Stimulus-response curves for the pulmonary vascular bed to hypoxia and hypercapnia.

1. In anaesthetized open-chest cats and dogs, blood flow and gas tensions were measured in a circuit inserted into a pulmonary vein while ventilating the lobe which it drained with low O(2) and high CO(2) mixtures.2. Both hypoxic and hypercapnic mixtures caused a reduction in blood flow from the lobe.3. Stimulus-response curves relating blood flow to pulmonary venous P(O2) and P(CO2) were obtained. Those for hypoxia were usually asymptotic in shape; the curves became steep below 100 torr and flow sometimes fell to zero. The mean reduction in blood flow for every 20 torr fall in P(O2) was 15.7% in cats and 11.8% in dogs. Those for hypercapnia were steep at first but levelled out at high P(CO2) values; the maximum reduction in flow was 40-60% as vasoconstriction was only observed over a limited P(CO2) range.4. Hypoventilation of the lobe led to a reduction in blood flow. This was mainly attributable to hypoxia though other factors such as hypercapnia may sometimes have contributed.5. Total occlusion of the bronchus of an O(2)-filled lobe caused blood flow to fall in two phases. The first phase could be attributed to a rise in P(CO2) and the second to a fall in P(O2).6. The results confirm the hypothesis that hypoxia is an important factor regulating local blood flow in relation to local ventilation.