The red pulp of the human spleen. Structural basis of blood filtration.

The paper summarizes the authors' findings indicative of an open blood circulation in the human spleen, particularly those obtained by immune and enzyme histo- and cyto-chemical methods, and by electron microscopy. In the red pulp the blood gets into the extravascular spaces of the pulp cords, where the individual blood components have to pass between numerous macrophages to reach the sinuses. The sinus wall is composed of elongated endothelial cells surrounded by waved annular or ring fibers of the basement membrane. In some areas annular fibers are joined by longitudinal fibers, giving rise to the filtration lattice, the fenestrated basement membrane. The sinus wall represents the last filter barrier which decises whether the blood elements get back into the blood or not. Extravasation of blood secures that all foreign as well as the altered own components, particularly cellular and particle ones naturally along with the normal constituents get from the circulating blood into extravascular spaces. In the next phase, however, the normal ones return into the circulation, whereas the abnormal components are removed from the extravascular tissue by means of the macrophagic and immune system of the spleen.