What is the medullary cavity in a long bone used for?

The medullary cavity is a continuous, hollow space located within the central region, or diaphysis, of long bones.1

This cavity serves as a reservoir for yellow bone marrow, which consists primarily of fat cells. While yellow marrow functions mainly as a storehouse for fats, it retains the capacity to be converted into red marrow under specific physiological conditions, such as severe blood loss or fever. In adult humans, the medullary cavities of long bones are typically occupied by this fatty tissue, distinguishing them from other areas of the skeleton—such as the vertebrae, ribs, and the ends of long bones—where red marrow remains the primary site for blood cell formation.23