Anatomy of a Long Bone

Long bones like your femur (thigh bone) or humerus (upper arm) have distinct regions that serve specific purposes. The diaphysis is the elongated central shaft, while the epiphyses are the enlarged ends that participate in joints. Connecting these regions are the metaphyses, which house the growth plates in developing bones.

The external structure includes the periosteum, a two-layered membrane surrounding the bone. Its outer fibrous layer provides protection, while the inner osteogenic layer contains bone-forming cells essential for growth and repair. The bone ends are covered with smooth articular cartilage that reduces friction and absorbs shock at joints.

Internally, bones have both compact bone (dense outer layer providing strength and protection) and spongy bone (lightweight inner structure made of trabeculae). The central medullary cavity contains yellow bone marrow (mostly fat), while spongy areas house red bone marrow where blood cells develop. The endosteum lines internal cavities and contains bone-forming cells.

Did you know? Bones are highly vascular! The nutrient artery enters through a small hole (foramen) and branches throughout the bone, supplying oxygen and nutrients. These arteries maintain high blood pressure to reach all areas of this seemingly solid structure.