cortical centres facilitate urination by initiating the micturition reflex
Relaxes external sphincter
Filling the urinary bladder
Bladder fills at 1ml/min at normal hydration
As bladder fills, pressure increases
Partially full bladder: contractions relax spontaneously after a few seconds
Increasingly full bladder: contractions more frequent, intense and last longer
Bladder and sphincter innervation
Hypogastric nerve
- sympathetic
- involuntary control
- L2
Pelvic nerve
- parasympathetic
- involuntary control
- S2,3
Pudenal nerve
- somatic
- voluntary control
- S2,3
The guarding reflex promotes continence: during bladder filling
Distension of bladder stimulates pelvic nerve via stretch receptors in bladder wall and internal sphincter
Activation of pelvic nerve leads to stimulation of hypogastric nerve
Hypogastric nerve causes
- relaxation and reduced excitability of bladder detrusor muscle
- constriction of internal sphincter
External sphincter held closed by pudendal nerve
As bladder continues to fill with urine
Stretch receptors continue to stimulate to pelvic nerve
Stimulation of pelvic nerve also causes
- contraction of detrusor muscle
- relaxation of the internal sphincter
Periodic reflex micturition contractions also stimulated above 200ml
Micturition contractions continue to be stimulated and relax but at >300ml bladder contractions begin to predominate
Full bladder sensation conserved to thalamus and then to cerebral cortex- desire to urinate increases
Voluntary relaxation of external sphincter via pudendal nerve
Micturition occurs
Paraplegia
Complete severing of nerve inputs from cerebral cortex
- micturition reflexes return, but without cortical control
- periodic but unannounced bladder emptying
- ‘automatic bladder’
Partial spinal cord damage with loss of inhibitory descending signals
Frequent urination as excitatory impulses from cerebral cortex remain unopposed
Known as ‘uninhibited bladder’
Crush injury of dorsal roouts
Afferent nerve destruction- micturition reflexes lost despite complete efferent system
Bladder fills to capacity and overflows dropwise- ‘overflow incontinence’
Known as ‘atonic bladder’
Problems with micturition reflex
- Control of micturition can be lost ( stroke injury, Alzheimer’s, cerebral cortex or hypothalamus problems)
- Bladder sphincter muscle lose tone (urinary incontinence)
- Urinary retention may develop in males if enlarged prostate gland compresses the urethra and restricts urine flow
Functional classifications
Failure to store urine- incontinence
Failure to empty- renetion
Urinary incontinence
Loss of sensory neurones (due to injury)
- bladder fills to capacity
- no signals from stretch receptors in bladder
- overflow incontinence occurs
Involuntary bladder contractions (due to injury)
- urge incontinence or increased frequency
Heightened urge incontinence (sensitive bladder
- spicy food (capsaicin)
- caffeine/ chocolate (xanthines)
- citrus fruits (citric acid)
- carbonated beverages (sugar. sweeteners)
Anticholinergic actions
Actions: inhibit bladder contractions
Facilitate involuntary contractions of internal bladder sphincter
e.g. oxybutinin
Unwanted effects
- dry mouth
- blurred vision
- palipitations
- drowsiness
- fascial flushin
Tricyclic antidepressants
Used at low doses short term for nocturnal enuresis in children >10
Muscle relaxant effects on bladder, inhibits re-uptake of noradrenaline and serotonin
Can cause behavioural problems
e.g. impramine
Urinary retention
Acute urinary retention
- catheterisation
- surgery
Chronic urinary retention
- pharmacological intervention
- surgery
a- adrenergic blocking drugs used in urinary retention
Actions
- antagonist action a1A adrenoreceptors in bladder neck
- relaxes smooth muscle at bladder neck and increases urine flow
Cautions
- can reduce blood pressure
- avoided in patients with postural hypotension
Unwanted effects
- hypotension
- drowsiness
- dpression
- headache
- dry mouth
- GI disturbances
Parasympathomimetics (choline esters)
Actions
- agonist action at muscarinic ACh receptors
- increased contractions of the bladder detrusor muscle
Cautions
- avoid in patients with cardiac disorders
- avoid in GI ulceration, asthma, hypotension, epilepsy, parkinsonism, pregnancy
Unwanted effects
- nausea and vomiting
- intestinal colic
- bradycardia
- blurred vision
- sweating
e.g. bethanecol
