Bladder physiology Flashcards
What is the flow of urine?
The nephron takes filtrate down to the ureter - the upper end is dilated to form the renal pelvis.
Urine flows down the ureter into the bladder, and out of the body through the urethra.
How does urine flow.
Both gravity and hydrostatic pressure contribute, as well as:
Atypical smooth muscle cells in the renal pelvis initiate peristaltic waves, which propagate down the ureter, to encourage the flow of urine,
This allows the filling of the bladder against gravity.
What are obstructions to the flow or urine in a ureter?
Kidney stones can painfully obstruct flow, formed by precipitation of barely-soluble constituents of urine.
The pressure builds up behind the obstructions, causing stretch and pain.
Treated by percutaneous nephrostomy - a tube placed to relieve the pressure, or extracorporeal shock wave lithotripsy to break up the stones.
What is the urinary bladder?
Can expand enormously in size, from an empty volume of 10-20ml, to 500-100ml.
The residual volume increases with age to 100ml.
The structure of the bladder allows for expansion.
What is the lower urinary tract?
Once in the lower urinary tract, urine is slightly changed, signalling factors and cells enter from the urothelium.
Infection can cause changes in the urine.
What is the structure of the bladder wall?
Serosa - Thin outer layer to prevent excessive expansion and rupture,
Detrusor - thick smooth muscle layer, contracts to increase pressure.
Lamina propria - connective tissue layer.
Urothelium - folds of epithelial cells, allows expansion of bladder.
What does the bladder wall look like?
See picture.
What is the urothelium?
It has high-resistance tight junction between its epithelial cells.
Greatly reduces permeability through the bladder wall.
Copes with hostile environment of urine - can be very concentrated or very dilute, or high urea.
Sense the contents of urine, signal to deeper layers of bladder.
What are umbrella cells?
The urine-facing surface of the urothelium has specialised umbrella cells that project out of the epithelium, and spread out cytoplasm over the surface.
Link together with the tight junctions to form a large surface area to prevent urine leakage.
What happens in a urinary tract infection?
A UTI causes pain, which is sensed by the urothelial cells, and signals to the nerves in the lamina propria, which signal to the brain.
The cells also generate frequency, which drives the bladder to empty and clear the infection.
Signals also increase thirst, which increases urine output to dilute the infection.
What is the lamina propria?
Contains blood vessels, lymphatics for drainage, nerves and interstitial cells in connective tissue.
The nerves detect chemical and mechanical stimuli (bladder filling) from the umbrella cells
The blood vessels supply urothelial and smooth muscle cells.
What is the detrusor?
Smooth muscle cells, forms bundles in bladder wall, that cross around and through each other in a basket-weave pattern, so uniform contraction of bladder and efficient emptying.
How is the detrusor innervated?
Main innervation by autonomic nerves - it is smooth muscle.
Has parasympathetic and sympathetic nerves
Sympathetic - noradrenaline acts on b3-adrenoceptors to cause relaxation and filling.
Parasympathetic - acetylcholine acts on M3 muscarinic receptor to contraction of bladder, and empty.
What is continence?
Continence - low pressure reservoir for storage of waste products
Continually active sphincter to prevent leakage.
What is voiding?
Relaxation of sphincters.
Increases pressure in reservoir (bladder).
Anal sphincter normally shut.
To defecate, need to relax it, and contract the rectum to empty.
What are the requirements for continence and voiding?
Sensory mechanisms to inform about filling
Higher control centres for voluntary voiding. (emptying when socially acceptable)
Reflex pathways to generate voiding
The right muscles to do it.
What is the pelvic floor anatomy?
See picture.
The pelvic floor sits underneath the bladder, to support the base of the reservoirs.
Damage to the pelvic floor will affect continence.
What is the female urogenital tract?
The bladder and urethra are close to the vagine.
Damage to the wall of the vagina or pelvic floor can affect continence.
The outflow tract is short, damage is common in child birth, so women will suffer from low resistance forms of incontinence - leaks.
See picture.
What is the male urogenital tract?
The prostate gland surrounds the urethra.
The outflow tract is long, so has high resistance.
The prostate gland enlarges with age and has problems with high resistance - an inability to empty the bladder fully.
See picture.
What does the human bladder look like?
The urethra is surrounded by striated skeletal muscle.
The skeletal muscle forms the external urethral sphinter, which is under somatic control.
The internal sphincter is smooth muscle, under autonomic control.
See picture.
What does the femal human urethra look like histologically?
The lumen is on the inside, urothelial cells on the outside, surrouned by smooth muscle, which is further surrounded by striated muscle.
See picture.
What is the innervation for continence?
The somatic system keeps the striated external sphincter closed.
The somatic system constricts the smooth muscle sphincter so there is high resistance.
The pelvic floor is innervated so the outflow is shut.
The bladder smooth muscle is kept relaxed.
The parasympathetic system is inactive.
What is the change from continence to voiding?
The somatic system and sympathetic system are switched off, the sphincters relax.
The parasympathetic system is activated, so the bladder walls contract.
What are the nerves in the bladder system?
Sensory nerves from the bladder enter various parts of the spinal cord.
Motor nerves from the CNS enter the bladder and innervate it and the urethra.
Somatic nerves go to the external urethral sphincter.
What are the efferent nerves in the bladder system?
Efferent nerves come from the thorical lumbi, autonomic so are sympathetic.
Sacral nerves go from sacral segments to the bladder, so are parasympathetic.
What are the afferent nerves in the bladder system?
The sensory afferent nerves follow the autonomic nerves.
The cell body is in the dorsal ganglia, and innervate the dorsal horn.
The autonomic nerves come from the lateral horn.
What are the storage and voiding reflexes?
The sensor nerves feedback signals to the pontine micturition centre, where neurones coordinatet he reflex.
The brain receives sensory inputs form the periphery about the filling states of the bladder.
The brain activates the descending system to switch off sympathetic nerves, and nerves to the external urethral sphincter.
Parasympathetic nerves to the bladder are switched on.
What are normal human urodynamics?
The bladder pressure increases slowly, once the urethra is relaxed, there is a strong contraction of the bladder, and urine flows.
What is the pathology of urinary continence?
UTI - chemical stimuli increase bladder activity, and the urge to void.
Spinal cord disorder, by injury or multiple sclerosis.
Stroke.
Damage to pelvic floor.
Detrusor overactivity.
Atonic bladder - decrease in contraction, may be secondary to autonomic neuropathy.
What is the effect of abdominal pressure in women?
There is a short outflow tract which has low resistance.
Damage to the pelvic floor or anterior vaginal walls, increases the risk of leaking - stress incontinence.
An increase in abdominal pressure increases the vaginal pressure in the bladder, causes leaking.
What is the treatment for stress incontinence?
Mainly surgical to support the outside urethral tract.
What is the pharmacological management of an overactive bladder?
Antimuscarinic drugs to mediate parasympathetic system, e.g. oxybutynin.
Boutlinum toxin, decreases force of contraction.
B3 adrenoceptor agonists, inhibits receptors innervated by sympathetic system to cause relaxation, e.g. mirabegron.
What is urinary outflow obstruction?
Caused by benign prostatic hypertrophy - increase in prostate size which blocks outflow.
Treated surgically - transurethral resection of the prostate.
a-adrenoceptor antagonists e.g. terazosin can relax the smooth muscle of the prostate.
5a-reductase inhibitors e.g. finasteride reduce hypertrophy.
