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## What does bone do?

Bone has many important jobs in the body:
                          - The skeleton is made of bone to provide a strong framework to support and protect the soft organs (such as the brain, heart, and lungs) from injury.
                          - Bone works with muscle to hold up the body when we stand and to move the body when we walk or run. 
                          - Bone houses the bone marrow, which makes blood cells.
                          - Bone stores growth factors and minerals such as calcium.
                          - Bone releases factors into the blood that are necessary for normal functioning of soft organs such as the kidney.

The human body needs calcium to build and maintain bones and to ensure all cells in the body work properly. Not only do bones require calcium for growth and health, bones require other factors and nutrients such as vitamin D to function normally. Exercise is also very important for normal bone growth and for adult bone health. Our skeletons require the stresses and loading that occurs with movement and exercise to keep our bones healthy throughout life.

## What is bone made of?

Bone is made of protein, collagen, and minerals, especially calcium. Collagen provides a framework for the incorporation of mineral, mainly calcium phosphate into the collagen framework. The mineral makes bone hard and strong while the collagen provides flexibility so that the bone can resist breaking.

Each bone has two types of bone tissue to ensure strength: The dense, hard outer layer is called compact or cortical bone while the inner, less dense, lattice-like bone is called cancellous, trabecular or spongy bone that is surrounded by bone marrow.

## What is bone remodeling?

Three types of cells are involved in removing old bone tissue and replacing it with fresh bone tissue (bone remodeling):
                              - Osteoblasts: make new bone.
                              - Osteoclasts: break down and reabsorb bone.
                              - Osteocytes: long-lived cells inside bone that direct osteoblasts and osteoclasts.

In remodeling, osteoclasts remove bone through a process known as resorption. This is followed by osteoblasts that lay down new collagen on the resorbed bone surface that mineralizes to form new bone. Osteocytes play a role in directing osteoclasts to resorb bone and direct osteoblasts to make or stop making new bone.  This depends on whether the bone is “loaded” due to exercise or “unloaded” due to inactivity/sedentary conditions.

Bone is made up of living tissue that is constantly changing, a process known as remodeling (see box for a description of this process).

The remodeling process changes with age:
                            - In children and teens, the body adds more bone than it takes away. Bones gradually become larger, heavier, and denser as children grow.
                            - In young, healthy adults, about the same amounts of bone are removed and replaced.
                            - As adults get older and get certain diseases, remodeling can become unbalanced. More bone can be removed than the amount that is laid down. As a result, bones can become weaker, and certain bone diseases can develop.

Following are examples of diseases of unbalanced bone remodeling:
                          - Osteoporosis : In osteoporosis, old bone is removed more quickly, and new bone is laid down more slowly resulting in bone that is more susceptible to breaking.
                          - Osteopetrosis : The rate of bone removal is slower in osteopetrosis, so bones get too dense.

Following are examples of bone diseases with defective bone quality:
                          - Osteogenesis imperfecta : People with osteogenesis imperfecta have a genetic defect where they do not make enough collagen or their collagen is made incorrectly.
                          - Paget’s disease of bone:  With Paget’s disease, more bone is laid down than the amount that is taken away, and new bone is not formed correctly.
                          - Fibrous dysplasia : Fibrous dysplasia replaces normal bone with fibrous (scar-like) tissue.

For more information on the role of calcium in bone, see Calcium and Vitamin D: Important for Bone Health.

 

This content was created by the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) with contributions from:
                        - National Institute on Aging
                        - National Institute of Diabetes and Digestive and Kidney Diseases
                        - NIH Office of Research on Women’s Health

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#### Related Information

Osteoporosis

Bone Mineral Density Tests: What the Numbers Mean

Calcium and Vitamin D: Important for Bone Health

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Last Reviewed:             May 2023