II: Renal Clearance Flashcards
Define the Filtration Fraction (FF).
It is the ratio b/t the GFR and RPF (GFR/RPF), or the fraction of the RPF that is filtered by the glomerular capillaries.
What is the normal filtration fraction, and what happens with the remainder?
FF is normally ~20%, and the 80% of the RPF that is not filtered enters the efferent arteriole, then the peritubular capillaries.
Which type of substances give an accurate measurement of GFR?
substances that are freely filtered, and neither secreted nor reabsorbed
How is GFR calculated?
(Urine conc. x urine flow) / (plasma conc.)
What are the disadvantages of using inulin to estimate GFR?
- it must be infused to obtain constant blood titers (not feasible in clinic)
- it is expensive
What are the disadvantages of using creatinine to estimate GFR?
it is not reabsorbed but IS secreted; therefore, must correct by over-estimating plasma concentration of creatinine by 10%
How can we estimate renal plasma flow?
by measuring the clearance of PAH, as PAH is freely filtered and 90% is excreted (we are able to estimate plasma flow due to high amount that is excreted)
What is PAH used for, and what is one of its disadvantages?
It is used to estimate renal plasma flow, and a disadvantage is that 10% is retained in the venous system.
What is the effective renal plasma flow (ERPF) used for?
It is used to cancel the venous sample when estimating RBF using PAH, as 10% of PAH is normally retained in the venous system.
What happens with glucose in terms of F+S=R+E?
Glucose IS filtered, but it gets totally reabsorbed back into the capillary system (we do NOT want glucose in the urine).
Are plasma proteins filtered?
Only small proteins are able to be filtered (usually less than 65 kDa, like albumin), but they are immediately reabsorbed in the proximal tubule.
Why does serum creatinine tend to over-estimate GFR by 10%?
This is because there’s roughly 10% secretion of creatinine back into the renal tubules after filtration.
