Net Tubular Reabsorption or Secretion of a Substance Can Be Calculated From Filtered and Excreted Amounts

The rate at which the kidney tubules reabsorb a substance can be calculated if we know how much is filtered and how much is excreted per unit time. If the filtered load of a substance exceeds the rate of excretion, the kidney tubules must have reabsorbed the substance. The equation is:

Treabsorbed Px

where T is the tubular transport rate.

The rate at which the kidney tubules secrete a substance is calculated from this equation:

Note that the quantity excreted exceeds the filtered load because the tubules secrete X.

In equations 8 and 9, we assume that substance X is freely filterable. If, however, substance X is bound to the plasma proteins, which are not filtered, then it is necessary to correct the filtered load for this binding. For example, about 40% of plasma Ca2+ is bound to plasma proteins, so 60% of plasma Ca2+ is freely filterable.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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