The hematogenous red pulp is (with the white pulp) one of two types of tissue within the spleen making up the splenic pulp. The splenic tissue of the red pulp is characterized by sinus cavities awash in blood and filled with macrophages that cleanse the blood of pathogens, especially bacteria, and dead red blood cells (erythrocytes). For this reason, most of the immune system work of the spleen occurs within the red pulp. This red pulp is made up of splenic cords (also called the cords of Billroth), sheets of connective tissue that separate the splenic sinuses (sinusoids or sinoids, which are specialized blood vessels) through which the blood flows. Thin cell walls allow the blood to flow relatively freely either within the splenic sinuses or out into the space between the cords. While the red and white pulp are intermixed throughout the spleen, the red pulp tends to be found further away from the arterioles that flow through the organ and makes up about three-quarters or more of the entire splenic pulp. The large supply of blood within the red pulp of the spleen allows it to serve as a reservoir for additional blood supplies when the body is in need.