Leukocyte traffic in the lung.

An adult human with a cardiac output of 5 liters/min pumps approximately 7200 liters of blood through the pulmonary circulation in 24 h. An additional 1 % of the cardiac output (72 liters over 24 h) flows through the bronchial circulation to supply the conducting airways and blood vessels, lymphatic collections, and portions of the pleural surface (22). The bronchial and pul­ monary venous outflow becomes confluent in the peripheral lung and drains into the left heart. However, the bronchial venous drainage from the central bronchovascular structures and lymphatics empty into the azygous and hemi­ azygos system and then into the right heart. As each liter of circulating blood contains approximately 109 leukocytes, their traffic through the bronchial and pulmonary microvessels is enormous. The purpose of this brief review is to discuss the similarities and differences between these two lung vascular beds.