Tubular Reabsorption and Secretion

Two additional processes that participate in urine formation are reabsorption and secretion. Reabsorption defines movement of solute or water from the tubule lumen to the blood, whereas secretion denotes transport from the blood to the tubule lumen. For many solutes, such as organic acids, transport proceeds in both directions. Net transport is determined by the dominant flux. As described later, the tubular secretion of some diuretics is critical for their action. The nephron sites where ions and organic solutes are transported are spatially separated. Figure 21.1 illustrates the various nephron segments, the primary sites of solute transport, and the magnitude of sodium reabsorption. In some instances, as with sodium, several transport mechanisms mediate its reabsorption. Importantly, each mechanism is spatially separated within different nephron segments. This is important in understanding diuretic action, which is specific to particular sodium transport mechanisms. Furthermore, some common side effects caused by diuretics, such as potassium wasting, develop as a direct consequence of the mechanism and the particular location of diuretic action at sites upstream from

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