A 65-year-old man with mild hea failure is treated with a loop diuretic. A few days later the man complains of muscle weakness. Laboratory results are shown below. Aerial PCO2: 48 mm Hg Aerial pH: 7.49 Plasma HCO3-: 35 mEq/L Which of the following is most likely decreased in this man?

The lab repos indicate that the man has developed metabolic alkalosis (increased PCO2, pH, and HCO3-), which occurs commonly with overuse of diuretics (thiazides and loop diuretics). The overuse of a loop diuretic increases the excretion of sodium and potassium by the kidneys. The increase in potassium excretion leads to a decrease in plasma potassium levels. The sodium depletion stimulates renin secretion, which in turn raises angiotensin II levels in the plasma (which also stimulates aldosterone secretion).