The bone marrow has been the “blood factory” of the body, especially for red blood cells and platelets. Although all bone cavities contain marrow, it is only certain bones in adults that have active, blood-producing, red marrow. These include the spongy parts of long bones such as the femur, and the flat bones of the ribs, breastbone, vertebrae, and skull.

Where Red Blood Cells are Formed?A bone shaft, in cross section (left), reveals the spongy internal structure of a bone. A closer look (above) shows blood vessels distributed within the bone.

About 3 million red blood cells are produced by an adult per second. The intricate bone marrow structure shown in this micrograph is the site of blood production. It is here, too, that some of the iron from worn-out red cells is recycled in new red-cell production.