Mechanism of pulmonary gas exchange and CO₂ transport during breath holding

¹ Cardio-Pulmonary Laboratory, Mary Imogene Bassett Hospital, Cooperstown, New York

Experiments describe the changes in Pa_COCO2 and lung volume shrinkage during breath holding with O₂ in man and the Pa_COCO2, pH and CO₂ content of arterial and mixed venous blood during breath holding in the dog. An explanation is offered for the aberrations in CO₂ transport and exchange which occur during apnea. A self-perpetuating cycle is established during breath holding which is initiated by the arrest of the ventilatory output of CO₂. The arterial P_COCO2 rises rapidly as a result of decreased clearance of CO₂ from venous blood, the concentrating effect of lung volume shrinkage and the Haldane effect from oxygenation of hemoglobin. The venous P_COCO2 rises more slowly because of the uptake of CO₂ by the tissues and the Haldane effect from reduction of oxyhemoglobin. By this mechanism the CO₂ output into the lungs progressively falls and eventually stops. The cycle then is reversed and CO₂ moves from lungs to arterial blood.