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Ionized calcium
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Potassium
Protein-bound calcium
Sodium
Total calcium
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This hiker is hyperventilating, which is the normal physiologic response to altitude and would result in a respiratory alkalosis. In alkalotic states (high pH), there is increased binding of calcium ions to protein, resulting in a decrease in ionized calcium levels. Calcium exists in physiologically active and inactive forms in the human body. Ionized calcium is the physiologically active form, whereas protein-bound calcium is inactive. Calcium and hydrogen ions competitively bind to serum proteins. Therefore, an excess of hydrogen ions (i.e., low pH) results in reduced calcium binding to proteins, allowing more calcium to exist in the ionized state. The opposite is true in states of low hydrogen ion concentration (high pH). Homeostatic calcium regulation depends on ionized, not total, calcium concentrations and is mediated by parathyroid hormone. Incorrect Answers: Answer 2: Potassium levels are decreased during states of metabolic alkalosis due to intracellular shifts of potassium in response to pH, where intracellular protons are transported into the serum down their concentration gradient in exchange for potassium import into cells via a proton/potassium ion exchanger. In contrast, potassium levels increase in response to respiratory alkalosis due to increased alpha-adrenergic activity, which stimulates hepatic calcium-dependent potassium channels. Answer 3: Protein-bound calcium levels would increase during alkalotic states as the reduced hydrogen ion concentration allows for increased calcium binding to protein. Answer 4: Sodium levels are not expected to change with hyperventilation. Hyponatremia might be caused by states of fluid overload (e.g., heart failure, cirrhosis, nephrotic syndrome), syndrome of inappropriate diuretic hormone, or poor dietary intake. Answer 5: Total calcium level is not expected to change with hyperventilation. The various forms of calcium are inter-converted (i.e., ionized calcium into protein-bound calcium) while the total body calcium remains steady. Bullet Summary: Hyperventilation induces a respiratory alkalosis that increases calcium binding to serum proteins, resulting in decreased ionized calcium.
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