Ultimately, Our Ability to Assess the Function of the Kidneys

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The Answer is In its simplest form, excretion rate can be measured as an ion's filtration rate + secretion rate - reabsorption rate. Filtration rate can be determined as filtered load (GFR × plasma concentration of an ion). Excretion gives an indication of the net direction of movement of an ion. If the amount of a solute excreted per minute is less than its filtered load, then the solute was reabsorbed. If the amount of a solute excreted is greater than filtered load, then that solute was secreted. However, this measurement reflects which force (reabsorption or secretion) is dominating for a particular solute, which means that both are usually occurring at the same time. The rate at which a solute is excreted can be described in terms of clearance: a measure of the volume of plasma from which a solute is completely removed. Clearance is calculated as excretion rate/plasma concentration. Clearance gives an indication of how the kidneys are handling one solute relative to another. Clinically, clearance is determined by three variables: (urine concentration of a solute × urine flow rate)/plasma concentration of a solute. These are easily measured clinically. Clearance also provides a way to measure GFR. If a substance is freely filtered and is neither reabsorbed nor secreted, then the amount of that substance in the urine must be equal to the amount filtered (filtered load). In this case, the clearance of this substance is equal to GFR. Inulin is a polysaccharide that is neither secreted nor reabsorbed. In this case, GFR = (urine concentration of inulin × urine flow rate)/plasma concentration of inulin. Another less invasive method for measuring GFR involves measuring creatinine, a byproduct of muscle metabolism. Only a small amount of creatinine is secreted and none is reabsorbed.

Ultimately, Our Ability to Assess the Function of the Kidneys

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