The Lower Urinary Tract Flashcards
Exit of urine out the kidneys
Tubular fluid exits collecting duct at tip of renal pyramid - also known as renal papilla
Into renal pelvis and ureter
Minor and major calyces lead to renal pelvis
Fluid deposition into renal pelvis stretches smooth muscle
Distention triggers peristaltic contractions at hilus
Fluid moves down ureter in pulses towards bladder for storage and controlled release
Ureters - Structure
Tubes approximately 30 cm long
Mucosal layer: transitional epithelium – 3-8 cells thick, impermeable to urine
Supported by layers of smooth muscle:
o inner: longitudinal muscle (L)
o outer: circular/spiral muscle (C)
o extra outer layer of longitudinal muscle
Ureters - function
Dilation of renal pelvis generates action potential from pacemaker cells in hilum
Peristaltic waves generated – between 1 to 6 per minute
Number of contractions enhanced by parasympathetic NS, and inhibited by sympathetic nervous system
Ureters and Peristalsis
Consist of successive waves of contractions and relaxation of longitudinal and circular smooth muscle:
Longitudinal muscle contracts first followed by circular muscle relaxation (1)
Longitudinal muscle then starts to relax allowing a bolus to form followed by circular muscle contraction which pushes against the bolus (2)
Pattern of contraction is repeated resulting in slow but progressive movement of a pulse of urine along the ureter = PERISTALSIS (also known as “vermiculation”)
Entrance of urinary bladder
Ureters attach to posterior wall of urinary bladder
Pass through bladder wall at oblique angle for 2-3 cm into bladder - ureteral openings are slit-like rather than rounded
- together this helps prevent backflow of urine up ureters during contraction of bladder
Urinary bladder - structure and function
Structure:
A hollow muscular organ, consisting of fundus (body) and neck
Outer “Detrusor” Muscle layer: consists of longitudinal, circular/spiral muscles
Inner Mucosal layer: transitional epithelium, folded into “rugae” when bladder empty; highly elastic – expands as bladder fills
Trigone - triangular area bounded by openings of ureters and entrance to urethra; acts as a funnel to channel urine towards the neck of the bladder
Functions: temp storage of urine (1L), stimulated to contract by parasympathetic NS
Urinary bladder - exit of urine
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 urogenital 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; external sphincter not as well developed (incontinence following childbirth)
Males: Urethra passes through prostrate gland and through uro-genital diaphragm and penis; Prostate gland enlarges in 50% of males >60 yrs (along with hypertrophy of detrusor muscle) - may require surgical or hormone treatment; prostate cancer (death rate 3%)
Micturition
- Bladder progressively fills until pressure within bladder reaches a threshold level
- this elicits the “micturition reflex” which produces a conscious desire to urinate
The micturition reflex is an autonomic reflex which is inhibited by higher centres in the brain (stimulates continual tonic contraction of external sphincter) and facilitated by cortical centres in the brain (relax external sphincter):
The internal sphincter relaxes at the same time and urination occurs
Filling the bladder
Bladder fills at 1 mL/min at normal levels of hydration
As bladder fills, (intravesical) pressure increases
Micturition contractions are periodic reflex contractions of short duration which occur above approximately 200 mL urine volume
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 control
Pelvic nerve - parasympathetic
Pudendal nerve - somatic control
Micturition is inhibited by activity in the hypogastric and pudendal nerves and facilitated by activity in the pelvic nerves
The guarding reflex promotes Continence
Bladder filling: progressive bladder distension stimulates the pelvic nerve via activation of stretch receptors in bladder wall and internal sphincter
Activation of pelvic nerve = stimulation of hypogastric nerve
Hypogastric nerve stimulation causes: relaxation and reduced excitability of the bladder detrusor muscle (2); constriction of the internal sphincter (3); Also, the external sphincter is held closed by pudendal nerve (4)
These all promote continence
The Micturition Reflex
As bladder continues to full with urine: stretch receptors in bladder continue to stimulate the pelvic nerve, causing contraction of the detrusor muscle and relaxation of the internal sphincter
Periodic reflex micturition contractions are also stimulated above 200mL
These contractions continue but at >300mL bladder contractions begin to predominate
Full bladder sensation conveyed to thalamus and then to cerebral cortex -> desire to urinate increases
At appropriate time, voluntary relaxation of external sphincter occurs via pudendal nerve. Micturition occurs, aided by lowering of diaphragm, contraction of abdominal muscles and opening of internal sphincter
Neural disruption of micturition
Paraplegia: complete severing of nerve inputs from cerebral cortex (1): Micturition reflexes return, but without cortical control; Periodic but unannounced bladder emptying – “Automatic bladder”
Partial spinal cord damage with loss of inhibitory descending signals (2): Frequent urination as excitatory impulses from cerebral cortex remain unopposed - Known as “Uninhibited bladder”
Crush injury of dorsal roots (3): Afferent nerve destruction - micturition reflexes lost despite complete efferent system; Bladder fills to capacity and overflows dropwise - “overflow incontinence”; Known as “Atonic bladder”
Infants lack voluntary control over urination until corticospinal connections are established
Problems with micturition reflex
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)
Bladder sphincter muscles can lose tone (e.g. after pregnancy) - leading to urinary incontinence
Urinary retention may develop in males if enlarged prostate gland compresses the urethra and restricts urine flow
