Fast transit of red cells and plasma in contracted versus relaxed spleens.

Nine-tenths of the inflowing blood passes through the normal relaxed spleen via a fast arteriovenous pathway whereas the remainder travels slowly via the red pulp. However, flow through this fast pathway diminishes to zero when the organ is distended by perfusion at high venous pressure. The present investigation, in which a slug-injection procedure was used to determine the volumes of distribution of red cells and 125I-labelled albumin in isolated, Ringer-perfused cat spleens during a single transit, has shown that in spleens contracted by noradrenaline the entire inflow passes via the fast arteriovenous pathway. The peak outflow concentration of 125I-labelled albumin occurred much later (factor 2.03) than that of red cells. This difference is too great to be accounted for solely by axial accumulation of red cells within a closed system of vessels in vivo and indicates that albumin was distributed throughout a significantly larger space than red cells. These findings are explained in terms of the classical observations of MacKenzie and co-workers on red cell movement in transilluminated mouse spleens during contraction. Whether the splenic retention of damaged red cells would change as a result of the altered flow pathways is not yet known.