Branches of the splenic artery enter the white pulp from the trabeculae. The central artery sends branches to the white pulp itself and to the sinuses at the perimeter of the white pulp called marginal sinuses (see Fig. 14.29). The central artery continues into the red pulp, where it branches into several relatively straight arterioles called penicillar arterioles. The penicillar arterioles then continue as arterial capillaries. Some arterial capillaries are surrounded by aggregations of macrophages and are thus called sheathed capillaries. Sheathed capillaries then empty directly into the reticular meshwork of the splenic cords rather than connecting to the endothelium-lined splenic sinuses. Blood entering the red pulp in this manner percolates through the cords and is exposed to the macrophages of the cords before returning to the circulation by squeezing through the walls of the splenic sinuses (Fig. 14.31). This type of circulation is referred to as open circulation, and it is the only route by which blood returns to the venous circulation in humans. In other species such as the rat and dog, some of the blood from the sheathed capillaries passes directly to the splenic sinuses of the red pulp. This type of circulation is referred to as closed circulation.
FIGURE 14.30 • Splenic sinus and splenic cord structure. a. This scanning electron micrograph shows a cross section of a splenic sinus (SS), revealing the lattice structure of its wall. Through the multiple openings in the wall, processes of macrophages (arrows) are inserted into the sinus lumen. The remainder of the micrograph shows characteristically smooth-surfaced processes of reticular cells (RC). The spaces of the reticular cell framework contain neutrophils (N), macrophages (M), and blood platelets (P). 4,400.
b.
Schematic diagram of the reconstructed structure of splenic sinus. Note the direction of blood flow in open and closed circulations.
c.
Scanning electron micrograph of the splenic sinus, showing the architecture of the sinus wall as seen from its luminal side. Rodlike endothelial cells run in parallel and are intermittently connected to each other by side processes. A nuclear swelling is shown at lower right. The tapered ends of a few of the rod cells are seen. The macrophage, neutrophil, and lymphocyte are outside the sinus.
5,300. (Reprinted with permission from Fujita T, Tanaka K, Tokunga J. SEM Atlas of Cells and Tissues. Tokyo: Igaku-Shoin, 1981.)
FIGURE 14.31 • Schematic diagram of open and closed splenic circulations. In the open circulation, which occurs in humans, penicillar arterioles empty directly into the reticular meshwork of the cords rather than connect to the endotheliumlined splenic sinuses. Blood entering the red pulp then percolates through the cords and is exposed to the macrophages residing there. In the closed circulation, which occurs in other species, the penicillar arterioles empty directly into the splenic sinuses of the red pulp. PALS, periarterial lymphatic sheath.
Open circulation exposes the blood more efficiently to the macrophages of the red pulp. Both transmission and scanning electron micrographs often show blood cells in transit across the endothelium of the sinus, presumably reentering the vascular system from the red pulp cords. The blood collected in the sinuses drains to tributaries of the trabecular veins that converge into larger veins and eventually leaves the spleen by the splenic vein. The splenic vein in turn joins the drainage from the intestine in the hepatic portal vein.
The spleen performs both immune and hemopoietic functions.