The bones

The bones do lots of different things. They:

  • support and protect your internal organs
  • work with the muscles to move the body
  • store minerals, such as calcium
  • contain bone marrow, which is where blood cells are made.

Bones are strong and rigid, but not heavy. They are made from a protein called collagen and minerals such as calcium.

Bones are constantly renewing themselves. Cells called osteoclasts break down and remove old bone. Cells called osteoblasts build new bone.

Our bones stop growing in length when we are about 18 years old. They continue to increase in thickness though until we are in our late twenties. After our mid-thirties, bone density starts to decrease. This is called bone loss or bone thinning. It means that, over time, our bones become more fragile and can break.

Osteoporosis can develop in older people. This is when bone density is much lower than normal. As the bones are weaker, they are more likely to break.

Your doctor can test the health of your bones. If needed, they can treat your bones to prevent a fracture.

What do the bones do?

Bones have several functions. They:

  • provide support and protection for your internal organs
  • work with your muscles to allow your body to move
  • store calcium and other minerals
  • contain bone marrow, which is where your blood cells are made.

Bones have developed so that they are strong and rigid without being heavy. Bone is made from a protein called collagen, which is strengthened by calcium and other minerals.


Types of bone

There are two types of bone – compact bone and cancellous bone.

Compact bone is the hard, rigid shell on the outside of bones. Inside this shell is cancellous bone, which is arranged like a honeycomb or mesh with lots of spaces. It is sometimes called spongy bone, because the holes in it make it look a bit like a sponge.

The structure of a long bone
The structure of a long bone

View a large version

Read a description of this image


Bone growth and repair

Bone tissue is alive. It has a blood and nerve supply to stay healthy. Bones are constantly renewing themselves. This helps maintain their strength and shape.

Inside the bones, there are two types of bone cell:

  • osteoclasts, which break down and remove old bone
  • osteoblasts, which build new bone.

Our bones stop growing in length by the time we are about 18 years old. But bones continue to increase in thickness (density) until our late twenties.

Bone density then stays about the same into our mid-thirties. After this, osteoclasts remove more bone than osteoblasts make. This causes bone density to slowly decrease.


Bone thinning

As we age, our bone density slowly decreases. This is called ‘bone loss’ or ‘bone thinning’.

The hard, outer shell of the bones (compact bone) thins and the holes in cancellous (spongy) bone get larger. In time, bones become more fragile. This is why bone fractures are more common after the age of 65.

Bone mass at different ages
Bone mass at different ages

View a large version

Read a description of this image


Osteoporosis

Some people develop osteoporosis as they get older. Osteoporosis means bone density is much lower than normal. The bones are weaker and more likely to break (fracture). The most common places to have fractures are the wrists, hips and spine.

Osteoporosis does not cause symptoms until a weakened bone breaks (fractures). But your doctor can do tests to assess your risk of osteoporosis and to check the health of your bones. The results of these tests help doctors decide if you need treatment to lower your risk of a fracture.

A healthy bone and a bone with osteoporosis
A healthy bone and a bone with osteoporosis

View a large version

Read a description of this image

Back to Bone health

Exercise and bone health

Try to find a type of exercise that you enjoy. This means you are more likely to keep doing it.