The bone marrow is made from millions of blood vessels crowded together to form myeloid tissue. In spongy bones, there are adequate sponge-like cavities called lamella. They form a trabecular matrix where the blood vessels can condense to form the bone marrow. Therefore they are porous enough to deliver adequate glucose, lipids, amino acids, and other trace minerals necessary to create new red blood cells. This process is called erythropoiesis. Further, when red blood cells are old or damaged, they return to sites like the spleen, liver or reach back to the bone marrow. The old cells are phagocytized by macrophages and removed from the bloodstream.In compact cells, the osteons are densely packed, leaving no space for nutrients and minerals to reach blood vessels. Therefore erythropoiesis cannot take place in compact bones. Instead, they have a yellow bone marrow majorly consisting of fat.