GFR and Renal Clearance Flashcards
What is GFR?
Glomerular Filtration Rate:
Amount of fluid filtered from the glomeruli into the Bowman’s capsules per unit time (ml/min)
What is a freely filtered substance?
Concentration in plasma is the same as the filtrate (unaffected by glomerular filtration)
How do you calculate GFR of inulin/creatinine?
GFR = C = (V x U) / P
What are the 5 properties of inulin that make it ideal for GFR calculation?
- Freely filtered
- No reabsorption
- No secretion
- Non-toxic
- Measurable in plasma and urine
What are 2 limitations of inulin?
- Can’t administer orally - will be digested
- Need catheterisation - to measure urine production
rate
What is C?
Renal Clearance:
C = (V x U) / P
What can the difference between C of a freely filtered substance and inulin indicate?
Csubstance < Cinulin
- more reabsorption than secretion of substance
Csubstance = Cinulin
- same reabsorption than secretion of substance
Csubstance > Cinulin
- less reabsorption than secretion of substance
What is special about inulin/creatinine?
GFR = C
What is special about PAH?
RPF = C
- all PAH removed from plasma via filtration and
secretion
How can you calculate FF?
Filtration Fraction
- proportion of plasm entering glomeruli that is
filtered
- FF = GFR / RPF
= C(inulin) / C(PAH)
Why is inulin concentration in the renal vein different?
Water reabsorption —> dilutes blood in renal vein —> lowers inulin concentration (80%)
Why is PAH clearance not used if a renal disease is suspected?
Renal disease —> affect secretion —> C(PAH) ≠ RPF —> FF inaccurate
What is now replacing inulin/creatinine measurements and why?
Use of substances with the same properties as inulin that emit radiation eg. 51CrEDTA —> plot time against log(plasma activity) —> find gradient
- only need single dose and 2-3 samples
- initial steep fall of graph —> CrEDTA diffusing into
ECF - inulin —> accurate but inconvenient
- creatinine —> convenient but inaccurate
