measuring GFR and renal clearance; Lect 3

Question 1

decreased GFR is a key sign of what condition

Answer 1

renal disease

Question 2

equation for filtered load and GFR

Answer 2

filtered load (FLx) = GFR x Plasma concentration (Px)

Question 3

give the two equations to determine excretion rate of X

Answer 3

• ERx = urine concentration of X (Ux) x urine rate (v)
• ERx = FLx + secretion rate (Sx) - reabsorption rate (Rx)

Question 4

use the lase of conservation of mass in relation to GFR

Answer 4

• if a material is neither reabsorbed or secreted by the nephron, then what goes in must come out
• if Rx and Sx = 0, then ERx = FLx
  
  • excreted rate of X = filtered load of X

Question 5

what is the equation to determine GFR using urine concentration of X

Answer 5

GFR = (Ux)(V) / Px

• Ux = urine concentration of X
• V: urine rate
• Px: plasma concentration of X

Question 6

inulin clearance is used to measure what?

Answer 6

GFR

• inulin clearance = GFR

Question 7

describe the perfect GFR marker

Answer 7

• freely filtered
• neither reabsorbed nor secreted

Question 8

what is the gold standard to measure GFR? What are frequently used as indicators of GFR in clinical practice

Answer 8

• gold standard: inulin clearance
• clinical practice: creatinine clearance and plasma creatinine clearance

Question 9

what is creatinine and why is it a good measurement for GFR

Answer 9

• produced by skeletal muscle at a relatively constant rate from breakdown of creatine phosphate
  
  • already at a steady state concentration in the blood
• it is freely filtered, not reabsorbed, and is secreted only slightly

Question 10

creatinine clearance equation to determine GFR

Answer 10

C_Cr = (U_Cr)(V) / P_Cr = GFR

Question 11

we can use creatinine to estimate GFR in what two ways

Answer 11

1. calculate GFR by determining creatinine clearance (C_Cr)
2. use P_Cr to estimate GFR using a standard predictive equation

Question 12

A single plasma creatinine can provide an estimate of GFR. How does plasma creatinine concentrate change with a reduced GFR

Answer 12

plasma creatinine rises

Question 13

list the algorithms that exist for estimating GFR based on P_Cr

Answer 13

• Cockcroft - Gault equation
• MDRD 4

Question 15

what is the clearance equation

Answer 15

Cx = (Ux)(V) / Px

• clearance represents the volume of plasma that is cleared of solute X per minute

Question 16

how does clearance of a secreted substances and a reabsorbed substance compare to GFR

Answer 16

• clearance of a secreted substance will be greater than C_Cr, thus greater than GFR
• clearance of a reabsorbed substance will be less than C_Cr, thus less than GFR

Question 17

Cx = 12.5; GFR = 125. Is the substance secreted or reabsorbed

Answer 17

reabsorbed

Question 18

Cx/C_inulin < 1.0, is this substance secreted or reabsorbed

Answer 18

reabsorbed

Question 19

Cx/Cinulin > 1.0, is this substance secreted or reabsorbed

Answer 19

secreted

Question 20

what is the equation for clearance ratio

Answer 20

clearance ratio = Cx/C_inulin

Question 21

what substance is a measure of RPF? Why

Answer 21

Para-aminohippurate (PAH)

• PAH is filtered and then the rest of it is secreted and excreted thus the All the blood is cleared of PAH

Question 22

give clearance equation for PAH

Answer 22

C_PAH = (U_PAH)(V) / P_PAH

Question 23

Calculate RBF using RPF

Answer 23

RBF = RPF / (1-Hct)

Question 24

give a substance that is filtered and then reabsorbed 100% in a healthy nephron

Answer 24

glucose