Merck Manual

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Quick Facts



The Manual's Editorial Staff

Last full review/revision May 2020
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What are bones?

Bones are strong, rigid structures made of proteins and minerals such as calcium.

Your bones:

Bones have a hard, dense outer layer that gives them strength. The inside of bones is porous (full of tiny holes) to let blood flow through. The inside of bones also contains the bone marrow. The outside of bones is covered by a thin sheet of tissue called the periosteum, which helps nourish the bone.

How do bones work?

Bones give your body form and structure and allow you to move around.

What are joints?

Joints are where two bones meet.

Joints would fall apart unless they were held together by ligaments.

  • Ligaments are tough, flexible bands of tissue that grow across a joint from one bone to another

Joints have several different ligaments that hold them together. Ligaments and the way the ends of your bones are shaped let joints move only in certain directions. For example your knee joint can bend and straighten but not move sideways.

Some joints, such as your shoulder, can move a lot. Other joints, such as those between your ribs and spine, move only a little.

Like ligaments, muscles connect two bones across a joint. When muscles contract, they pull the joint open or closed.

How do bones grow?

When you see a skeleton you might think bones are not alive, like pipes or sticks of wood. However:

  • Bones are living tissue

  • Like all living tissue, bones need a constant supply of blood to provide oxygen and nutrients

Children's bones grow by getting longer from the ends. There is a soft part at the end of a child's bones called a growth plate. Growth plates form new bone that makes bones longer. During adolescence, children's growth plates turn into hard bone so their bones stop getting longer.

  • In remodeling, old bone tissue is gradually replaced by new bone tissue

  • Every bone in your body is completely remodeled about every 10 years

For bones to grow and remodel, they need a steady supply of calcium, other minerals, vitamin D, and certain hormones.

What does bone marrow do?

What can go wrong with bones?

Problems with bones include:

Bone tumors may be cancerous or noncancerous (benign). Some cancerous tumors spread to your bones from other organs.

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