Renal assessment and physiology of micturition Flashcards
What are 2 situations GFR is useful?
renal disease
drugs
What does the total GFR =?
Sum of all filtration by functioning nephrons so progression of disease indicated by decrease in GFR
Why is GFR important in terms of drugs?
eg antibiotics removed by filtration - excretion
drug concentration in plasma could become toxic
adjust dose
What tests are done to measure GFR (renal function)?
plasma clearance test
What does the plasma clearance test measure?
ability of kidney to clear plasma of various substances
What is important in plasma clearance tests - plasma or urine?
volume of plasma
What is the gold standard for plasma clearance tests?
inulin
Why is inulin the gold standard for plasma clearance tests?
no reabsorption or secretion
100% filtered is excreted
not metabolised by kidney
inulin clearance - GFR
If substances are filtered and reabsorbed what will their clearance be compared to inulin?
lower
If substances are filtered and secreted what will their clearance be compared to inulin?
higher
GFR value
125mls/min
What causes changes in GFR?
women - 10% less due to SA
lose 1ml/min/year after 30
In clinical practice what was the next best after inulin to measure GFR?
radioactive substance handled same as inulin
Why is the direct ways to measure GFR not used anymore?
Too complex
injections
expensive
What is routinely used to estimate GFR?
creatinine clearance
What is creatinine?
endogenous product of muscle creatine breakdown
Problem with creatinine clearance to measure eGFR
not a linear correlation
need to lose about 50% of GFR to notice difference in levels
3 factors affecting serum creatinine
muscle mass
dietary intake
drugs
Normal approx. GFR
100mls/min/1.73m squared
Glucose clearance and why
0 - all reabsorbed
Urea clearance and why
less than inulin as some is reabsorbed
What is PAH used to measure?
renal plasma flow
Why is PAH used to measure RPF?
freely filtered and remaining is secreted
more than 90% excreted in 1 transit
Renal plasma flow value
660ml/min
Penicillin clearance and why
higher than inulin as some secreted
How does urine travel from kidneys to ureters?
peristaltic contraction of smooth muscle of ureters
Does urine composition change once it leaves the kidneys?
no
Bladder muscle name
detrusor muscle - spiral, longitudinal and circular
Is the external or internal urethral sphincter a true sphincter?
external
Epithelium in bladder
transitional
Relations of bladder
midline
posterior to pubic bones
anterior to rectum and repro. organs
What makes up the trigone?
2 vesicoureteric orifices
urethral opening
Would urethral and ureter problems cause bilateral or unilateral kidney problems?
urethra - bi, ureter - uni
Describe the pressure-volume curve of bladder
characteristic shape
long flat segment with initial urine
sharp rise as micturition reflex triggered
Normal daily urine production
750-2500mls
Motor innervation of bladder - parasympathetic
pelvic nerves contract detrusor muscle
S2-4
Motor innervation of bladder - sympathetic
hypogastric nerves L1-3
inhibit bladder contraction and close internal sphincter
Main function of hypogastric nerves
prevent semen reflux into bladder in ejaculation
Motor innervation of bladder - somatic motorneurons
external urethral sphincter - keep it closed
S2-4
Sensor innervation of bladder
stretch receptors –> spinal cord
Consequence of activating stretch receptors in bladder
a - parasympathetic flow
b - inhibit sympathetic
c - inhibit somatic motorneurons to external sphincter
d - pathways to sensory cortex - sensation of fullness
Volume needed to initiate micturition reflex
300-500mls
How is delay of micturition accomplished?
descending pathways which stimulate somatic nerves, inhibit parasympathetic and over ride input from bladder
What does voluntary initiation of micturition consist of?
stimulate parasympathetic
inhibit somatic motor neurons
What does relaxing pelvic floor muscles do?
may cause sufficient downward tug of detrusor
Significance of perineal and external sphincter muscles contracted voluntarily
prevent stopping midstream and prevent urine flow down urethra
How does female urethra empty after urination?
gravity
How does male urethra empty after urination?
contractions of bulbocavernous muscle
3 major types of neural abnormalities of micturition
problems with afferent
problems with afferent and efferent
interruption of facilitatory and inhibitory descending pathways
Significance of neural abnormalities of micturition
bladder contracts but not enough to expel all urine - high residual volume
Ways paraplegics can have some control over micturition
rub/pinch thigh - mild mass reflex
mild noxious stimuli on skin
