# Alveolar gas equation

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**Editor-In-Chief:** C. Michael Gibson, M.S., M.D. [1]

## Overview

The alveolar pO_{2} is not routinely measured but is calculated from blood gas measurements by the Alveolar gas equation:

<math>p_AO_2 = p_IO_2 - p_ACO_2/R + F \, </math> where:

- R is the Respiratory quotient (normally about 0.8)
- p
_{A}O_{2}is the Alveolar pO_{2} - p
_{I}O_{2}is the Inspired pO_{2}, equal to about 150 mm Hg (0.21 x 713 mmHg at sea level). The given pressure at sea level is due to atmospheric pressure (760 mmHg) minus the partial pressure of water vapor (47 mmHg), as alveolar gas is completely saturated with water. The mole fraction of oxygen is about 0.21 in dry atmospheric gas. - p
_{A}CO_{2}is the Alveolar pCO_{2}(assumed to be equal to the measured arterial pCO_{2}) - F is a correction factor (usually less than 2 mmHg)