The effect of cardiac output on pulmonary shunt was studied in living lungs of human patients and dogs which had alveoli and blood vessels, and also in artificial lung (bubble-type oxygenator) which, had no such structural factors. In spite of such a structural difference, quite similar results were observed in both living and artificial lungs, and this suggests that some common factors other than the change in blood vessels result in the change in pulmonary shunt. It was observed that pulmonary shunt increased as the saturation of mixed venous blood became higher. It was clarified that the oxygen receiving capacity of blood decreased as the oxygen saturation became higher, i.e., the amount of reduced hemoglobin became less. The reduction in oxygen-receiving capacity of blood results in the leduction in oxygen-transfer efficiency from alveoli to blood and hence pulmonary shunt increases, and this can be regarded as a kind of shunt-like effect. It can be concluded that the change in pulmonary shunt by cardiac output is not due to the change in the lung function but is caused by the change in the oxygen-receiving capacity of the mixed venous blood
