Determination of tissue volume and carbon dioxide dissociation slope of the lungs in man

¹ Department of Physiology, Graduate School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania

The volume of N₂O absorbed by the lung can be measured during inspiration before the gas is taken up in significant amounts by the capillary blood flow by a plethysmographic technique. If the alveolar fraction of N₂O and its solubility coefficient are known, the combined pulmonary tissue and capillary blood volume (Vt+c) can be calculated. Vt+c averaged 517 ml, sd 61 ml, in nine adult males, values in good agreement with those obtained by other methods. The pulmonary tissue volume alone, Vt, in six of these subjects was 438 ml, sd 58 ml. A modification of the technique, using CO₂ as the test gas, permitted calculation of the CO₂ dissociation slope of the lung tissue. In five subjects, the CO₂ dissociation slope of the lung, without the buffering effect of pulmonary capillary blood, over a range of Pa_COCO2 from 31 to 55 mm Hg, averaged 0.30 ml CO₂ STPD/100 ml lung tissue per mm Hg, or including physically dissolved CO₂, 0.38 ml CO₂ STPD/100 ml lung tissue per mm Hg. Because Vt is about four times larger than Vc, the lung tissue is of greater importance than the pulmonary capillary blood in buffering fluctuations in alveolar carbon dioxide during respiration.

Submitted on August 26, 1963

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