Yellow bone marrow is a type of bone marrow found in the cavities of long bones, such as the femur and humerus. It primarily consists of adipose (fat) tissue and serves as an energy reserve for the body. Unlike red bone marrow, which produces blood cells, yellow bone marrow is less active in hematopoiesis, the process of blood cell formation. As people age, some of their red bone marrow is gradually replaced by yellow bone marrow. This transition is a normal part of development and helps the body store fat, which can be converted to energy when needed. In certain conditions, yellow marrow can revert to red marrow to increase blood cell production.