Renal Blood Flow and Glomerular Filtration Rate Flashcards
Definition of clearance
Volume of plasma which is cleared of the substance in unit time (mL/min)
Equation for clearance
Cx=(UxV)/Px
Where: Cx = clearance of x Ux = urine concentration of x V = urine flow (mL/min) Px = plasma concentration of x
In what conditions are clearance measurements not possible?
Where there is no urine being produced (anuria)
What substance can be used to measure GFR?
Inulin
What properties of inulin make it a suitable molecule for measuring GFR?
It passes freely through the glomerular filter, but is neither reabsorbed, secreted, synthesised nor metabolised by the kidney and so all the filtered inulin is excreted and all the urine which is excreted has entered the urine only by filtration at the glomerulus
What is inulin clearance equal to?
GFR
What substance is normally used to measure GFR?
Creatinine
What is creatinine?
A product of muscle metabolism
What makes creatinine a suitable substance for measuring GFR?
As long as the subject remains at rest, the plasma creatinine level stays reasonably constant
If a solute has a clearance value higher than the inulin clearance, what does it mean?
The solute must get into the renal tubules not only by glomerular filtration but also by tubular secretion
If a solute has a clearance value less than that of inulin, what does it mean?
a) the solute may not be freely filtered at the glomerulus
b) the solute is freely filtered but is then reabsorbed from the tubule
c) both of these may occur for the same substance (e.g. albumin)
What substance is used to measure Renal Plasma Flow?
PAH
What makes PAH a suitable substance for measuring Renal Plasma Flow?
It is not only secreted, but is also filtered at the glomerulus –> so the amount excreted is equal to the sum of the amount filtered plus the amount secreted
