In the closed circulatory route, blood enters the spleen via the splenic artery and follows this artery as it branches into arterioles. The arterioles are in close contact with the venous sinuses into which the blood flows. The venous sinuses connect to venuoles, and the splenic vein, which exits the spleen and empties into the portal vein. In the open pathway, the arterioles empty into the pulp cords, not the sinuses. In open circulation, the blood cells must pass through the additional barrier of the network of reticular fibers which make up the cord, before they can squeeze into the venous sinuses.
Mechanical filtration occurs at the junction between the arterioles and the venous sinuses.
In order to travel into the splenic vein, the blood cells must first filter through the walls of the venous sinuses. Only healthy cells are capable of deforming to the extent required to pass through the endothelial cells of the sinuses. In addition to this measure, the macrophages of the pulp cords are actively involved in blood filtration. This filtration process is a regulation of the blood content passing through the circulation. Insofar as the spleen fulfils its role as a filter, this organ is a prime example of the interdependence of structure and function.