Microcirculatory pathways and blood flow in spleen: new insights from washout kinetics, corrosion casts, and quantitative intravital videomicroscopy.

The dynamics of splenic microcirculatory blood flow and the role of the spleen with respect to red blood cells (RBCs) have been elucidated by means of several different experimental approaches. First, the organ was treated as a 'black box' and input/output relationships were studied, such as the kinetics of RBC and plasma washout during Ringer perfusion. Second, microcorrosion casts were prepared from minimal amounts of injected material, such that 'fast' pathways for flow were selectively identified. Third, high-resolution video-microscopy was carried out in rat and mouse spleens transilluminated in vivo, with subsequent quantitative analysis of the data. New insights were obtained regarding fast and slow pathways for RBC flow; the origin of the high intrasplenic hematocrit; immature RBCs and the spleen; the pH, O2 tension and glucose concentration within the reticular meshwork; microcirculatory pathways bordering the white pulp; entry of blood into venous sinuses, both by open ends in the marginal sinus/zone and via interendothelial slits in sinus walls; spontaneous cyclic contractions of capillary walls in vivo, impeding RBC flow (i.e. endothelial contractility); and leukocyte interactions with walls of venous vessels. The different experimental approaches have provided complementary information and have clarified a number of important issues about which uncertainty existed in the literature. Exploiting the quantitative analysis of high-resolution intravital videomicroscopic recordings has an exciting potential for gaining new insights into the workings of this complex and neglected organ.