Renal Clearance Flashcards
What is renal clearance?
the volume of plasma per unit time from which all of a substance has been removed.
What are the units of clearance?
Volume of plasma cleared of solute per unit time
How do we measure clearance?
Measure plasma [C] and urine [C]
What is the equation for renal clearance?
Cx • Px = V̇ •Ux;
Cx = clearance of X Px = plasma concentration of X V̇ = urine flow rate Ux = urine concentration of X
Why is inulin used to calculate GFR?
Inulin is only filtered- it is not secreted or reabsorbed so the amount appearing in the urine must have been filtered at the glomerulus.
Is clearance = to the amount of substance secreted?
No, but related
What does it mean if the clearance of a substance is higher than that of inulin? Lower?
Higher = some more excreted
Lower = some absorbed
What do we use to estimate GFR, rather than infusing all of our patients with inulin, and waiting for a steady state of [inulin] to be achieved in the office? How close is this to reality?
Creatinine–this slightly overestimates
Why does creatinine overestimate GFR? Who is it more overestimeated in–pts with a low GFR, or high?
Small amount is constantly excreted in the nephron tubules
Low GFR pts will overestimate more, because secretion contributes a higher fraction of total
To obtain a true GFR with creatinine, you’d have to compare the amount in plasma vs the amount inf urine excreted and do some math. This is not actually done in reality. How is it actually calculated?
Measured on a average curve
What happens to plasma [creatinine] if you decrease GFR by 50%?
Goes up by 50%
What are the three factors that are taken into account in the Cockcroft and Gault formula for creatinine clearance?
Age, lean body weight, M/F
Why is lean body weight used in calculating the creatinine clearance rather than straight body weight?
Fat does not make creatinine
What happens to GFR with age? Why?
Decreases, since there is a steady decrease in glomeruli throughout life
What happens to the amount of nephrons as a person ages?
In most people there is about a 10% decrease in the number of functional nephrons each 10 years after age 40. This causes a progressive decrease in both renal blood flow and GFR.
