<P> 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; in an adult having 65 kilograms of mass (143 lb), bone marrow typically accounts for approximately 2.6 kilograms (5.7 lb). 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 body's systemic circulation . Bone marrow is also a key component of the lymphatic system, producing the lymphocytes that support the body's immune system . In addition to hematopoietic cells, the marrow is composed of marrow adipose tissue as well as trabecular bone . The interplay between these different cell types and the local factors they produce can have an effect on hematopoietic cells within the hematopoietic stem cell niche . </P> <P> Bone marrow transplants can be conducted to treat severe diseases of the bone marrow, including certain forms of cancer such as leukemia . Additionally, bone marrow stem cells have been successfully transformed into functional neural cells, and can also potentially be used to treat illnesses such as inflammatory bowel disease . </P> <P> The two types of bone marrow are "red marrow" (Latin: medulla ossium rubra), which consists mainly of hematopoietic tissue, and "yellow marrow" (Latin: medulla ossium flava), which is mainly made up of fat cells . Red blood cells, platelets, and most white blood cells arise in red marrow . Both types of bone marrow contain numerous blood vessels and capillaries . At birth, all bone marrows are red . With age, more and more of it is converted to the yellow type; only around half of adult bone marrow is red . Red marrow is found mainly in the flat bones, such as the pelvis, sternum, cranium, ribs, vertebrae and scapulae, and in the cancellous ("spongy") material at the epiphyseal ends of long bones such as the femur and humerus . Yellow marrow is found in the medullary cavity, the hollow interior of the middle portion of long bones . In cases of severe blood loss, the body can convert yellow marrow back to red marrow to increase blood cell production . </P> <P> The stroma of the bone marrow is all tissue not directly involved in the marrow's primary function of hematopoiesis . Yellow bone marrow makes up the majority of bone marrow stroma, in addition to smaller concentrations of stromal cells located in the red bone marrow . Though not as active as parenchymal red marrow, stroma is indirectly involved in hematopoiesis, since it provides the hematopoietic microenvironment that facilitates hematopoiesis by the parenchymal cells . For instance, they generate colony stimulating factors, which have a significant effect on hematopoiesis . Cell types that constitute the bone marrow stroma include: </P>

What part of the body is bone marrow taken from