Red marrow, also called red bone marrow, is found mainly in the axial flat bones and the proximal ends of some long bones in adults. Specifically, it is located in the:
- Flat bones such as the cranial bones of the skull, clavicle, sternum (breastbone), ribs, scapula (shoulder blades), vertebrae, and pelvis
- Proximal ends (near the body) of the long bones such as the humerus (upper arm) and femur (thigh bone)
In infants and young children under 5 years old, red marrow is abundant in all bones, including the medullary cavity of long bones. However, as a person ages, much of this red marrow gradually converts into yellow marrow, which primarily stores fat. In adults, red marrow is mostly confined to the axial skeleton and parts of the proximal long bones. Red marrow is the hematopoietically active tissue that produces red blood cells, white blood cells, and platelets essential for blood formation and immune function.