Filtered Load

A term we use in other chapters is filtered load. It is the amount of substance that is filtered per unit time. For freely filtered substances, the filtered load is just the product of GFR and plasma concentration. Consider sodium. Its normal plasma concentration is 140 mEq/L, or 0.14 mEq/mL. (Note: 1 mEq of sodium is 1 mmol.) A normal GFR is 125 mL/min, so the filtered load of sodium is 0.14 mEq/mL × 125 mL/min = 17.5 mEq/min. We can do the same calculation for any other substance, being careful in each case to be aware of the unit of measure in which concentration is expressed. The filtered load is what is presented to the rest of the nephron to handle. A high filtered load means a substantial amount of material to be reabsorbed. The filtered load varies with plasma concentration and GFR. A rise in GFR, at constant plasma concentration, increases the filtered load, as does a rise in plasma concentration at constant GFR.