What resides in the medullary cavity of long bones?
A.
Spongy bone only.
B.
Compact bone only.
C.
None of these.
D.
Bone marrow.
The Correct answer and Explanation is:
The correct answer is D. Bone marrow.
Explanation:
The medullary cavity, also known as the marrow cavity, is a central space within the diaphysis (shaft) of long bones. This cavity is primarily filled with bone marrow, which plays a crucial role in the body’s hematopoietic system.
Bone Marrow and Its Types:
- Red Bone Marrow: This type of marrow is involved in hematopoiesis, the process of producing red blood cells, white blood cells, and platelets. It is highly vascularized and contains hematopoietic stem cells. Red bone marrow is predominantly found in the flat bones (such as the sternum and pelvis) and in the spongy (cancellous) bone of the proximal ends of long bones in adults. In infants and children, red marrow is more widespread throughout the skeleton.
- Yellow Bone Marrow: As individuals age, red marrow in the medullary cavities of long bones is gradually replaced by yellow marrow, which primarily consists of adipocytes (fat cells). Yellow marrow serves as an energy reserve and can be converted back into red marrow if needed, such as during periods of severe blood loss or anemia.
Bone Structure Context:
- Spongy Bone: Spongy bone, or cancellous bone, is found within the epiphyses (ends) of long bones and between layers of compact bone in other bones. It provides structural support and houses red marrow in certain areas but does not reside in the medullary cavity.
- Compact Bone: Compact bone forms the dense outer layer of the bone and surrounds the medullary cavity. It is designed to withstand compressive forces and provides strength and protection to the bone.
Therefore, the medullary cavity is specifically associated with the presence of bone marrow. The function and composition of bone marrow are vital for blood cell production and overall skeletal health, distinguishing it from spongy and compact bone types that have different structural and functional roles within the bone.