1. To test the relative sensitivity of serotonin and propranolol pulmonary extraction measurements to changes in pulmonary vascular surface, we sequentially subjected anaesthetized dogs to left upper lobe atelectasis, left lung atelectasis and left pulmonary artery occlusion.

2. We used a triple-indicator dilution technique to simultaneously measure the pulmonary extraction of serotonin and propranolol. After an initial series of measurements, dogs received dopamine and dextran to slightly increase pulmonary artery pressure and prevent further recruitment of capillaries.

3. Left upper lobe atelectasis did not modify the pulmonary extraction of serotonin and propranolol. Left lung atelectasis provoked a reduction in the serotonin extraction ratio by only 4%, whereas the propranolol extraction ratio decreased by 13%. After left pulmonary artery occlusion, propranolol and serotonin extraction ratios decreased by 16% and 5% respectively.

4. We conclude that the pulmonary extraction of propranolol is more sensitive to a decrease in pulmonary vascular surface than that of serotonin.

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