Elevation of organ resistance due to leukocyte perfusion

Leukocytes are larger and considerably less deformable than erythrocytes, thereby causing a disproportionate effect on local blood flow, which despite their low concentration, may be significant at the organ level. To investigate the degree to which circulating leukocytes affect whole organ resistance, a hemodynamically isolated rat gracilis muscle was perfused in situ under well-controlled conditions. Comparison of organ pressure-flow data from vasodilated vasculature using normal physiological cell concentrations and perfusion pressures indicates that leukocytes (60-75% neutrophils) provide approximately 22% of the whole blood resistance despite their relatively small cell volume fraction of approximately 0.1%. On a single cell basis, each leukocyte imposes a resistance elevation equivalent to that of approximately 750 erythrocytes. Furthermore, when leukocytes are activated via pretreatment using N-formyl-methionyl-leucyl-phenylalanine or endotoxin, they show a higher resistance, accounting for 50-60% of the total resistance. These findings indicate that leukocytes play a significant role in normal skeletal muscle organ perfusion and may be a major determinant of organ perfusion during disease states.