After injection of microspheres into the pulmonary artery approximately 50% of spheres 2.8–4.0 µ in diameter can be found in the systemic circuit during the first circulation, but only 6% of those 8 µ or larger get through the pulmonary circuit when compared with simultaneously injected tagged erythrocytes. If vessels are impeding the flow of spheres and are circular in cross section, then more than half of the erythrocytes must be distorted while passing through the pulmonary circuit. A continuous infusion of norepinephrine (1–4 µg/kg/min) brings about a reduction in the percentage of the various sizes of spheres passing through the pulmonary vessels. This suggests vasoconstriction. Acetylcholine iodide (13–40 µg/kg/min) usually diminishes the percentage of spheres 2.8 µ and smaller, which can go through the pulmonary circuit, but increases the percentage of microspheres 5.7 µ and larger. During prolonged inspiration the percentage of microspheres passing through in each group studied was less than during expiration. If all microspheres injected are to pass through the pulmonary circuit in inspiration they must be 1.4 µ or smaller.

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Microspheres of styrene divinyl benzene varying in size from about 1 µ to 16 µ have been injected into the pulmonary artery of dogs. The proportion of the spheres of various sizes which reach the systemic circuit has been determined. The spheres in the groups less than 5 µ in diameter which pass through the pulmonary circuit are diminished when norepinephrine or serotonin is infused but are not affected by the infusion of acetylcholine or inhalation of carbon dioxide in oxygen. Spheres of sizes larger than 5 µ pass through the pulmonary circuit in larger proportions when CO2 is inhaled or acetylcholine is infused but are unaffected by norepinephrine and reduced by infusion of serotonin. The central volume is usually increased in dogs given serotonin but is unchanged during inhalation of CO2.

The dye T-1824 and red cells marked with P32 were injected into the pulmonary artery of dogs. The characteristics of arterial dilution curves after blocking the descending aorta, after hypoxia and after transfusion following hemorrhage were studied. After pulmonary pressure was raised by blocking the aorta, the figure for peak concentration time divided by appearance time (PTC/AT) was increased, the concentration of labels rose to three-fourths of peak concentration more quickly and the figure for peak concentration divided by mean concentration (PC/MC) diminished. The results following hypoxia were variable. In only one out of eight dogs studied were the changes similar to those described above. In five opposite changes were found and in two the changes were mixed. Transfusion following hemorrhage resulted in changes which showed no regular pattern. The changes with blocking of aorta are those which we would predict if pulmonary shunts open. Those with hypoxia indicate that the opposite changes have occurred.