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 lbs), bone marrow typically accounts for approximately . 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.