Journal of Cell and Developmental Biology

Bone marrow is the flexible tissue in the interior of bones. In humans, red blood cells are produced by cores of bone marrow in the heads of long bones in a process known as hematopoiesis. On average, bone marrow constitutes 4% of the total body mass of humans. The hematopoietic component of bone marrow produces approximately 500 billion blood cells per day, which use the bone marrow vasculature as a conduit to the bodys systemic circulation. Bone marrow is also a key component of the lymphatic system, producing the lymphocytes that support the bodys immune system.