renal assessment and micturition Flashcards
why is ability to measure GFR important
- in patient w renal disease, progression of underlying disease process results in nephron destruction and dec nephron function - progression indicated by reduction in GFR
- many drugs are removed from body by excretion by filtration. When GFR falls, excretion falls so may need to adjust dose appropriate to decreasing renal function
total GFR
sum of all filtration by functioning nephrons
measurement of GFR
plasma clearance tests are used to measure renal function. They measure ability of kidney to clear plasma of various substances
gold standard plasma clearance test
insulin
loading IV dose of insulin, allow time to equilibriate, then sample simultaneousluy plasma and urine
why is insulin a god measurement of GFT
insulin is freely filter at the glomerulus and neither reabsorbed nor secreted
it is not metabolised by the kidney and doesn’t interfere with normal renal function and so insulin clearance is measure of GFR
normal GFR in man
125ml/min
what is used in practice to measure GFR
not insulin - too cumbersome
51Cr-EDTA has been used instead - suitable radioactive subastance handled by kidney in same way insulin
why is GFR now not measured prropery
too complex and expencive
takes several hours
requires injection of istope
what is used routinely to estimate GFR
creatinine
creatinine is endogenous (breakdown produce of muscel creatine)
good agreement with insulin clearance
factors affecting creatinine
muscle mass: athletes vs malnutrition
dietary intake: creatine supplements vs veggies
drugs: some lead to increses
normal GFR
approx 100mls/min/1.73m2
across range of adults, sexes, kidney function and size
glucose clearance
usually zero because all is normally reabsorbed
clearance of urea
less than that of insulin because some urea is reabsorbed
PAH clearance (organic anion para-amino-hippuric acid)
used to measure renal plasma flow (RPF)
why is PAH clearance used to measure renal plasma flow
PAH is freely filtered at glomerulus and then PAH remaining in plasma is actively secreted into tubule so that >9-% of plasma is cleared of its PAH content in one transit of kidney
PAH clearance is a measure of all plasma flowing through kidneys in given time = renal plsma flow
what is PAH clearance a measure of
all the plasma flowing through the kidneys in a given time = renal plasma flow
normal renal plasma flow
~660mls/min
penicillin clearance
greater clearance than insulin because filtered and secreted
how does urine get to bladder
flows from kidneys to ureters via peristaltic contraction of the smooth muscles of the ureters
enter bladder at an oblique angle (prevent reflux of urine)
bladder
bag of smooth muscle, arranged in spiral, longitudinal and circular bundles: detrusor muscle
contraction of this muscle is mainly responsible for empyting bladder during micturtition
internal urethral sphincter
not a true sphincer
where the smooth muscle at start of urethra acts as a sphincter when smooth muscle is relaxed
external urethral sphincter
true sphincter
made of skeletal muscle under voluntary somatic control
where is bladder
lies in midline posterior to pubic bones
anterior to reproductive system and rectum
lining of bladder
transitional epithelium
trigone of bladder
2 vesicoureteric openings
urethral opening
urethral obstructions cause
bilateral renal problems
ureter obstruction causes
unilateral renal problems
normal daily urine production
750-2500mls
control of micturition: motor innervation
- rich parasympathetic supply inc which cause contraction detrusor muscle - inc pressure in bladder
- sparse sympathetic supply inhibit bladder contraction - main function to prevent reflux semen into bladder during ejaculation
- somatic motoneurones (pudendal nerve) innervate skeletal muscle that forms external urethral sphincer,keeps it closed even against strong bladder contractions
control of micturition: sensory innervation
stretch receptor afferent from bladder wall, as it fills inc discharge in afferents to spinal cord -
via interneurones;
1. exctitation parasympathetic
2. inhibition sympathetic
3. inhibition somatic motoneurones to external sphincter
4. pathways to sensory cortex - sensation of fullness
in adults - volume of urine in bladder required to initiate spinal reflex
300-350mls
how is delay in micturition accomplished
descending pathways from many brain centres which:
- inhibit parasympathetic
- stimulate somatic nerves to external sphincter, thus overiding input from bladder stretc receptors
mechanism of voluntary micturition
descending pathways wich
- stimulate parasympathetic
- inhibit somatic motor neurones
3 major types of micurition abnormaliies due to neural lesions
- interuption of afferent nerves
- interuption of both afferent and efferent nerves
- interuption of facilitatory and inhibitory descending pathways frombrian
all 3 types of micturition abnormalities result in
bladder contracts but contractions are generally insufficient to empty bladder completely and urine is left in bladder
