Learning Objective:
Understand and apply the Alveolar Gas Equation to calculate the alveolar oxygen tension (PAO₂), which helps assess gas exchange efficiency.

Formula:

Explanation:

• = Alveolar oxygen partial pressure (mm Hg)
• = Inspired oxygen partial pressure (≈ 150 mm Hg at sea level on room air)
• = Arterial CO₂ partial pressure (mm Hg)
• = Respiratory quotient (CO₂ produced / O₂ consumed; usually 0.8)

The equation calculates expected alveolar O₂, accounting for CO₂ levels and metabolism.

Concept Summary:

• Normal PAO₂ ≈ 100 mm Hg on room air
• Helps evaluate A–a gradient:
  • A–a gradient = PAO₂ – PaO₂
  • ↑ indicates impaired gas exchange (V/Q mismatch, diffusion defect, shunt)

Example:

Given:

✅ Answer: PAO₂ = 100 mm Hg