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Flashcards in Lecture 12 Deck (17):

What is the tip of the renal pyramid called?

renal papilla


Describe the exit of urine 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 contractions at hilus

Fluid moves down ureter in pulses towards bladder for storage and controlled release


Describe the structure and function of the ureters

Mucosal layer: transitional epithelium
– 3-8 cells thick, impermeable to urine

Supported by layers of smooth muscle:
- inner: longitudinal muscle (L)
- outer: circular/spiral muscle (C)
- extra outer layer of longitudinal muscle

Dilation of renal pelvis generates action potential from pacemaker cells in hilum

Peristaltic waves generated – between 1 to 6 per minute…

…the number of contractions can be modulated by nervous system:
- parasympathetic NS: enhanced
- sympathetic NS: inhibited


Describe the structure and function of the urinary bladder

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

Outer “Detrusor” Muscle layer:
- consists of longitudinal, circular/spiral

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 a funnel to channel urine towards neck of bladder

Temporary storage of urine

Up to 1 L capacity

Stimulated to contract by parasympathetic NS


Describe the exit of urine from the urinary bladder

The urinary bladder is guarded by two sphincters:

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 uretheral 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


Describe the female and male elimination of urine

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

Urethra passes through prostate gland and through urogenital diaphragm and penis

Longer urethra compared to females provide some protection against UTIs

Prostate gland enlarges in
50% of males >60 yrs (along with hypertrophy of detrusor muscle)
- may require surgical or hormone treatment

-Prostate cancer – one of the commonest cancers affecting older men


Describe micturition

Bladder progressively fills until pressure within bladder reaches a threshold level…
…this elicits the “micturition reflex” which produces a conscious desire to urinate or eventual emptying of the bladder

The micturition reflex is an autonomic reflex which is
(i) inhibited by higher centres in the brain and
(ii) facilitated by cortical centres in the brain:

Higher centres keep the micturition reflex under inhibition…
…prevents micturition by stimulating continual tonic contraction of the external sphincter

Cortical centres facilitate urination by initiating the micturition reflex and relaxing the external sphincter

Internal sphincter relaxes at the same time and urination occurs


Describe filling the urinary bladder

Bladder fills at 1 mL/min at normal levels of hydration

As bladder fills, (intravesical) pressure increases

– these 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


Describe the bladder and sphincter innervation

Hypogastric nerve-Sympathetic and involuntary control

Pelvic nerve- parasympathetic and involunatry cotrol

Pudendal nerve- Somatic ad volutary cotrol

Micturition is inhibited by activity in the hypogastric and pudendal nerves
It is facilitated by activity in the pelvic nerves


Describe how the guarding reflex promotes cotinece

During bladder filling:
Progressive bladder distension stimulates the pelvic nerve via activation of stretch receptors in bladder wall and internal sphincter (1)

Activation of the pelvic nerve leads to stimulation of the hypogastric nerve

Hypogastric nerve stimulation causes:
(i)relaxation and reduced excitability of the bladder detrusor muscle (2)
(ii)constriction of the internal sphincter (3)

Also, the external sphincter is held closed by pudendal nerve (4)

These responses are known as the “guarding reflexes” which promote continence


Describe the micturition reflex

Micturition is facilitated
by activity in the pelvic nerve

Stretch receptors in bladder continue to stimulate the pelvic nerve

Stimulation of the pelvic nerve also causes:
(i)contraction of the detrusor muscle (1)
(ii)relaxation of the internal sphincter (2)

Periodic reflex micturition contractions are also stimulated above 200 mL. These relax spontaneously after a few seconds

These micturition contractions continue to be stimulated and relax but at > 300 mL bladder contractions begin to predominate

Full bladder sensation conveyed to thalamus and then to cerebral cortex. Desire to urinate starts to increase…

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


Describe the neural disruption of micturition

Paraplegia: complete severing of nerve inputs from cerebral cortex (1)
Mictuiation 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


Explain the problems with the 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)

(2)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

Functional classifications

Failure to store urine = incontinence

Failure to empty bladder = retention

- Due to bladder dysfunction

- Due to problem with outlet of urine


Describe the three main types of urinary incontinence

Loss of sensory nerves – due to injury
Bladder fills to capacity
No signals from stretch receptors in bladder
Overflow incontinence occurs (Atonic bladder)

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)


Describe the drugs used in urinary incontinence

Anticholinergics (muscarinic ACh receptor antagonists, i.e. Antimuscarinic drugs)
Actions:- inhibit bladder contractions
- facilitates involuntary contraction of internal bladder sphincter

Examples:OXYBUTININ, Tolterodine, Flavoxate (less side effects but less effective)

Unwanted effects: Dry mouth, blurred vision, palpitations, drowsiness, facial flushing (Atropine-like)

Tricyclic antidepressants, e.g. IMIPRAMINE can be used at low doses for short-term treatment for nocturnal enuresis in children > 10 years but can have side effects, e.g. behaviour problems on withdrawal

Mechanisms: - anticholinergic and direct muscle relaxant effects on the bladder but also inhibits re-uptake of
noradrenaline and serotonin increasing their levels
DULOXETINE also increases serotonin and noradrenaline levels – for moderate to severe stress urinary incontinence

DESMOPRESSIN (ADH analogue) – also useful in children for nocturnal bed-wetting

MIRABEGRON – selective beta-3 receptor agonist – useful for overactive bladder syndrome

Purified bovine collagen implants – useful for urinary incontinence


Describe the alpha-adrenergic blocking drugs

Actions:- Antagonist action at 1A adrenoceptors in bladder neck
- Relaxes smooth muscle at bladder neck and increases urine flow rate

Examples: Alfuzosin, Doxazosin, Indoramin, Prazosin, Tamsulosin, Terazosin

Cautions: - Can reduce blood pressure so careful dosing required in patients already receiving antihypertensive treatments
- Should be avoided in patients with postural hypotension

Unwanted effects: Hypotension, drowsiness, depression, headache, dry mouth, GI disturbances


Describe the Parasympathomimetics (Choline Esters)

Actions: - Agonist action at muscarinic ACh receptors
- Increases contraction of the bladder detrusor muscle
- Limited role in relief of urinary retention – now superseded by catheterisation


Cautions: - Use with care or avoid using in patients with cardiac disorders (e.g. arrhythmias)
- Avoid in cases involving GI ulceration, asthma, hypotension, epilepsy, Parkinsonism, pregnancy

Unwanted effects: Nausea, vomiting, intestinal colic, bradycardia, blurred vision, sweating

Neostigmine, Pyridostigmine – inhibit breakdown of acetylcholine – promotes incontinence

Finasteride, Dutasteride are inhibitors of androgen synthesis (5α–reductase inhibitors) useful for
relieving urinary retention caused by prostate hyperplasia