It is evident to the most superficial observer that under ordinary conditions of breathing some of the expired air must be drawn back again into the lungs. A few experiments to determine the proportion of the breath that is reinspired have been made during the last fifteen years, but no really thorough study of the question was carried out until the recent work of Crowder.1 This investigator has studied by ingenious methods the effect of such factors as change of position, body motion, different types of breathing and different temperatures, and in addition has determined the conditions that obtain on the sleeping-porch and in the open air. Nasal breathing is the type especially studied; in mouth-breathing there is little or no reinspiration.
The Reinspiration of Expired Air. JAMA. 2013;310(20):2205. doi:10.1001/jama.2013.5432