The medullary cavity, also called the marrow cavity, is the central, hollow space running along the length of a bone. This internal chamber functions as a specialized biological site for various tissues. The cavity’s contents are responsible for producing blood cells and storing energy reserves, making it a dynamic component of human physiology.
Location Within Bone Structure
The medullary cavity is primarily found within the shaft, or diaphysis, of long bones, such as the femur in the thigh or the humerus in the arm. This strategic placement along the bone’s axis helps to reduce overall bone weight while maintaining structural strength against bending forces. The cavity is not empty but is filled with a soft tissue known as bone marrow, which is highly vascularized with numerous blood vessels that supply the bone and transport its products.
The walls of this central space are formed by a dense layer of compact bone, also called the cortical bone. Lining the internal surface of the medullary cavity is a thin, vascular membrane called the endosteum. The endosteum contains specialized cells, including osteoblasts and osteoclasts, which are responsible for the constant remodeling and maintenance of the bone tissue. This lining regulates the internal diameter of the cavity as the bone grows and develops.
The Types of Bone Marrow Within
The medullary cavity houses two distinct types of bone marrow: red marrow and yellow marrow. Red bone marrow is characterized by hematopoietic tissue, containing the stem cells responsible for generating all types of blood cells. This marrow is rich in reticular fibers and sinusoids, giving it its characteristic color due to the presence of developing red blood cells.
Yellow bone marrow, conversely, is predominantly composed of adipose cells, or fat cells, which serve as a long-term energy reserve. The yellowish hue comes directly from the stored triglycerides within these adipocytes. In infants and young children, nearly all of the medullary cavity is filled with red marrow to support the high demand for new blood cells during growth.
As a person matures, the red marrow in the medullary cavities of the long bones is gradually replaced by yellow marrow. By adulthood, red marrow is largely restricted to flat bones, such as the pelvis, sternum, and ribs, and the ends of the long bones. This conversion shifts the primary function of the long bone medullary cavity from blood cell production to fat storage.
Essential Roles in Health
The contents of the medullary cavity perform two primary functions. The red bone marrow is the site of hematopoiesis, the process that produces billions of new blood cells daily. Hematopoietic stem cells within this marrow differentiate into erythrocytes (red blood cells), leukocytes (white blood cells), and platelets, which are then released into the bloodstream.
Red blood cells carry oxygen, white blood cells support the immune system, and platelets are necessary for blood clotting. The yellow bone marrow’s primary role is energy storage, as the fat reserves can be mobilized and metabolized when needed. Furthermore, yellow marrow is a reserve tissue that can convert back to red marrow under conditions of severe blood loss or anemia to increase blood cell production rapidly. The surrounding bone tissue also acts as a reservoir for minerals, particularly calcium and phosphorus, which are released into the blood to maintain concentrations required for nerve and muscle function.