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Filtration Fraction


The filtration fraction (FF) is defined as the ratio of the GFR to the RPF. Under normal conditions this represents about 20% of the plasma volume passing through the kidneys or approximately 180 L /day and is calculated as the ratio of Cinulin to CPAH as illustrated in the figure. The average adult produces a urine volume of 1 to 2 L in the same period. This means that greater than 99% of the filtrate must be reabsorbed by the tubules. The FF plays a role in determining tubular reabsorptive efficiency, particularly in the proximal tubule. As FF increases the protein concentration in the plasma entering the peritubular capillaries also increases. The peritubular capillary oncotic pressure increases enhancing fluid reabsorption from the renal interstitial space to the capillary. This, in turn, decreases renal interstitial pressure favoring reabsorption across the tubular epithelium and minimizing back flux from the renal interstitial space to the tubule lumen. As FF decreases the reverse occurs.

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