Bones are living organs that support the body, protect organs, store minerals, and help make blood cells. A long bone such as the femur has a strong outer shell, a lighter inner network, and a marrow cavity inside. Its structure balances strength with low mass, which helps the skeleton handle forces from standing, walking, and jumping.
Understanding bone structure also helps explain fractures, growth, and diseases such as osteoporosis.
Bone tissue is constantly renewed by remodeling, a process in which old bone is removed and new bone is added. Osteoclasts break down bone, while osteoblasts build new bone matrix that later hardens with calcium phosphate minerals. In children and teenagers, long bones grow longer at growth plates near the ends of bones.
When growth plates close after puberty, bones can still remodel and repair, but they no longer lengthen.
Key Facts
- Compact bone is dense outer bone that provides strength and protection.
- Spongy bone is a porous inner network that reduces weight and helps absorb stress.
- The marrow cavity in long bones contains marrow, which can store fat or produce blood cells.
- Osteoblasts build bone, osteoclasts break down bone, and osteocytes maintain mature bone tissue.
- Bone remodeling rate depends on the balance: bone gain = osteoblast activity minus osteoclast activity.
- Longitudinal growth occurs at epiphyseal plates, also called growth plates, before they close at maturity.
Vocabulary
- Compact bone
- Compact bone is dense, hard bone tissue that forms the strong outer layer of most bones.
- Spongy bone
- Spongy bone is a lightweight network of bony struts called trabeculae found inside many bones.
- Marrow
- Marrow is soft tissue inside bones that can produce blood cells or store fat depending on the type.
- Growth plate
- A growth plate is a layer of cartilage near the end of a long bone where lengthwise growth occurs in young people.
- Osteocyte
- An osteocyte is a mature bone cell that helps maintain bone tissue and sense mechanical stress.
Common Mistakes to Avoid
- Thinking bone is dead material. Bone is living tissue with cells, blood vessels, nerves, and constant remodeling.
- Confusing compact bone with spongy bone. Compact bone is dense and forms the outer shell, while spongy bone is porous and mostly inside the ends of long bones.
- Assuming bones only grow by stretching. Long bones grow at growth plates where cartilage is replaced by bone, not by the whole bone simply pulling longer.
- Mixing up osteoblasts and osteoclasts. Osteoblasts build bone, while osteoclasts break down bone during remodeling.
Practice Questions
- 1 A long bone grows 1.6 cm in length during a year. If both growth plates contribute equally, how much length did each growth plate add?
- 2 A bone remodeling site has osteoblasts adding 12 units of bone matrix and osteoclasts removing 9 units in the same time period. What is the net bone change?
- 3 A runner increases training gradually over several months. Explain how bone remodeling can make the runner's bones better suited to repeated impact.