filtration fraction = GFR/RPF glomerular filtration rate/renal plasma flow = 20% (i.e., 80% of plasma is not filtered) prevents hyperviscosity of blood in efferent arterioles and peritubular caps.
filtration barrier to negatively charged molecules and to molecules > 70,000 kDa (albumin is 65,000 kDa but is negatively charged - 200 neg. charges per molecule)
GFR = 125 ml/min = 180 L/day (Urine production = 1.8 L/day; 99% reabsorption)
Angiotensin II constricts efferent arteriole more than afferent arteriole.
How is GFR measured? There is one way into the kidney (blood) and two ways out (blood or urine). By using substances that are filtered from blood BUT not reabsorbed and not secreted into urine. Clearance of substance X from plasma: GFR [X]pv = Vu[X]u In English, the amount of X leaving plasma = the amount of X in urine. Amount = volume x concentration. for plasma, volume = GFR and concentration = [X]p
for urine, volume = Vurine and concentration = [X]u.
amount leaving plasma = amount in urine = GFR [X]p = Vu[X]u or GFR = Vu[X]u/[X]p
note that the graph of [X]p = f(GFR) = y=1/X = hyperbola.
Example of GFR calculation using lab data:
[Cr]u = 75 mg/dL [Cr]p = 2.5 mg/dL Vu = 1440 ml/day or 1/ml/min.
GFR = 75 x 1/2.5 = 30 ml/min
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