your skeleton and prosture

Your skeleton has to be strong, tough and flexible. The materials in your skeleton make this possible. Your skeleton consists of cartilage and bone. A bone is not a solid thing: bone cells use food and oxygen to get energy for growth and repair. These substances are brought to the bone cells by blood vessels that run through holes inside the bone. Just like cartilage, bone is a type of connective tissue. It is hard but also slightly flexible. Bone is able to grow and repair itself. Most of your skeleton is made of bone. Let’s look at a typical bone, for example a bone in your leg, see figure 4.21. It is made of a long part (called a shaft) and two heads. The outer part of this bone is made of a hard material called compact bone. The inside of the heads is made of softer spongy bone, which has a lot of holes in it to make the bone weigh less. The shaft is hollow and filled with marrow, which produces blood cells. The bone is covered by a tough outer layer called the periosteum. This is the place where new bone cells are made to renew the bone. The heads have another layer around them which is made of cartilage, for protection.Bones are strong, but also flexible. Two components in bone make this possible. They are called calcium and collagen (a glue-like substance). Calcium is for strength and collagen gives flexibility.

As you get older the balance between calcium and collagen changes. A child has less calcium and more collagen (and also more cartilage) in its skeleton than an adult. This is why a child is very flexible. To grow strong bones, a child has to take foods that contain a lot of calcium (such as milk, beans and grains). An older person has more calcium but less collagen. As you grow older, your bones become stronger, but they lose their flexibility.

Also, after your mid-thirties, you slowly begin to lose bone tissue. It means that the holes inside your bones slowly become larger. This is called osteoporosis,Every eight to ten years your skeleton is completely renewed! During a life time of eighty years, a person will grow at least eight new skeletons. The bones you had when you were three are not really the same bones as the ones you have now. But how are your bones renewed?

The periosteum contains different kinds of cells that keep the bone healthy: osteoclasts break down old or damaged bone. They ‘eat’ their way through your skeleton. This means they can also help change the shape of your bones as you grow. Other cells called osteoblasts make new bone. Osteoclasts and osteoblasts work together so that there is a fine balance between breaking down old bone and building up new bone. Figure 4.23 shows the process of breaking down and building up new bone.