Diffusing Capacity of the Respiratory Membrane |
The ability of the respiratory membrane to exchange a gas between the alveoli and the pulmonary blood can be expressed in quantitative terms by its diffusing capacity, defined as the volume of a gas that diffuses through the membrane each minute for a pressure difference of 1mmHg.
For Oxygen, the diffusing capacity under resting conditions averages 21ml per minute per mmHg. As the average pressure difference is of 11mmHg, the amount of oxygen that diffuses per minute would average 21 x 11 = 230ml.
For Carbon Dioxide, the diffusing capacity has never been measured as this gas diffuses so quickly that the aveerage pCO2 in the pulmonary blood is not far different from the pCO2 of the alveoli (around 1mmHg). But as it has been shown that the diffusing capacity is directly proportional to the diffusion coefficient of the gas, it can be safely assumed that the diffusing capacity for CO2 is at least 20 times that of oxygen (400 to 450ml/minute/mmHg).
The diffusing capacity of the respiratory membrane can be modified during exercise by opening or dilating closed capillaries.