A long bone, like the femur or tibia, has a hollow shaft and wider ends. It is constantly rebuilt by cells that add and remove bone throughout life.
Parts of a long bone
The shaft is the diaphysis and the ends are the epiphyses. In children, a growth plate of cartilage between them lets the bone grow longer until it closes after puberty.
Compact and spongy bone
Compact bone forms the hard outer wall and is strongest along the shaft. Spongy bone fills the ends with a mesh of thin struts that holds red marrow.
Periosteum
A tough membrane covers the outside of the bone and is rich in nerves and blood vessels. It causes much of the pain of a fracture and supplies cells for healing.
Bone cells
Osteoblasts build new bone, osteoclasts break old bone down, and osteocytes are mature cells trapped in the bone that sense stress. The balance between building and removal keeps bone strong.
| Cell | Job | When it matters |
|---|---|---|
| Osteoblast | Builds new bone | Growth and fracture healing |
| Osteoclast | Removes old bone | Remodeling, high calcium needs |
| Osteocyte | Senses load, maintains bone | Bone adapts to exercise or disuse |
At the bedside
In children, a fracture through a growth plate needs careful follow-up because it can affect growth. When osteoclasts outpace osteoblasts, as in osteoporosis, bones break with minor falls, often at the hip, wrist and spine.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).