Bone marrow is the soft spongy tissue that lies within the hollow interior of long bones. In birds and mammals, Bone marrow in large bones produces new blood cells. Bone marrow forms around 4% of total body weight. There are two types of bone marrow: red marrow that is responsible for producing red blood cells, white blood cells and platelets; and yellow marrow consisting mainly of fat cells. It is composed of hematopoietic cells, marrow adipose tissue, and supportive stromal cells. In Humans, bone marrow produces approximately 500 billion blood cells per day.