Figure 293

Mechanisms of potassium secretion and sodium reabsorption by the principal cells of the late distal and collecting tubules.

and make up about 90 per cent of the epithelial cells in these regions. Figure 29-3 shows the basic cellular mechanisms of potassium secretion by the principal cells.

Secretion of potassium from the blood into the tubular lumen is a two-step process, beginning with uptake from the interstitium into the cell by the sodium-potassium ATPase pump in the basolateral membrane of the cell; this pump moves sodium out of the cell into the interstitium and at the same time moves potassium to the interior of the cell. The second step of the process is passive diffusion of potassium from the interior of the cell into the tubular fluid. The sodium-potassium ATPase pump creates a high intra-cellular potassium concentration, which provides the driving force for passive diffusion of potassium from the cell into the tubular lumen.The luminal membrane of the principal cells is highly permeable to potassium. One reason for this high permeability is that there are special channels that are specifically permeable to potassium ions, thus allowing these ions to diffuse across the membrane.

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Brain Blaster

Brain Blaster

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