Which is the primary mechanism causing respiratory alkalosis?

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Respiratory alkalosis is driven by excessive CO2 loss due to increased ventilation. When you hyperventilate, you blow off more carbon dioxide from the lungs, reducing the amount of dissolved CO2 in the blood. Because CO2 is in equilibrium with carbonic acid and H+ (CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3−), falling CO2 shifts the balance toward fewer hydrogen ions, raising the blood pH. So the primary mechanism is increased alveolar ventilation—hyperventilation.

Deficient alveolar diffusion would mainly affect oxygen transfer and may distort gas exchange in other ways, but it doesn’t cause the rapid CO2 loss that produces alkalosis. Parasympathetic inhibition doesn’t directly alter CO2 removal, and deficient pulmonary perfusion is a perfusion issue that typically affects gas exchange differently and is not the driver of CO2 loss leading to alkalemia.