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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, creatinine, and other endogenously produced substances as indicators of changes in glomerular filtration rate.

  • Describe other clinical measures of renal function.

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THE CLEARANCE CONCEPT

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Image not available.As outlined in Chapter 1, the body constantly removes metabolic waste products, ingested substances, and excess salt and water from the body 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 of any substance can be expressed in several ways, a common one being the plasma half-life, that is, the time it takes for the concentration in the plasma to be reduced by 50%. 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 other hand, means that the substance is removed from the plasma only by the kidneys and is either excreted in the urine or catabolized by the renal tubules.

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Clearance measures the volume of plasma from which all of a substance is removed in a given time.

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Compare how the body handles two substances with similar-sounding names but very different properties: inulin and insulin. Insulin is the familiar pancreatic hormone involved in regulating blood glucose. It is a protein with a molecular weight of 5.8 kDa and is small enough to be freely filtered by the glomerulus. Once in Bowman’s space, it moves along with every other filtered substance into the proximal convoluted tubule, where it is largely taken up by endocytosis and degraded into its constituent amino acids. Very little insulin escapes this uptake, and very little of the filtered insulin makes it all the way to the urine. Thus, ...

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