Regional differences in neutrophil margination in dog lungs

We investigated the relationship between polymorphonuclear leukocyte (PMN) retention and erythrocyte (RBC) velocity in the lungs of mongrel dogs. Regional velocity was estimated by measuring regional RBC transit times and was correlated with the retention of PMN found in the same lung sample 10 min after the injection of a bolus of labeled cells. Data from the whole lung showed that the total number of cells marginated in the pulmonary vasculature was 2.4 times as great as the number present in the circulation and that this pool turned over at a rate of 1%/s. The regional data showed increased retention, indicating slower PMN turnover in the upper lung regions, which have longer transit times and therefore slower blood velocities than the RBC is attributed to a greater discrepancy between PMN and RBC is attributed to a greater discrepancy between PMN and capillary size and the fact that PMN are less deformable than RBC. The large number of capillary segments present in the lung allows neutrophils to move more slowly while RBC stream around them. We conclude that there are approximately 2.5 times as many PMNs marginated in the lung as there are in the total circulating blood volume of the dog and that the pulmonary marginated pool turns over at approximately 1%/s with slower turnover in the upper compared with the lower regions of the lung.