Red bone marrow has more blood cells than yellow bone marrow, including red blood cells and platelets. Yellow marrow also has some white blood cells, but its color is due mostly to its having more fat cells. It can also change into red bone marrow, if needed.
Red bone marrow also has stem cells, including cells that produce new blood cells. Hundreds of millions of blood cells are made every day. When they are mature, they are allowed to pass into the circulatory system.
Red and yellow bone marrow are also more prevalent in different parts of the body. Red marrow is found in places like the skull, the ribs, the bones of the spine, the shoulder blades and the end of long bones in the arms and legs. Yellow bone marrow is found in places where red marrow has been depleted. These places are mostly found in the appendages. People are born exclusively with red bone marrow, and the body begins to produce yellow marrow as they age. Eventually, the nearly 6 pounds of bone marrow in the human body is half red. Red bone marrow is also known as medulla ossium rubra. Yellow marrow is medulla ossium flava.