Micturition Flashcards
Why do Ureters open at an oblique angle to the bladder wall?
Ureters are compressed to prevent reflux of urine back up into ureters (even at ↑ pressures)
- passive flap-valve effect
What does myogenic control mean in terms of ureteric peristalsis?
ie. it originates in smooth muscle
What is micturition?
The basic act of urination (emptying the bladder)
What are the stages of micturition?
Urine made in kidneys (1 ml/min)
Urine stored and released from bladder
What are the functions of the bladder and associated sphincters?
Storage of urine
Release of urine
What is the rate of urine production?
Urine is formed continuously at a rate of 1 ml/min in normally hydrated subjects
Describe the flow of urine through the kidneys
- Urine collected from all CDs of nephrons
- Empty into renal pelvis
- Urine enters ureters
What is the renal pelvis?
The funnel-like dilated proximal part of the ureter in the kidney
How does urine move into the ureters?
Contraction of the smooth muscle of the pelvis aids movement of urine into the ureter
What occurs when urine enters ureters?
- Urine enters ureter
- Ureter distends; contracting surrounding circular smooth
muscles - Junction at pelvis + ureter closes
- This pushes urine further into ureter, causing distension
and further contraction - Peristaltic wave initiated + propagated along length of
ureter until urine propelled into bladder
At what frequency do peristaltic waves occur in the ureters?
Peristaltic waves in ureter occur at a frequency of ~1-6 contractions / min
What pressure is urine subjected to in the ureters?
Ureters squeeze urine to a pressure of 10 - 20 mmHg
How is the peristaltic action of ureters regulated?
Changing volume of urine co ordinates the peristalsis
What are kidney stones?
(renal calculi) – most common disorder of urinary tract & develop from crystals that separate from urine within urinary tract
*not same as Gallstones
How does urine normally prevent the formation of kidney stones?
Normal urine contains inhibitors (citrate) to prevent this
What are kidney stones made up of?
Calcium is present in nearly all stones (80%), usually as calcium oxalate or less often as calcium phosphate. Others made up of uric acid (<10%), struvite (<10%), cystine (<5%)
What causes the formation of kidney stones?
- poor urine output/obstruction
- altered urinary pH
- low [inhibitors]
- Infection
- excess dietary intake of stone-forming substances
Where do kidney stones form?
Kidney stones can form anywhere within urinary tract: kidney, ureter or in bladder
- Ureterolithiasis
What are the symptoms of Ureterolithiasis ?
Dysuria (painful urination)
Haematuria
Loin pain/back pain
Reduced urine flow
Urinary tract obstruction: pressure reaches 50 mmHg - causes considerable pain “renal colic”
If stone approaches tip of urethra – intense pain can inhibit micturition – “strangury”
How do kidney stones cause pain in the genitalia?
If ureter is blocked by kidney stone, then pressure in ureter rises sharply due to the continuing peristaltic contractions – causes considerable pain – usually referred to small of back and/or tip of penis or vulva
What is strangury?
Sometimes continuing peristalsis can dislodge the stone into the bladder.
If the stone approaches the tip of the urethra, intense pain can stop flow of urine – known as “strangury”
What is Pyelonephritis?
kidney infection
Describe how volume of the bladder causes change in pressure?
Bladder can be almost empty or contain upto 400ml without much increase in pressure
How does the bladder withstand such changes in volume without altering pressure?
Result of it’s structure – spherical; even though tension in wall may increase as bladder fills, so does the radius
Describe the structure of the bladder
Has 3 structural layers:
1. Mucosal lining (transitional epithelium)
- Muscle coat (detrusor muscle)
- Mucosal layer (trigone)
Describe the structure and function of the mucosal lining
Transitional epithelium is capable of stretching without damage and consists of ridges that flatten out as bladder fills
How is exchange of substances prevented at the mucosal lining?
very impermeable to salts and water ⇒ no exchange between urine and capillaries of bladder wall
What is the muscle coat of the bladder composed of?
muscle coat around lining epithelium is made of bundles of smooth muscle interlacing and running in various directions – considered a single structure known as the detrusor muscle
Where is the mucosal layer of the bladder?
Mucosal layer generally loosely attached to underlying muscle except at the base of the bladder
What is the trigone?
where entrance of 2 ureters and exit of urethra form a triangle
mucosa is firmly attached – thickest and least distensible part of bladder
How is urine outlet into the urethra regulated?
Guarded by 2 sphincters: internal and external
Explain how the internal sphincter’s structure allows the passage of urine
- Extension of detrusor muscle
⇒ NOT under voluntary control - formed by loop of muscle extending from detrusor
- when detrusor contracts, fibres forming this loop shorten
and open the sphincter
Explain the structure of the external sphincter’s role in urine excretion
2 striated muscles
- compressor urethrae & bulbocavernosus
- surround urethra ⇒ responsible of continence
⇒ under conscious, voluntary control
- composed of skeletal muscle and is continuous with levator ani
Whar is the levator ani?
broad, thin muscle, situated on either side of the pelvis
How does the passage of urine differ between males and females?
Females require both sphincters for effective continence, but males manage with either sphincter intact due to different anatomical arrangement around the urethra
Describe the structure of female urethra
The female urethra is shorter + simpler than male urethra as it carries only urine
In women these structures around the neck of the bladder are the end of the system and point of exit of urine from body
Why are women more prone to incontinence?
External sphincter muscle is poorly developed and women are more prone to incontinence particularly after childbirth – keep doing pelvic floor exercises
Apart from urine, what other substance does the male urethra carry?
The male urethra also serves as a duct for the ejaculation of semen - as part of its reproductive function
Explain the location of the male urethra
In men the urethra continues through the penis – urine remaining in urethra can be expelled by contractions of the bulbocavernosus muscles. These different arrangements mean that men and women develop different pathologies
Summarise the structure and function of the bladder
- Lining – transitional epithelium
- Bladder muscle - detrusor
- Impermeable to salt & water
- Permeable to lipophilic molecules
NB composition of urine does not change in bladder
Summarise the outlet from the bladder into the urethra
Internal Sphincter – smooth muscle, involuntary control
External Sphincter – striated muscle, voluntary control
Define micturition in terms of neural innervation
process by which neural circuits in the brain and spinal cord coordinate the activity of the smooth muscle in the bladder and urethra
In what way do neural circuits control micturition?
These circuits act as on/off switches to alternate the lower urinary tract between 2 modes of operation:
- Storage
- Elimination
What 3 sets of peripheral nerves innervate the lower urinary tract?
- Parasympathetic (pelvic n)
- sympathetic (hypogastric n)
- somatic (pudendal n)
Describe how bladder innervation is sensory
gives sensation (awareness) of fullness and also pain from disease
Why is bladder innervation described as motor?
Causes contraction / relaxation of detrusor muscle + external sphincter to control micturition
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Describe the parasympathetic motor innervation of the bladder
- Arises in ventral/anterior horn at sacral region of spinal (S2-S4)
cord - Preganglionic fibres synapse onto postganglionic fibres
on wall of bladder + internal sphincter
Describe the sympathetic motor innervation of the bladder
- Arise in lateral/posterior horn at lumbar region of spinal
cord (T11-L2) - Complex route
- Preganglionic fibres synapse onto postganglionic
neurones in hypogastric ganglia - Some postganglionic neurones supply internal sphincter,
- Others innervate with P/s ganglia in bladder wall (inhibit
neurotransmission) - A few sympathetic neurones end in detrusor muscle
- more found in trigone region nearer internal sphincter
Describe the somatic motor innervation of the bladder
Arise from sacral region
Supply striated muscle of the external sphincter
What is the parasympathetic neurotransmitter?
ACh & ATP – cause detrusor to contract
What is the effect of sympathetic motor innervation?
NA – inhibits transmission at parasympathetic ganglia ⇒ indirectly causes detrusor to relax
NA - also directly via β-Rs (also in trigone area) causes detrusor to relax
Describe the effects of the parasympathetic innervation on the sphincters
Nitric Oxide (NO) & ACh - relaxes internal sphincter
What is the effects of the sympathetic motor innervation of the bladder sphincters?
Noradrenaline (NA) – contracts internal sphincter
What effect does somatic ACh have on sphincters?
ACh – tonic (continual) activity holds external sphincter closed
Summarise the effects of the P/s innervation of the bladder
Parasympathetic neurones: Encourage micturition
Contract detrusor
via ACh (muscarinic R) & ATP (purigenic R)
Relax internal sphincter
via NO (cGMP) & ACh (nicotinic R?)
Summarise sympathetic motor innervation of the bladder
Sympathetic neurones: inhibit micturition
Relax detrusor
indirectly via NA (α-R) & directly via NA (β-R)
Contract internal sphincter
via NA (α-R)
Summarise the somatic neuronal effects on the bladders motor function
Somatic neurones: inhibit micturition Contract external sphincter via Ach (nicotinic R)
What is the sensory innervation of the bladder?
The pelvic, hypogastric and pudendal nerves contain sensory afferent axons that transmit information from lower urinary tract to the lumbosacral spinal cord
What is the role of the sensory afferents?
Nerves travelling by this route signal bladder wall signal tension.
Describe the sensory innervation via the pelvic nerve
Main afferent pathway is via pelvic nerve (parasympathetic):
Small myelinated Aδ–fibres ⇒ micturition reflex
- Stretch receptors ⇒ signal wall tension
- Volume receptors ⇒ signal bladder filling
Outline the sensory innervation via nociceptive C fibres
Unmeylinated C fibres ⇒ endings in/near epithelium
Nociceptors ⇒ pain (e.g. during infection of bladder lining – cystitis; excessive distension)
What are the hypogastric afferents?
Hypogastric (sympathetic) & Pudendal (somatic) pathways
Nociceptors
Flow receptors (external sphincter)
What do the afferent A fibres sense?
A fibres: sense tension in detrusor:
i. Filling of bladder
ii. Detrusor contraction
⇒ bladder fullness, discomfort
What is the role of the afferent C fibres?
C fibres: respond to damage & inflammatory mediators
PAIN (urgent desire to micturate)
Describe the bladder activity when empty
Initially – bladder empty
Sphincters closed
(tonic activity sympathetic & somatic pudendal nerves)
Bladder pressure low
What happens when urine enters the bladder?
Arrival of urine from ureters Detrusor relaxes progressively (sympathetic activity inhibiting parasympathetic transmission) Little increase in pressure Sphincters still closed
What is receptive relaxation?
phenomenon of a hollow organ relaxing as it fills (also seen in stomach)
When do we become aware of the bladder filling?
when there’s 100-150 ml of urine in the bladder
When does bladder filling become problematic?
Discomfort begins at 300 - 400 ml
Further distension beyond this causes mounting distress and eventually frank pain
Threshold volume differs from person to person
How do we empty the bladder?
Via Micturition; an autonomic reflex
e.g. in babies (<18 months), adults with spinal cord transection above sacral region
How is micturition reflex regulated as we age?
Reflex is modified by voluntary control
Inhibited or initiated by higher centres in the brain
Maturation of bladder complete by >6 years
How do we regulate emptying the bladder once trained?
Basic circuits act as on / off switches to alternate between 2 modes of operation: storage and elimination
What can disrupt the voluntary control of urine passage?
Disease / injury / ageing to nervous system in adults disrupts voluntary control of micturition
bladder hyperactivity & urge incontinence
stress incontinence
How is bladder emptying initiated?
Receptors detect tension in bladder wall – excited afferent activity – triggers activity in parasympathetic efferents which supply detrusor and internal sphincter by spinal reflex arc
What action causes the bladder to empty?
Detrusor muscle contracts (ACh); reinforced by ATP.
Other parasympathetic fibres relax internal sphincter (Ach & NO) and urine flows out
What causes sphincter relaxation to allow the passage of urine?
Entry of urine into first part of urethra, triggers afferent nerves
These inhibit somatic nerves which tonically contract external sphincter
Summarise how the sacral region allows passage of urine
Detrusor muscle is contracting, both internal and external sphincters are relaxed and urine flow is established
These all depend on sacral reflex (pons and cerebrum silence symp. activity and allow p/s to takeover)
Where is the bladder located?
The bladder is contained in the floor of the abdominal cavity
What effect does abdominal muscle contraction have on the bladder?
Increased intra-abdominal pressure transmitted to bladder and to normally supported urethra
Reflex contraction of periurethral striated muscles helps compress urethra ⇒ micturition reflex aided
What is the importance of bladder emptying?
Normally sterile
Occasional bacterial entry
Complete emptying restores sterility
Bacteria in retained urine seeds fresh urine
Retained urine ⇒ clinical infection (UTI)
Normally all urine emptied approx. 5-10ml left in bladder
What is a UTI?
A urinary tract infection, or UTI, is an infection that can happen anywhere along the urinary tract
Outline the different UTI’s dependent on where they occur
- Bladder; infection here is called cystitis / bladder
infection - Kidneys; infection of one/both kidneys = pyelonephritis
- Ureters – rarely the site of infection
- Urethra – infection of the urethra is called urethritis
What is the occurence of UTI’s in men and women?
More common in women because of short urethra
Common in men over 40 due to prostatic disease, causing bladder outflow obstruction
What are the risk factors of UTIs?
Diabetes mellitus; long-term catheterisation; pregnancy; enlarged prostate; prolonged immobility; kidney stones; bowel incontinence; advanced age
What is the effect of an ageing bladder?
Slow urine stream
Prostate enlargement (BPH -benign prostatic hyperplasia)
most common cause of lower urinary tract symptoms in men (occurring in 25% of men > 40yrs)
What is the consequence of a slow urine stream?
incomplete emptying → infection
Explain what occurs to an ageing bladder
Incontinence
Weakening of sphincters (e.g. stress incontinence)
Common in women after child-birth, weakened pelvic floor muscles
Failure of nervous control
Overactive bladder (OAB) – detrusor contracts spastically – results in sustained high bladder pressure – urge incontinence
How can we treat bladder incontinence?
Medication: antimuscarinics ⇒ relax smooth muscle & ↓ detrusor contraction within 2 weeks
(eg non-specific muscarinic receptor antagonist Oxybutynin
Side effects:
What are the side effects of antimuscarinics used to treat baldder incontinence?
dry mouth Constipation Headache blurred vision dry eyes Hypertension Drowsiness urinary retention Bladder retraining (used for stress & urge incontinence)
What are the long term treatments for Bladder incontinence?
surgery
sacral nerve stimulation (SNS)
stem cell therapy
tissue engineered bladder
