The Carbon Dioxide Equilibrium in Alveolar Air and Arterial Blood

It is a familiar thesis that if the diffusing apparatus of the lung permits 95 per cent saturation of the blood with oxygen, substantial equilibrium between the carbon dioxide pressures on opposite sides of the alveolar membrane will be attained. The principal supporting argument is the fact that carbon dioxide diffuses 20 to 30 times as rapidly as oxygen. It is assumed that the transfer of each gas is a diffusion phenomenon. That such an equilibrium exists in resting subjects has been demonstrated experimentally by Bock and Field (1) and Dautrebande (2). Krogh and Lindhard (3) have studied the changes in alveolar carbon dioxide pressure in exercise, but, in common with many others, doubt the accuracy with which such determinations portray the arterial carbon dioxide pressure. It has been our purpose to investigate the possibility of collecting, during exercise, alveolar air which will measure the average carbon dioxide pressure of arterial blood.