Bone marrow, the flexible tissue found inside the bones of the body and responsible for blood cell creation, comes in two types. Red marrow is made up primarily of hematopoietic tissue, the tissue that makes up blood cells. Fat cells account for almost all yellow marrow.
Red bone marrow creates almost all of the body’s blood cells and platelets, with the exception of a few white blood cells known as lymphocytes, which reach maturity in the lymphatic tissues of the body. Red marrow exists most commonly in bones that are flat, such as hip bones, breast bones, the skull, ribs, shoulder blades and the vertebrae of the spine. Red blood cells, which carry oxygen from the lungs to other organs, are essential to keeping the body healthy. Red marrow ensures that the body always has the proper level of blood cells.
The main purpose of yellow bone marrow is to store fat. This is the body’s natural way of protecting itself from extreme starvation, because the body will consume the fat in yellow marrow as a last resort. Yellow marrow can also turn itself back into red marrow in cases of severe blood loss or anemia. This transformation can happen in as little as an hour, protecting the body from red blood cell loss and organ damage stemming from a lack of oxygen. This type of marrow is stored in the hollow center of the long bones of the body.
Both types of marrow are prone to certain diseases, especially red marrow. The body’s stem cells are contained primarily in red marrow, so any disease affecting it can dampen the immune system's ability to function. The most common red bone marrow disease is the cancer leukemia, although exposure to chemotherapy or radiation can also damage the stem cells in the marrow. Bone marrow transplants are almost always needed in such cases.
Bone marrow makes up as much as 4 percent of the total body weight of a human. People are born with all red marrow, but the body begins converting it to yellow marrow around age 5 and, by adulthood, nearly half of the marrow in the body is yellow marrow. Red bone marrow is one of the most important parts of the human body, but yellow marrow works as an effective backup if any problems arise with the red marrow.