In a long bone, the medullary cavity in the diaphysis is primarily occupied by which substance in adults?

In a long bone, the medullary cavity in the diaphysis is primarily occupied by which substance in adults?

• A. Yellow marrow
• B. Red marrow
• C. Periosteum
• D. Articular cartilage

Answer: (A)

Inside a long bone, the hollow central cavity in the shaft—the medullary cavity—holds marrow. In adults, this space is mainly yellow marrow, which is adipose tissue that stores fat. Red marrow, which produces blood cells, is mostly located in the ends of long bones (epiphyses) and in some flat bones. Periosteum is the outer membrane surrounding the bone, and articular cartilage covers joint surfaces, not the medullary cavity. The shift from red to yellow marrow as you mature explains why the diaphyseal cavity is filled with yellow marrow in adults.

Inside a long bone, the hollow central cavity in the shaft—the medullary cavity—holds marrow. In adults, this space is mainly yellow marrow, which is adipose tissue that stores fat. Red marrow, which produces blood cells, is mostly located in the ends of long bones (epiphyses) and in some flat bones. Periosteum is the outer membrane surrounding the bone, and articular cartilage covers joint surfaces, not the medullary cavity. The shift from red to yellow marrow as you mature explains why the diaphyseal cavity is filled with yellow marrow in adults.