1. The Skeletal System

2. Living Bones
Bone tissue is made of cells that take in nutrients and use energy.
The skeletal system is made up of bones. There are 5 major functions of the skeletal system:
1. The skeleton gives shape and support to your body.
2. Bones protect your internal organs.
3. Major muscles are attached to bone and help them move.
4. Blood cells are formed in the center of many bones in soft tissue called red marrow.
5. Major amounts of calcium and phosphorous are stored in the skeleton for later use. They make the bones hard.

3. Bone Structure
Bones are many different shapes and sizes. Bones are not smooth – they have bumps, edges, round ends, rough spots, and many pits and holes.
Muscles and ligaments will attach to bumps and pits. Blood vessels and nerves enter and leave through holes.
A bone’s surface is covered in periosteum – a tough, tight fitting membrane. It contains small blood vessels that carry nutrients to the bone. Also cells that are responsible for growth and repair are found in the periosteum.
Under the periosteum are 2 types of bone tissue – compact bone and spongy bone.

4. Compact Bone
This is a hard, strong layer. This layer gives the bone strength. This area has deposits of calcium phosphate that make the bone hard.
Bone cells and blood vessels are also found in this layer.

5. Spongy Bone
This is located at the ends of long bones. It has many small, open spaces that makes bones lightweight. In the center of long bones are openings called cavities. This spaces have bone marrow in them. Some marrow is yellow and made of fat cells. Some are red and make red blood cells.

6. Cartilage
The ends of bones are covered in cartilage – smooth, slippery, thick layer of tissue. This area does not have blood vessels or minerals.
Cartilage is flexible and important in joints because it acts like a shock absorber. It also reduces friction between 2 bones rubbing together.
Cartilage can be damaged by disease, injury, or years of use.

7. Bone Formation
Before you were bone your skeleton was made of cartilage. The cartilage breaks down and is replaced by bone before you are born.
Bone forming cells, called osteoblasts, deposit calcium and phosphorous in bones making the tissue hard.
At birth you have more than 300 bones. As you grow the bones will fuse or grow together so that you only have 206 bones now.
Healthy bone tissue is always being formed and re- formed.
Another bone cell called osteoclast breaks down bone tissue in other areas of the bone. This releases calcium and phosphorous into the blood stream, which is important for the movement of your muscles.