The clinical significance of the pulmonary collateral circulation.

THE NORMAL LUNG contains two distinct circulations of strikingly different origins, proportions, and functions. On the one hand is the huge pulmonary circulation, which originates in the right heart and pours the entire venous cardiac output into the lungs for gas exchange; on the other, is the diminutive systemic circulation, which delivers oxygenated blood to the lungs for the sustenance of its tissues, nerves, vessels, and conducting airways. Vascular connections between the two circulations are sparse and require elaborate injection technics for their display. Among the most elusive of these communications are the precapillary vessels that join pulmonary to bronchial arteries (fig. 1). The systemic blood supply of the lung undergoes a remarkable proliferation in various disorders of the heart and lungs :1-4 old vessels enlarge and become tortuous; new vessels appear and join with the old to form bizarre medusan patterns. And, in contrast to the normal lung, in which precapillary communications between the two circulations are difficult to demonstrate, the enlarged precapillary anastomoses between the pulmonary and systemic circulations are readily apparent. There are several popular synonyms for the systemic circulation of the lung. In this presentation, two of these-" bronchial circula-