Microscopic studies on the influence of erythrocyte concentration on the post-junctional radial distribution of leukocytes at small venular junctions.

The influence of flow patterns and erythrocyte concentration on the post-junctional radial position of leukocytes was studied in microvascular junctions in an isolated mesocaecum preparation and in vivo in the rat cremaster muscle. Main branch and side branch dimensions (mesentery) were approximately 60 micrograms and approximately 20 micrograms, respectively. For the analysis the main branch was divided into three equally wide sectors. Firstly, the observations indicate that the main branch and side branch flows remain separated downstream of a junction and, secondly, that the blood cells mainly leave the junction within their original streams. At zero hematocrit 71-87.5% of the leukocytes from the side branch flowed in the closest main branch sector; 12.5-29% reached the central sector-the lowest numbers were seen in the central sector when the flow velocity ratio, main branch-side branch was 2:1. No cells became marginated. At 20% and 40% hematocrit, all leukocytes from the side branch flowed in the closest main branch sector. Marginated leukocytes in the side branch or upstream in the main branch remained marginated only if passing the junction on the uninterrupted wall. In the cremaster muscle, 3-capillary confluences were studied. The blood from the capillaries seemed to flow in three separate streams in the venule. Since these streams were narrower than those observed in the mesentery, the leukocytes in the capillaries producing the peripheral streams continued to flow in contact with the vascular wall, i.e. they became marginated in the venules without any lateral movement. On the basis of this study and previous work it is proposed that the radial position of leukocytes downstream of a junction is determined not only by leukocyte-erythrocyte interactions but also by the specific flow patterns that arise in microvascular confluences.