Diffusing Capacity of the Respiratory Membrane
The ability of the respiratory membrane to exchange a gas between the alveoli and the pulmonary blood is expressed in quantitative terms by the respiratorymembrane’s diffusing capacity, which is defined asthe volume of a gas that will diffuse through the mem-brane each minute for a partial pressure difference of 1 mm Hg. All the factors discussed earlier that affectdiffusion through the respiratory membrane can affect this diffusing capacity.
Diffusing Capacity for Oxygen. In the average young man,the diffusing capacity for oxygen under resting condi-tions averages 21 ml/min/mm Hg. In functional terms, what does this mean? The mean oxygen pressure difference across the respiratory membrane during normal, quiet breathing is about 11 mm Hg. Multipli-cation of this pressure by the diffusing capacity (11 x 21) gives a total of about 230 milliliters of oxygen dif-fusing through the respiratory membrane each minute; this is equal to the rate at which the resting body uses oxygen.
Change in Oxygen Diffusing Capacity During Exercise. During strenuous exercise or other conditions that greatly increase pulmonary blood flow and alveolar ventilation, the diffusing capacity for oxygen increases in young men to a maximum of about 65 ml/min/ mm Hg, which is three times the diffusing capacity under resting conditions. This increase is caused by several factors, among which are (1) opening up of many previously dormant pulmonary capillaries or extra dilation of already open capillaries, thereby increasing the surface area of the blood into which the oxygen can diffuse; and (2) a better match between the ventilation of the alveoli and the perfusion of the alveolar capillaries with blood, called the ventilation-perfusion ratio, which is explained in detail later. Therefore, during exercise, oxygenation of the blood is increased not only by increased alveolar ventilation but also by greater diffusing capacity of the respiratory membrane for transporting oxygen into the blood.
Diffusing Capacity for Carbon Dioxide. The diffusing capac-ity for carbon dioxide has never been measured because of the following technical difficulty: Carbon dioxide diffuses through the respiratory membrane so rapidly that the average PCO2 in the pulmonary blood is not far different from the PCO2 in the alveoli—the average difference is less than 1 mm Hg—and with the available techniques, this difference is too small to be measured.
Nevertheless, measurements of diffusion of other gases have shown that the diffusing capacity varies directly with the diffusion coefficient of the particular gas. Because the diffusion coefficient of carbon dioxide is slightly more than 20 times that of oxygen, one would expect a diffusing capacity for carbon dioxide under resting conditions of about 400 to 450 ml/min/ mm Hg and during exercise of about 1200 to 1300 ml/ min/mm Hg. Figure 39–10 compares the measured or calculated diffusing capacities of carbon monoxide, oxygen, and carbon dioxide at rest and during exercise, showing the extreme diffusing capacity of carbon dioxide and the effect of exercise on the diffusing capacity of each of these gases.
Measurement of Diffusing Capacity—The Carbon Monoxide Method. The oxygen diffusing capacity can be calculatedfrom measurements of (1) alveolar PO2, (2) PO2 in the pulmonary capillary blood, and (3) the rate of oxygen uptake by the blood. However, measuring the PO2 in the pulmonary capillary blood is so difficult and so impre-cise that it is not practical to measure oxygen diffusing capacity by such a direct procedure, except on an exper-imental basis.
To obviate the difficulties encountered in measuring oxygen diffusing capacity directly, physiologists usually measure carbon monoxide diffusing capacity instead and then calculate the oxygen diffusing capacity from this. The principle of the carbon monoxide method is the following: A small amount of carbon monoxide is breathed into the alveoli, and the partial pressure of the carbon monoxide in the alveoli is measured from appro-priate alveolar air samples. The carbon monoxide pres-sure in the blood is essentially zero, because hemoglobin combines with this gas so rapidly that its pressure never has time to build up. Therefore, the pressure difference of carbon monoxide across the respiratory membrane is equal to its partial pressure in the alveolar air sample. Then, by measuring the volume of carbon monoxide absorbed in a short period and dividing this by the alveolar carbon monoxide partial pressure, one can determine accurately the carbon monoxide diffusing capacity.
To convert carbon monoxide diffusing capacity to oxygen diffusing capacity, the value is multiplied by a factor of 1.23 because the diffusion coefficient for oxygen is 1.23 times that for carbon monoxide. Thus, the average diffusing capacity for carbon monoxide in young men at rest is 17 ml/min/mm Hg, and the diffusing capacity for oxygen is 1.23 times this, or 21 ml/min/mm Hg.