Final answer:
If a substance is neither reabsorbed nor secreted, its excretion rate is equal to the glomerular filtration rate (GFR). This measure indicates the initial quantity of a substance that is filtered into the kidney, such as in the case with inulin or creatinine for clinical assessment of renal function.
Step-by-step explanation:
If a substance is neither reabsorbed nor secreted by the kidneys, then its excretion rate is equal to the glomerular filtration rate (GFR). This concept is important in understanding kidney excretory function. The GFR measures how much of a substance is initially filtered into the kidney. Inulin, a plant polysaccharide, is a classic example of a substance that matches this criterion. Substances like inulin appear in the urine in direct proportion to their filtration rate, as they pass through the kidneys unaltered. Furthermore, the measurement of naturally occurring substances like creatinine, which is not significantly reabsorbed and only minimally secreted, can provide a reliable estimate of GFR in clinical settings.
The equation that summarizes urinary excretion is Excretion = Filtration - Reabsorption + Secretion. In this case, as the substance is neither reabsorbed nor secreted, excretion equals filtration. Therefore, understanding the rate at which substances are excreted by the kidneys plays a critical role in managing drug dosages, particularly for those drugs eliminated primarily through the kidneys, to avoid toxic accumulations due to impaired renal function.