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Figure 29-7

Primary mechanisms by which high potassium intake raises potassium excretion. Note that increased plasma potassium concentration directly raises potassium secretion by the cortical collecting tubules and indirectly increases potassium secretion by raising plasma aldosterone concentration.

Primary mechanisms by which high potassium intake raises potassium excretion. Note that increased plasma potassium concentration directly raises potassium secretion by the cortical collecting tubules and indirectly increases potassium secretion by raising plasma aldosterone concentration.

increased extracellular potassium concentration to elevate potassium excretion when potassium intake is raised (Figure 29-7).

Blockade of Aldosterone Feedback System Greatly Impairs Control of Potassium Concentration. In the absence of aldosterone secretion, as occurs in patients with Addison's disease, renal secretion of potassium is impaired, thus causing extracellular fluid potassium concentration to rise to dangerously high levels. Conversely, with excess aldosterone secretion (primary aldosteronism), potassium secretion becomes greatly increased, causing potassium loss by the kidneys and thus leading to hypokalemia.

The special quantitative importance of the aldos-terone feedback system in controlling potassium concentration is shown in Figure 29-8. In this experiment, potassium intake was increased almost sevenfold in dogs under two conditions: (1) under normal conditions and (2) after the aldosterone feedback system had been blocked by removing the adrenal glands and placing the animals on a fixed rate of aldosterone infusion so that plasma aldosterone concentration could neither increase nor decrease.

Note that in the normal animals, a sevenfold increase in potassium intake caused only a slight increase in potassium concentration, from 4.2 to 4.3 mEq/ L. Thus, when the aldosterone feedback system is functioning normally, potassium concentration is precisely controlled, despite large changes in potassium intake.

When the aldosterone feedback system was blocked, the same increases in potassium intake caused a much larger increase in potassium concentration, from 3.8 to almost 4.7 mEq/L. Thus, control of potassium concentration is greatly impaired when the aldosterone feedback system is blocked. A similar

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Essentials of Human Physiology

Essentials of Human Physiology

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