Acid-base homeostasis · USMLE Step 1

Which condition describes a primary respiratory alkalosis on arterial blood gas analysis?

  1. Low PaCO₂, high HCO₃⁻
  2. High PaCO₂, high HCO₃⁻
  3. Low PaCO₂, low HCO₃⁻
  4. High PaCO₂, low HCO₃⁻
Show answer and explanation

Correct answer: Low PaCO₂, low HCO₃⁻

In primary respiratory alkalosis, hyperventilation lowers PaCO₂, shifting equilibrium to reduce H⁺, causing pH rise; HCO₃⁻ falls later due to compensatory renal excretion.

Difficulty: Medium Question 10 of 19

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