Alveolar gas equation

The alveolar gas equation is used to predict the alveolar concentration of oxygen based on the alveolar concentration of carbon dioxide. The alveolar gas equation is expressed as the alveolar concentration of carbon dioxide divided by the respiratory quotient (0.8 in standard cases) and subtracted from inspired concentration of oxygen. Basically, the alveolar gas equation takes into account that an individual inspires a certain concentration of oxygen that reaches the alveoli, but that concentration is diluted by an amount of carbon dioxide generated by metabolism and released into the lungs. The generation of carbon dioxide is  proportional to a standard rate of exchange of carbon dioxide and oxygen called the respiratory quotient, equal to 0.8 or 80%.