Bone structure and function
Bones are living organs that support and protect the body, store minerals, and make blood cells.
Functions of the skeleton
The skeleton supports the body, protects organs (the skull protects the brain, the ribs the heart and lungs), allows movement as muscles pull on bones, stores minerals (about 99% of the body's calcium), makes blood cells in red marrow, and stores fat in yellow marrow.
Bone shapes
Long bones are longer than wide, such as the femur and finger bones. Short bones are cube-like, such as wrist bones. Flat bones are thin and often curved, such as the skull and sternum. Irregular bones, such as vertebrae, have complex shapes. Sesamoid bones, such as the kneecap, form within tendons.
Parts of a long bone
A long bone has a shaft (diaphysis) and two ends (epiphyses). The shaft has a central medullary cavity containing marrow. The outside is covered by periosteum, a membrane with blood vessels and bone-forming cells. Joint surfaces are covered with articular cartilage. In children, an epiphyseal (growth) plate between the shaft and ends allows lengthening.
Compact and spongy bone
Compact bone is dense and forms the outer layer; it is built of osteons, cylinders of concentric rings around central canals carrying blood vessels. Spongy bone, inside the ends of long bones and in flat bones, is a lattice of thin struts (trabeculae) aligned along lines of stress, light but strong, with red marrow in its spaces.
Bone cells
Osteoblasts build bone by laying down matrix; osteocytes are mature bone cells trapped in the matrix, sensing stress; osteoclasts break down bone, releasing calcium. The balance between osteoblasts and osteoclasts constantly reshapes bone throughout life; the whole adult skeleton is replaced roughly every 10 years.
Bone composition
Bone matrix is about one-third organic, mainly collagen, which gives flexibility, and two-thirds mineral, mainly calcium phosphate crystals (hydroxyapatite), which give hardness. Without collagen, bone is brittle; without minerals, it bends. In the disease rickets, poor mineralisation leaves bones soft and bowed.
Bone growth
Most bones form by replacing a cartilage model with bone (endochondral ossification); skull bones form directly within membranes. Length increases at the growth plates until they close in late adolescence; then growth in height stops. Bones grow wider by adding new layers under the periosteum.
Worked example
An X-ray of a 12-year-old's wrist shows dark gaps near the ends of the forearm bones. These are growth plates of cartilage, which show up darker on X-rays than bone. Their presence tells doctors the child is still growing; in an adult these plates have turned to bone and closed.
Key takeaways
Bones support, protect, move, store minerals, and make blood cells. They are classified as long, short, flat, irregular, or sesamoid. Long bones have a diaphysis, epiphyses, marrow, periosteum, and growth plates. Compact bone is built of osteons; spongy bone of trabeculae. Osteoblasts build, osteoclasts break down, and osteocytes maintain bone.
Flashcard
Support, protect, move, store, make blood
Check your understanding
Which cells would be most active in breaking down bone to raise blood calcium?