3: Clearance and Measures of Renal Function

Question 2

Name 4 ways of metabolic clearance

Answer 2

• disposal in the urine
• disposal in the feces
• biochemical transformation in the liver (+ other organs)
• exhalation of volatile substances

metabolic clearance rate: quantitative measure of oll the above ways of substance removal

Question 3

What is the formal definition of renal clearance?

Answer 3

renal clearance is the volume of plasma per unit time from which all of a specfic substance is removed

Question 4

What is the unit of renal clearance?

Answer 4

volume per time

Question 5

what is the plasma half-life of insulin?

Answer 5

less than 10 minutes

Question 6

Describe why Inulin is the ideal substance to measure GFR?

Answer 6

Inulin is small (5kDa) and is therefore freely filtered in the glomerulus
Inulin is not reabsorbed, catabolized, or secreted by the nephron

i.e., all inulin getting filtered by the glomerulus will be completely excreted into the urine

the kidneys are the only excretion route

Question 7

Describe the equation for substance clearance

Answer 7

Cx = the rate at which substance X is cleared from plasma, i.e., cleared volume per time
Px = Plasma concentration

amount cleared = Cx x Px

amount cleared is equal to the amount in the urine over time, which is urine concentration Ux times urine volume (V)

so

Cx x Px = V x Ux

so

Cx = (V x Ux) / Px

i.e., clearance rate is the excretion rate in urine divided by the plasma concentration

Question 8

What does net secretion or net reabsorption mean?

Answer 8

net secretion: the renal clearance is higher than the GFR (i.e., tubules must be secreting substance additionally)

net reabsorption: the renal clearance is lower than the GFR (i.e., tubules must be reabsorbing some of the filtered substance)

Question 9

Why does creatinine clearance or plasma creatinine level overestimate GFR?

Answer 9

because some creatinine is also secreated by the proximal tubule

creatinine clearance overestimates GFR by about 10-20% –> this fraction becomes higher if GFR is very low

Question 10

Cystatin C is not excreted in the urine, so how can it be used to assess GFR?

Answer 10

cystatine C is completely filtered in the glomerulus, however, following filtration it is taken up by proximal tubule cells –> degraded into its constituent amino acids

Question 11

Briefly explain renal scintigraphy

Answer 11

IV injection of radiotrace –> counters measure appearance of radioactive counts in each kidney

enables assessing individuals kidneys function (not just both together)