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 weighing 65 kilograms (143 lb), bone marrow typically accounts for approximately 2.6 kilograms (5.7 lb).he 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.