Preferential distribution of leukocytes in rat mesentery microvessel networks

Distribution of leukocytes in rat mesenteric microvessel networks was studied using intravital fluorescence video microscopy. A digital image analysis system was used to measure vessel diameters, flow velocities and leukocyte fluxes in 306 capillaries of 8 networks. Capillaries were defined as vessel segments connecting divergent to convergent branch points. Their topological position within the network was quantified by a generation number defined as the number of bifurcations between the capillary and the arteriole feeding the network. Proximal capillaries (generation numbers 4 and 5) were slightly but significantly smaller in diameter (8.9±0.4) μm, mean ± SEM) than distal ones (generation numbers 20 and 21, 10.1±0.4 μm). Average capillary flow velocity decreased markedly from 2.0±1.0 mm·s^-1 in proximal to 0.41 ±0.06 mm·s^-1 in distal capillaries. Average leukocyte concentration was 3.4±0.5·10⁹ l^-1 and thus significantly below systemic values (6.0·10⁹ l^-1) in proximal capillaries, and above in distal ones (11.7±2.6·10⁹l^-1). The analysis of flow and leukocyte flux partition at 138 bifurcations showed preferential distribution of leukocytes to the daughter capillary with higher flow rate. This suggests a tentative explanation for the observed leukocyte accumulation along the microvascular tree: due to their low fractional flow, proximal capillaries draw relatively leukocytepoor blood from the arteriole feeding the network; this leads to an increased leukocyte concentration in distal capillaries. As a consequence of the concomitant increase of capillary diameter with increasing generation number, leukocytes are preferentially flowing through larger capillaries and are excluded from small ones.