The ability of the respiratory membrane to exchange a gas between the alveoli and the pulmonary blood is expressed in quantitative terms by the respiratory membrane's diffusing capacity, which is defined as the volume of a gas that will diffuse through the membrane each minute for a partial pressure difference of 1 mm Hg. All the factors discussed earlier that affect diffusion 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 conditions 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 mmHg. Multiplication of this pressure by the diffusing capacity (11 x 21) gives a total of about 230 milliliters of oxygen diffusing 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 ventilationperfusion ratio, which is explained in detail later in this chapter. 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 capacity 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 calculated from 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 imprecise that it is not practical to measure oxygen diffusing
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