The skeletal system

What Are Bones?• Bones may seem to be dry and brittle, but your bones are living

organs. Bones are the major organs of the skeletal system. Connective tissues, such as cartilage and ligaments, are also part of the skeletal system. The adult human skeleton has 206 bones.

What are the Functions of the Skeletal System?

The skeletal system has the following functions: • It protects other organs. For example, your rib cage protects your heart and lungs. • It stores minerals that help your nerves and muscles work properly. Long bones store fat that can be used as energy. • Skeletal muscles pull on bones to cause movement. Without bones, you would not be able to sit, stand, or run. • Some bones make blood cells. Marrow is a special mate- rial that makes blood cells.

What Is the Structure of a Bone?• A bone may seem lifeless. Like other organs, however, bone is a living

organ made of several different tissues

BONE TISSUE If you look inside a bone, you will see two kinds of bone tissue: compact bone and spongy bone. Compact bone has no large open spaces, but it does have tiny spaces filled with blood vessels. Spongy bone has many large open spaces that help the bone absorb shocks.

MARROW Some bones contain a tissue called marrow. There are two types of marrow. Red marrow makes both red and white blood cells. Yellow marrow stores fat.

CARTILAGE Most bones start out as a flexible tissue called cartilage. When you were born, you didn’t have much true bone. As you grew, most of your cartilage was replaced by bone. However, bone will never replace cartilage in a few small areas of your body.

What Is a Joint?A place where two or more bones meet is called a joint.

What Is the Structure of a Joint?

Joints are often placed under a great deal of stress. Joints are held together by ligaments. Ligaments are strong bands of connective tissue. Cartilage covers the ends of many bones and helps cushion the areas where bones meet.

Skeletal System Injuries and DiseasesInjures:• Bones may be fractured, or broken. • Joints can also be injured. A dislocated joint is a joint in which one or

more bones have been moved out of place. Another joint injury, called a sprain, happens if a ligament is stretched too far or torn.

Diseases:• Osteoporosis is a disease in which the bones become less dense.

Bones become weak and break more easily. Age and poor eating habits can make it more likely for people to develop osteoporosis. • A disease that affects the joints is called arthritis. Arthritis is painful.

Joints may swell or stiffen. As they get older, some people are more likely to have some types of arthritis.