How much oxygen is exhaled with each breath?
A. Air that is exhaled contains less oxygen and more carbon dioxide than atmospheric air, because of the gas exchange within the lungs. A typical breath with a volume of 500 mL consists of 350 mL of alveolar air (from deep within the lungs, where the gas exchange occurs) and 150 mL of air from the larger air passages (where no gas exchange occurs, and where the composition is similar to that of atmospheric air). The alveolar air is about 13% oxygen (versus 21% in atmospheric air) and 5% carbon dioxide (versus 0.04% in atmospheric air). As a result, the weighted (by volume) average for the total exhaled air is about 15.5% oxygen. (source: Claude A. Villee, Biology, W.B. Saunders Co., Philadelphia and London, copyright 1957.
