What is the medullary cavity primarily filled with?

• A. Blood vessels
• B. Red marrow
• C. Yellow, fatty marrow
• D. Connective tissue

Answer: (C)

The medullary cavity, found within the long bones, is primarily filled with yellow, fatty marrow. This yellow marrow serves essential functions, including fat storage and the production of certain hormones. This composition is particularly relevant as an individual ages; the yellow marrow composition increases with age, gradually replacing red marrow in certain bones.

In contrast, while red marrow is responsible for producing blood cells and is located in the trabecular (spongy) bone, it is not the primary filling of the medullary cavity in adults. Similarly, while blood vessels and connective tissue are present in the bone structure, they do not fill the medullary cavity to the same extent as fatty yellow marrow. Understanding the role of yellow marrow is critical for grasping bone physiology and the overall function of the skeletal system.