GFR Practical

Question 1

What is normal GFR?

Answer 1

120ml/min

normal range = 90-150

Question 2

Past the age of 40 how does GFR change?

Answer 2

Falls by 10ml/min every decade

Question 3

What is GFR?

Answer 3

The regulated rate of filtration carried out by the glomerulus to form primary urine

Question 4

What can GFR be measured using?

Answer 4

It can be measured with any substance in the plasma that is small enough to be freely filtered and neither absorbed nor secreted into the tubules.

Question 5

Give examples of substances that can be used to measure GFR

Answer 5

inulin

creatinine

Question 6

What are the properties of substances that can be used to measure GFR?

Answer 6

• Freely filtered at the glomerulus
• Not reabsorbed or secreted
• Not metabolised
• Not toxic

Question 7

What is the formula for GFR?

Answer 7

(V x Uy)/Py

Py = plasma conc of y
V = urine flow rate
Uy = urine conc of y

Question 8

What is clearance and the formula for it?

Answer 8

The volume of plasma completely cleared of a substance per unit time.

Clearance (Y) = (V x Uy) /Py

Question 9

What does it mean if the clearance of a substance is smaller or greater than inulin’s?

Answer 9

If a substance has a clearance greater than that of inulin, you can conclude secretion has occurred.

However, if renal clearance is lower than that of inulin, you cannot rule out secretion.

Question 10

What can PAH be used to measure and why?

Answer 10

• With PAH, the combination of filtration and secretion ensures all PAH enters the urine and none leaves via the renal vein.
• PAH clearance can therefore be measured to give an estimate of the volume of plasma perfusing the kidneys per unit time.

Question 11

What is the equation for RPF, PPAH, V and UPAH?

Answer 11

RPF x PPAH= V x UPAH

RPF = renal plasma flow rate
PPAH = plasma conc of PAH
Urine flow rate (V)
UPAH = PAH conc in urine

Question 12

Why is creatinine used over inulin?

Answer 12

It’s endogenous and it’s released into the blood at a pretty constant rate so plasma concentration remains fairly stable over time.

Question 13

If GFR is 3x lower than normal what should happen to the creatinine levels?

Answer 13

3x higher

Question 14

How can EDTA be used to measure clearance?

Answer 14

• As measuring the clearance of creatinine is not so accurate, we can radio-logically tag EDTA to create 51CrEDTA which is a gamma emitter
• You can then administer the substance as a single injection and measure the plasma activity of the substance at various intervals to see how much has been excreted
• A graph can then be plotted and the clearance is found by the gradient of the logarithmic version of the graph

Question 15

What would be seen in the EDTA graph of a person with renal failure?

Answer 15

Have a gentler gradient as they have less clearance and so higher plasma volume