Copyright © 2004, Elsevier Science (USA) All rights reserved.
Understanding the role of the kidney in maintaining the constancy of the internal environment requires an appreciation of the quantities of substances that are taken into the body, produced by metabolism, and excreted from the body, as well as the quantities exchanged among body fluid compartments. Because substances are exchanged continuously and rapidly among these compartments, static parameters such as the concentration of a solute do not provide sufficient information about the rates of material movement or the regulatory adjustments that are occurring constantly. To appreciate the rate of movement into or out of a compartment, it is necessary to think in terms of mass flow, that is, in terms of an amount per minute. This flow can be expressed, as examples, in terms such as milliliters of water per minute, millimoles (or milliequivalents) of Na+ per minute, or milligrams of creatinine per minute.
The total amounts of all solutes and water entering the kidney via the renal artery leave in the combined flows of the urine, the renal vein, and the lymphatics. Some of these solutes and water are filtered from the plasma in the glomerulus. Solutes and water are returned from the tubular fluid to the peritubular capillary circulation by the process of reabsorption. For some solutes, the amount filtered may be augmented by transport from the peritubular circulation into the tubular fluid, a process referred to as secretion. Thus, the rate at which a particular solute is excreted is given by the sum of the rate at which it is filtered and the rate at which it is secreted, minus the rate at which it is reabsorbed.
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