Response of Small Pulmonary Arteries to Unilobar Hypoxia and Hypercapnia

We measured the internal diameters of small muscular pulmonary arteries in the right and left lower lobes of lungs rapidly frozen in the anesthetized, open-thorax cat. In every cat the right lower lobe was ventilated with the test gas and all other lobes were ventilated with O2. In 4 cats, the test gas supplied to the right lower lobe was also O2 (control experiments); in 6, it was 100% N2 and in 6 it was 90% N2–10% CO2. In the 4 controls, there was no difference between the internal diameters of corresponding arteries in the right and left lower lobes. In the other 12 cats, there was a highly significant decrease in the diameter of arteries in the right lower lobe that correlated well with a large decrease in blood flow to that lobe, calculated using a shunt equation. Ventilation of the right lower lobe with 10% CO2 in air (4 cats) did not affect the diameter of arteries and ligation of the pulmonary artery to the right lower lobe resulted in only a slight decrease in arterial dimensions in 3 of 4 cats.These data show that in regional alveolar hypoxia without systemic hypoxia the muscular pulmonary arteries of the terminal respiratory units in the hypoxic region actively constrict. The constriction occurs with or without alveolar hypercapnia.