ISSN: 1755-5310 (Electronic)1755-5302 (Print)

Interventional Cardiology

Review Articles On Pulmonary Circulation

The pulmonary circulation conducts the entire cardiac output with a remarkably low driving pressure from the pulmonary artery (mean Ppa of 15 to 20 mm Hg) to the left atrium (Pla of 7 to 12 mm Hg). As in the airways, the branching pattern of vessels leads to an increase in total cross-sectional area as the alveolar vessels are approached, but unlike in the airways, this increase is not associated with a decrease in resistance. Total cross-sectional area increases at a branching point if the number of daughter branches (n) is greater than the ratio of the parent to daughter radii squared, (a/b)2, but resistance decreases only if n is greater than (a/b)4. The latter case occurs in the peripheral airways but not in the vessels, so although small peripheral airways contribute little to normal airflow resistance, pulmonary microvessels make up a substantial portion of vascular resistance. Efforts to partition the pressure drop longitudinally suggest that approximately 20% to 30% is in the arterial portion (including arterioles), 40% to 60% in the microvascular portion, and the remainder in the veins. With increases in flow, recruitment occurs mainly at the level of microvascular vessels, so their relative contribution to resistance becomes less.    

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