American Academy of Professional Coders (AAPC) Practice Exam

What is notable about flat bones in terms of bone marrow?

1. They have a dense bone marrow cavity

2. They lack a bone marrow cavity

3. They contain more marrow than other bone types

4. They only have compact bone marrow

The correct answer is: They lack a bone marrow cavity

Flat bones, such as the bones of the skull, ribs, and sternum, are notable for the absence of a central marrow cavity, which is commonly found in long bones. Instead of having a single, large cavity to house bone marrow, flat bones consist of two layers of compact bone with a spongy bone layer in between called diploë. This spongy layer contains red bone marrow, which is responsible for producing blood cells. The distribution of marrow in flat bones is more diffuse, contributing to this unique structure. Flat bones are designed for protection and to provide surfaces for muscle attachment rather than for storing bone marrow in a central cavity. This functional adaptation highlights how different types of bones are structured to meet specific physiological needs.