Spleen Red Pulp

The red pulp is mainly filled with venous sinuses, which contain red blood cells and occasionally white blood cells. The sinuses are lined by elongated, rod-shaped endothelial cells. In a sinus seen in cross-section the endothelial cells would extend out of the screen. Slits between the endothelial cells allow viable red blood cells to squeeze into the sinuses from the splenic parenchyma. These sinuses eventually drain into the splenic vein. Surrounding the sinuses is the parenchyma, which contains lymphocytes, macrophages, plasma cells, etc. Another term for this parenchyma tissue is the cords of Billroth.