Plasma Clearance Flashcards
what is plasma clearance
a measure of how effectively the kidneys can ‘clear’ the blood of a substance
what is the calculation to obtain plasma clearance of a substance
[substance]urine x volume urine
divided by
[substance]plasma
if clearance value is less than GFR what does this mean
the the substance is reabsorbed
if clearance value is equal to GFR what does this mean
that the substance is neither secreted nor absorbed
if clearance value is more than GFR what does this mean
that the substance in secreted in the tubules
what substances clearance can be measured to give GFR
inulin
ie inulin clearance = GFR
what substance is used to measure renal plasma flow(RPF)
para-amino hippuric acid(PAH)
why is PAH good for measuring renal plasma flow(RPF)
because ALL the PAH that escapes filtration is secreted into the peritubular capsule, so it is completely cleared from the plasma
what properties should a clearance marker ideally have
non-toxic, inert(ie not metabolised), easy to measure
what properties should a GFR marker ideally have
freely filtered and NOT secreted or absorbed
what properties should a RPF marker ideally have
should be filtered AND completely secreted
what is the filtration fraction
the fraction of plasma flowing through the glomeruli that is filtered into the tubule
what calculation is used for the filtration fraction
GFR
divided by
RPF
what is the general value of renal plasma flow(RPF)
around 650ml/min
what is the general value of GFR
125 ml/min
