1 - Glomerular Filtration 2 Flashcards
Objectives: How do you measure GFR
- Measured by inulin infusion via IV
- All that is filtered is excreted
- Equation:
- GFR = (V x UIN) / PIN
- V = Urine Flow
- UIN = [Inulin] in Urine
- PIN = [Inulin] in Plasma
- GFR = (V x UIN) / PIN
Objectives: Explain the concept of clearance
- Clearance: Volume of plasma from which that substance is completely cleared per minute
- Equation:
- CA = ( UA x V ) / PA
- C = clearance
- U = [Concentration] in Urine
- V = Urine Flow
- P = [Concentration] in Plasma
- CA = ( UA x V ) / PA
Objectives: Explain single and double ratios
- Single: Any change to [Inulin] in the nephron relative to plasma concentration of inulin reflects changes in water movement
- BSIN / PIN = 1 = TFIN / PIN
- Anytime this is not one, something has changed!
- Double: Used to evaluate presence of net reabsorption or net secretion in individual nephron segments
- (Tubular Fluid Q/Plasma Q)/(Tubular Fluid IN/PlasmaIN)
Objectives: Explain Fractional Excretion (FE)
- Fractional Excretion (FE): Method of calculating degree of net reabsorption or net secretion of a substance
- Equation:
- Fractional Excretion = (Mass Excreted) / (Mass Filtered)
- FEA = ( UA x V ) / ( GFR x PA )
- FEA = ##.# means ##% of A has been excreted; and 100-## = reabsportion
- FEB = 1.5, 50% more B is excreted than was filtered; secretion is occuring
What is the definition of renal clearance?
Volume of plasma from which all of a substance has been removed and excreted into urine per unit of time
What makes inulin a good candidate to measure GFR?
- Not bound to plasma proteins
- Not electrically charged
- Freely Filtered
- Not lost by reabsoprtion from tubular fluid to peritubular fluid
- Not gained by secretion from peritubular fluid into tubular fluid
- All that is filtered is excreted
What is unique about inulin clearance with regards to the clearance equation?
What endogenous substance can approximate GFR?
GFR = (UINxV)/PIN = CIN
- Inuline is independent of plasma inulin concentration and urine flow; e.g. it can be used to directly measure GFR
- 125 ml/min represents max volume plasma that can be cleared exclusively by filtration per minute
-
Creatinine can approximate GFR for same reasons
- GFR = CCR = ( UCRxV ) / PCR
- Filtered and actively secreteed at slow rate, slightly overestimates GFR
- Can give relative kindey fx change over long period
How is the clearance of inulin used as a baseline vs other substances?
Imagine two substances, A and B
- CA > CIN = A is cleared by filtration and secretion
- Excreted A = Filtered A + Secreted A
- CB < CIN = some of filtered B is reabsorbed
- Excreted B = Filtered B - Reabsorbed B
What does the renal handling of glucose reflect?
What happens to it?
What happens to water and how does that impact the [glucose]?
- Glucose: Filtration (in glomerulus) followed by reabsorption of glucose (in the tubule)
- Water: Filtration (in the glomerulus) followed by reabsorption of water (in the tubule);
- The loss of water (tublar fluid volume) causes the concentration of glucose to increase
- Thus: Ratio of Urine Glucose or Tubular Fluid Glucose Concentration / Plasma Concentration reflects the net effects of glucose reabsorption AND water reabsorption
How can you eliminate the effect of water transportfor measuring renal handling of glucose?
By comparing the single ratio for glucose with the single ratio for inulin
(TF/P)G / (TF/P)IN
What three things does the double ratio allow calculation of?
- Net direction of solute transport (rebasorbed / secreted)
- What fraction of the filtered mass of a substance the excreted mass of a substance represents
- Measure of fraction of filtered solute still remaining in the urine or tubular fluid
Clearance Ratio
= 1
< 1
> 1
CX / CIN
= 1 - On a net basis filtered X is not reabsorped or secreted
< 1 - on a net basis X undergoes rebsorption
> 1 - On a net basis X undergoes secretion
