Leukocyte margination in alveolar capillaries: interrelationship with functional capillary geometry and microhemodynamics.

The pulmonary capillary microvasculature harbors a large pool of intravascularly marginated leukocytes. In this study, we investigated the interrelationship of leukocyte margination with characteristics of functional capillary geometry and microhemodynamics in alveolar capillary networks. In 22 anesthetized rabbits we assessed functional capillary density, average capillary length, red blood cell velocity and leukocyte kinetics in alveolar capillary networks in vivo by intravital fluorescence microscopy. In alveolar wall areas of 12,800 +/- 1,800 microm(2), we detected 3.6 +/- 0.5 sticking leukocytes and 21.0 +/- 1.9 functional capillary segments with an average capillary length of 35.7 +/- 2.1 microm. We calculated that approximately 15% of functional capillary segments are blocked by marginated leukocytes. Leukocyte margination was predominantly observed in capillary networks characterized by a high functional capillary density, short capillary segments and low red blood cell velocities. The multitude of interconnected capillary channels in these networks may allow alveolar blood flow to bypass marginated leukocytes. Hence, this interrelationship may be relevant for maintenance of adequate alveolar perfusion and low capillary network resistance despite excessive leukocyte margination in the pulmonary microvasculature. Local microhemodynamic factors may play a regulatory role in the spatial distribution of leukocyte margination.