Bone marrow is the tissue found in the interior of bones. It is responsible for the production of red and white blood cells. There are two types of bone marrow: red bone marrow (high proportion of blood building cells) and yellow bone marrow (high proportion of fatty tissue). At birth, all bone marrow is red; with increasing age, this is gradually converted to yellow marrow. In adults, red marrow is predominantly found in flat bones (pelvis, sternum, ribs etc.) whereas the medullary cavity is filled with yellow marrow.