This is the alveolar gas equation on its own, calculating the oxygen level your alveoli should have based on how much oxygen you're breathing and your CO₂ level.
| Term | Meaning | Typical |
|---|---|---|
| FiO₂ | Fraction inspired O₂ | 0.21 room air |
| Patm | Atmospheric pressure | 760 mmHg sea level |
| 47 | Water vapor pressure | constant |
| 0.8 | Respiratory quotient | constant |
Given: room air, sea level, PaCO₂ 40 mmHg.
149.750The alveolar gas equation predicts the oxygen tension the alveoli should contain.
This stops at step one — the expected alveolar oxygen. Use the full A-a gradient tool once you also have a measured PaO₂, since the gradient is PAO₂ minus PaO₂.
About 100 mmHg on room air at sea level. It rises with supplemental oxygen and falls at altitude as atmospheric pressure drops.
The 0.8 is the respiratory quotient — carbon dioxide displaces oxygen in the alveolus, and dividing by the respiratory quotient accounts for the slight difference between CO₂ produced and O₂ consumed.
It's the partial pressure of water vapor (PH₂O) in the fully humidified airway at body temperature, subtracted from atmospheric pressure to give the pressure available for dry gases.