Lecture 11 - Renal Assessment and Micturition Flashcards
When are the two situations where is it particularly useful to be able to measure GFR?
- Patients with renal disease - progression of disease –> nephron destruction + reduced nephron function
- For drugs eliminated by the kidney, fall in GFR –> increase in plasma drug levels –> toxicity (therefore may req. dose adjustment in reduced renal function)
What tests are used to measure GFR?
Plasma clearance tests (measuring ability of kidneys to clear plasma of various substances)
What formula is used to measure plasma clearance of a substance X?
Cx = [Ux]V/[Px] Units are mls/min Ux = urine concentration of x V = urine flow rate Px = plasma concentration of x
What is the gold standard plasma clearance test?
Insulin clearance
Insulin freely filtered at glomerulus + neither reabsorbed nor secreted
Not metabolised by kidney and doesn’t interfere with normal kidney function
What is the GFR of a normal man?
125mls/min
How much does GFR decline per year after 30 years?
1ml/min/year
How is GFR clinically measured now as insulin is no longer used?
51Cr-EDTA (a radioactive substance that is handled in the kidney in the same way as insulin)
What is routinely used to estimate eGFR?
Plasma creatinine
What is creatinine?
Breakdown production of muscle creatine
Why can we use creatinine clearance to estimate GFR?
As it correlates well with insulin clearance
What is the big caution with using creatinine clearance to estimate GFR?
GFR can half before elevation of the plasma creatinine
SO there are formulae using serum cr that take into account confounding variables
What factors affect GFR?
Muscle mass
Dietary intake, e.g. creatine supplements vs vegetarians
Drugs
What is normal eGFR approximately?
100mls/min/1.73m2
This is usually expressed as a percentage of normal as normal value varies for age/sex etc.
What is the normal clearance of glucose and urea?
Glucose - 0 (usually all reabsorbed)
Urea - less than insulin (some reabsorbed)
Clearance of what substance can be used to measure renal plasma flow?
The organic anion para-amino-hippuric acid (PAH)
PAH freely filtered at glomerulus and then the PAH remaining in the plasma is actively secreted into the tubule so>90% of plasma is cleared of PAH in one transit to the kidney
What is the average renal plasma flow?
660mls/min
Contractions of which muscle are responsible for emptying the bladder during micturition?
Detrusor
Describe the composition of the two urethral sphincters
Internal - not a true sphincter, is where smooth muscle at start of urethra acts as sphincter when it is relaxed
External - true sphincter, skeletal muscle under voluntary somatic control
What kind of muscle is the detrusor?
Smooth
What makes up the trigone?
2 vesicoureteric openings
Urethral opening
What kinds of problems do urethral obstructions lead to?
Bilateral renal problems
What kind of problems do ureteric obstructions lead to?
Unilateral renal problems
What is the appearance of the pressure-volume curve of the bladder?
Long, flat segment as initial urine enters bladder, sudden sharp rise as micturition reflex triggered
What is the normal urine production?
750-2500mls
What is the effect of parasympathetic innervation to the bladder?
Increased contraction of detrusor –> increased pressure in bladder (S2-4)
what is the effect of sympathetic innervation to the bladder?
Inhibits bladder contraction
Closes internal urethral sphincter
(hypogastric nerves)
What is the main function of the sympathetic innervation to the bladder (regarding nerve roots L1-3)?
Prevent reflux of semen into bladder during ejaculation
What somatic nerves are responsible for keeping the external urethral sphincter closed?
Pundenal nerve
Describe the sensory innervation of the bladder
Stretch receptor afferents from bladder wall, as bladder fills –> increased discharge in afferent nerves to spinal cord –>
a) excitation of PNS
b) inhibition of SNS
c) inhibition of somatic motoneurons to external sphincter
d) pathways to sensory cortex –> sensation of fullness
Describe the operation of the local spinal reflex of micturition
As bladder fills + becomes distended, stretch receptors are stimulated until their output is great enough to cause bladder contraction via stimulation of PNS and relaxing the EUS via somatic motoneurons
In babies, the micturition reflex operates at this level because higher brain connections have yet to be established
Delay in micturition is accomplished by what?
Descending pathways from many brain centres which inhibit PNS and stimulate somatic nerves to EUS overriding the bladder stretch receptors
What is involved in voluntary initiation of micturition?
Descending pathways that stimulate the PNS and inhibit somatic motorneurons thus summating with stretch receptor effects
What is one of the initial events in voluntary urination?
Relaxation of pelvic floor muscles, which creates a downward tug on detrusor to initiate its contraction
How can urine flow be stopped after urination begins?
Perineal muscles + external sphincter can be contracted voluntarily
How does the urethra empty after urination?
Female - by gravity
Male - contractions of bulbocavernosus muscle
What 3 major neural lesions may lead to abnormalities of micturition?
- Interruption of afferent nerves
- Interruption of afferent and efferent nerves
- Interruption of facilitatory and inhibitory descending pathways from the brain
How do some paraplegic patients train themselves to initiate voiding?
Pinching/stroking on thighs - mildly noxious stimuli may irridate to autonomic centres and evoke bladder/rectal voiding
