The action of carbon dioxide on the collateral pathways of pulmonary ventilation

The major collateral ventilatory pathways between the segments of the lobes of the dog are lined with smooth muscle and course from bronchiole to alveolus and from bronchiole to bronchiole. In previous studies, we showed that the channels constricted with metacholine and dilated with epinephrine. Carbon dioxide elevations in the inspired air, as noted in this study, greatly increased the collateral flow, whereas 100 per cent oxygen did not affect it. The greatest dilatation occurred when the inspired carbon dioxide increased from 2.5 to 6 per cent, with little if any further change when increased to 15 per cent. When perfusion of the in situ lung was blocked, or when the lung was excised, carbon dioxide still had a marked bronchodilating effect. This indicates its direct action on smooth muscle. The fact that carbon dioxide acts as a dilator of the collateral channels, independent of neural and humoral influences, confirms its important role in ensuring maximal pulmonary ventilation.