READ: Body Systems Continued

The Skeletal System

Lesson Objectives

• Identify the main tissues and organs of the skeletal system.
• List four functions of the skeletal system.
• Describe three movable joints.
• Identify two nutrients that are important for a healthy skeletal system.
• Describe two skeletal system injuries.

Vocabulary

Your Skeleton

How important is your skeleton? Can you imagine your body without it? You would be a wobbly pile of muscle and internal organs, and you would not be able to move.

Your skeleton is important for many different things. Bones are the main organs of the skeletal system. They are made up of living tissue. Humans are vertebrates, which are animals that have a backbone. The sturdy set of bones and cartilage that is found inside vertebrates is called a skeleton.

The adult human skeleton has 206 bones, some of which are named in Figure below. Strangely, even though they are smaller, the skeletons of babies and children have many more bones and more cartilage than adults have. As a child grows, these “extra” bones grow into each other, and cartilage slowly hardens to become bone tissue.

Living bones are full of life. They contain many different types of tissues. Cartilage is found at the end of bones and is made of tough protein fibers called collagen. Cartilage creates smooth surfaces for the movement of bones that are next to each other, like the bones of the knee.

Ligaments are made of tough protein fibers and connect bones to each other. Your bones, cartilage, and ligaments make up your skeletal system.

Functions of Bones

Your skeletal system gives shape and form to your body, but it is also important in maintaining homeostasis. The main functions of the skeletal system include:

• Support. The skeleton supports the body against the pull of gravity, meaning you don't fall over when you stand up. The large bones of the lower limbs support the rest of the body when standing.
• Protection. The skeleton supports and protects the soft organs of the body. For example, the skull surrounds the brain to protect it from injury. The bones of the rib cage help protect the heart and lungs.
• Movement. Bones work together with muscles to move the body.
• Making blood cells. Blood cells are mostly made inside certain types of bones.
• Storage. Bones store calcium. They contain more calcium than any other organ. Calcium is released by the bones when blood levels of calcium drop too low. The mineral phosphorus is also stored in bones.

Structure of Bones

Bones are organs. Recall that organs are made up of two or more types of tissues. Bones come in many different shapes and sizes, but they are all made of the same materials.

The two main types of bone tissue are compact bone and spongy bone.

• Compact bone makes up the dense outer layer of bones.
• Spongy bone is found at the center of the bone, and is lighter and less dense than compact bone.

Bones look tough, shiny, and white because they are covered by a layer called the periosteum. Many bones also contain a soft connective tissue called bone marrow. There are two types of bone marrow - red marrow and yellow marrow.

• Red marrow makes red blood cells, platelets, and most of the white blood cells for the body (discussed in the Diseases and the Body’s Defenses chapter).
• Yellow marrow makes white blood cells.

The bones of newborn babies contain only red marrow. As children get older, some of their red marrow is replaced by yellow marrow. In adults, red marrow is found mostly in the bones of the skull, the ribs, and pelvic bones.

Bones come in four main shapes. They can be long, short, flat, or irregular. Identifying a bone as long, short, flat, or irregular is based on the shape of the bone, not the size of the bone. For example, both small and large bones can be classified as long bones. The small bones in your fingers and the largest bone in your body, the femur, are all long bones. The structure of a long bone is shown in Figure below.

Bone Growth

Your skeleton begins growing very early in development. After only eight weeks of growth from a fertilized egg, your skeleton has been formed by cartilage and other connective tissues.

At this point your skeleton is very flexible. After a few more weeks of growth, the cells that form hard bone begin growing in the cartilage, and your skeleton begins to harden. Not all of the cartilage, however, is replaced by bone. Cartilage remains in many places in your body, including your joints, your rib cage, your ears, and the tip of your nose.

A baby is born with zones of cartilage in its bones that allow growth of the bones. These areas, called growth plate, allow the bones to grow longer as the child grows. By the time the child reaches an age of about 18 to 25 years, all of the cartilage in the growth plate has been replaced by bone. This stops the bone from growing any longer.

Even though bones stop growing in length in early adulthood, they can continue to increase in thickness throughout life. This thickening occurs in response to strain from increased muscle activity and from weight-lifting exercises.

Lesson Summary

• Bones, cartilage, and ligaments make up the skeletal system.
• The skeleton supports the body against the pull of gravity.
• The skeleton provides a framework that supports and protects the soft organs of the body.
• Bones work together with muscles to move the body.
• Blood cells are mostly made inside the bone marrow.

Review Questions

1. What are the main organs of the skeletal system?

2. Name one tissue of the skeletal system.

3. List four functions of the skeletal system.