How do calcium and physical activity make bones stronger?

To make bones strong and to keep them strong, the body needs calcium, vitamin D, and physical activity.

• Calcium helps bones to grow right. When the body makes new bone tissue, it first lays down a framework of a protein called collagen. Then, calcium from the blood spreads throughout the collagen framework. The hard crystals of calcium attach to the bone structure. Calcium and collagen work together to make bones both strong and flexible.Calcium is also needed for many other activities within the body such as neural communication (the way your nerves and brain send signals to each other) and heart and lung functions. If the body doesn't get enough calcium from foods and drinks, it takes it from bones, which can make them weaker.
  

• Vitamin D helps the body use calcium. Your bones can’t do their job without vitamin D. Lots of foods have calcium, but vitamin D can be harder to find. You can get what you need for the day by drinking two glasses of fortified milk, and some cereals and yogurts are fortified with vitamin D as well.
  

• Physical activity helps bones become stronger and thicker, just as a muscle gets stronger and bigger the more you use it. Bones are living tissue. Bone-strengthening activity causes new bone tissue to form, making bones even stronger. It also makes muscles stronger, and muscles push and tug against bones to make them even stronger. Physical activity also makes you better coordinated, which can make you less likely to fall and break a bone.