Key Points

• Relatively small changes in plasma concentrations of K+, Ca2+, and Mg2+ can have important effects on neuromuscular excitability and cardiac rhythm. Balance of these cations is maintained largely by regulation of their excretion in the urine.

• Most of the K+ in the body is present in cells, so the only ~2% of the total body K+ is located in the extracellular fluid compartment. Thus, the balance between the uptake of K+ into cells and its loss from cells is the most important determinant of acute changes in the plasma K+ concentration.

• Changes in intracellular K+ generally parallel changes in extracellular K+. Thus, the transfer of K+ into or out of cells helps to buffer the effects of K+ intake or loss on the plasma K+ concentration.

• Alkalemia, ^-adrenergic agonists, insulin, and aldosterone enhance K+ uptake into cells. K+ is lost from cells in response to acidemia, hyperosmotic states, a-adrenergic agonists, and with exercise. Cell damage can cause the release of large amounts of K+ from cells.

• K+ is reabsorbed passively in the proximal tubule, and is reabsorbed passively and actively in the loop of Henle. The regulation of K+ secretion that occurs in the connecting tubule and collecting duct, i.e., the aldosterone-responsive distal nephron (ARDN), determines the rate of urinary K+ excretion.

• More than 99% of the total body Ca2+ is in bone and this portion does not exchange readily with free Ca2+ in the extracellular fluid.

• Calcium is reabsorbed passively in the proximal tubule at a rate that follows closely the rates of Na+ and water reabsorption. Ca2+ reabsorption in the loop of Henle occurs primarily in the ascending limb by passive diffusion via the paracellular pathway, and it is driven by a favorable concentration gradient and the lumen-positive voltage.

• Calcium is actively reabsorbed via Ca2+-selective channels in CNT cells in the connecting tubule,

Essential Medical Physiology, Third Edition

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