Leukocyte distribution to arteriolar branches: dependence on microvascular blood flow

Leukocyte distribution to arteriolar branches in the hamster cheek pouch microvasculature was studied by rendering the cells fluorescent via a constant intravenous infusion of acridine orange. A principal objective was to determine the major factors influencing the partitioning of the cellular flux between daughter branches at arteriolar branch points with internal diameters permitting single-file leukocyte flow. Leukocyte flux determinations (cells/sec) and individual cell velocities were made for over 8000 cells at 13 branch sites in 10 animals. The results indicate that for the branch flow ratios of 0.3 to 0.7 encountered in the present experiments, the predominant factor influencing the average leukocyte flux is the branch blood flow. Branch fractional flux dependence on fractional blood flow was analyzed using linear and nonlinear models. A linear characterization provided an excellent fit and description of the functional relationship but a slight nonlinear component was also predicted by the nonlinear sigmoidal model which also fit the data well. Because of the small degree of predicted nonlinearity and the overlap of the standard deviations of the parameter estimates, we were not able to statistically decide on which model, and hence which functional relationship, is operative. Thus, the answer to the question regarding the presence of any preferential distribution is inconclusive. However, the data and analyses suggest that if preferential distribution is operative, its magnitude over the flow range and conditions here studied is small, and that for many purposes the relationship between the in vivo average fractional cell flux and blood flow may be viewed as linear.