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OBJECTIVES

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  • Define the terms clearance and metabolic clearance rate, and differentiate between general clearance and specific renal clearance.

  • List the information required to calculate clearance.

  • State the criteria that must be met for a substance so that its clearance can be used as a measure of glomerular filtration rate; state which substances are used to measure glomerular filtration rate and effective renal plasma flow.

  • Given data, calculate the clearance of any excreted substance.

  • Predict whether a substance undergoes net reabsorption or net secretion by comparing its clearance with that of inulin or by comparing its rate of filtration with its rate of excretion.

  • Given data, calculate net rate of reabsorption or secretion for any substance.

  • Given data, calculate fractional excretion of any substance.

  • Describe how to estimate glomerular filtration rate from creatinine clearance and describe the limitations of this estimate.

  • Describe how to use plasma concentrations of urea and creatinine as indicators of changes in glomerular filtration rate.

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THE CLEARANCE CONCEPT

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Image not available. Metabolic waste products, ingested substances, and excess salt and water are constantly being removed from the body (cleared) by a number of means, including disposal in the urine, in the feces, biochemical transformation in the liver, and for volatile substances, exhalation. The rate of removal can be expressed in several ways, a common one being the plasma half-life. Another way to express removal rate is clearance, which is the volume of plasma per unit time from which all of a specific substance is removed. Clearance in a biomedical context has both a general meaning and a specific renal meaning. The general meaning is simply that a substance is removed from the plasma by any of the mechanisms mentioned above. Its quantitative measure is called the metabolic clearance rate. Renal clearance, on the contrary, means that the substance is removed from the plasma and excreted in the urine. As explained below the clearance of certain substances is a method to measure glomerular filtration rate (GFR). Evaluation of GFR via renal clearance is commonly used clinically as an overall assessment of renal health. Repeated assessments over a period of time can indicate whether renal function is stable or deteriorating.

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CLEARANCE UNITS

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Image not available. The units of renal clearance are often confusing to the first-time reader, so let us be sure of the meaning. First, the units are volume per time (not amount of a substance per time). The volume is the volume of plasma that supplies the amount excreted in a given time. For example, suppose each liter of plasma contains 10 mg of a substance X, and 1 mg of substance X is excreted in 1 hour. Then 0.1 L of plasma supplies the 1 mg that is excreted, that is, the renal clearance is 0.1 L/h. The reader should appreciate that removing all of a substance from a ...

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