Fractional values

There are two additional calculations that inform our understanding of renal function.  These are fractional excretion and fractional reabsorbtion.  These compare the amount of excretion or reabsorption with respect to the amount filtered.  Importantly, differences in fractional handling indicate differences in nephron function.

For example, you can evaluate the renal handling of Na⁺ in the absence and presence of amiloride, an inhibitor of the epithelial sodium channel (ENaC).  If the fractional excretion of Na⁺ is different in the presence of amiloride, this tells you that ENaC is playing a role in transporting Na⁺.

Fractional excretion

Fractional excretion = amount excreted / amount filtered

[latex]=  \frac{(U \cdot V)}{(P \cdot GFR)}[/latex]

For the example in the previous chapter:

[latex]=  \frac{0.33}{15.86} = 0.02[/latex]

Expressed as a percentage this would be 2.1%

Fractional reabsorption

Fractional reabsorption = amount reabsorbed / amount filtered

[latex]=  \frac{ (P \cdot GFR) – (U \cdot V)}{(P \cdot GFR)}[/latex]

For the example in the previous chapter:

[latex]=  \frac{15.53}{15.86} = 0.98[/latex]

Expressed as a percentage this would be 97.9%