U01.21.028 Glomerular Filtration Rate (GFR)

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Learning Objective:
Understand and apply the formulas for Glomerular Filtration Rate (GFR), which measures how much plasma is filtered by the glomeruli per unit time.

Formulas:

$C_{inulin} = GFR = \frac{U_{inulin} \times V}{P_{inulin}}$

and by Starling forces in the glomerulus:

$GFR = K_f \Big[ (P_{GC} - P_{BS}) - (\pi_{GC} - \pi_{BS}) \Big]$

Explanation:

$U_{inulin}$ = Urine concentration of inulin
$P_{inulin}$ = Plasma concentration of inulin
$V$ = Urine flow rate
$K_f$ = Filtration coefficient (product of capillary permeability and surface area)
$P_{GC}$ = Glomerular capillary hydrostatic pressure (favors filtration)
$P_{BS}$ = Bowman space hydrostatic pressure (opposes filtration)
$\pi_{GC}$ = Glomerular capillary oncotic pressure (opposes filtration)
$\pi_{BS}$ = Bowman space oncotic pressure (usually ≈ 0)

Concept Summary:

Inulin is the gold-standard marker for GFR because it is freely filtered and neither reabsorbed nor secreted.
Creatinine clearance is commonly used to estimate GFR in clinical practice (slightly overestimates due to secretion).

Example (Using Inulin):

Given:
$U_{inulin} = 100 \text{ mg/dL}$
$P_{inulin} = 5 \text{ mg/dL}$
$V = 1 \text{ mL/min}$