11.6: Skeletal System

Are bones alive?

From seeing a skeleton, you might think that bones are just dead, hollow structures. But in a living person, those hollow spaces are full of living cells. Bones have a blood supply and nerves. Bones are a living tissue.

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.

The adult human skeleton has 206 bones, some of which are named below (Figure below). Bones are made up of living tissue. They contain many different types of tissues. Cartilage, a dense connective tissue, is found at the end of bones and is made of tough protein fibers. 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.

Figure \(\PageIndex{1}\): The skeletal system is made up of bones, cartilage, and ligaments. The skeletal system has many important functions in your body. What bones protect the heart and lungs? What protects the brain?

Functions of Bones

Your skeletal system gives shape and form to your body, but it also plays other important roles. 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 come in many different shapes and sizes, but they are all made of the same materials. Bones are organs, and recall that organs are made up of two or more types of tissues.

The two main types of bone tissue are compact bone and spongy bone (Figure below).

• Compact bone makes up the dense outer layer of bones.
• Spongy bone is found at the center of the bone and is lighter and more porous 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 in the pores of the spongy bone. Bone marrow is where blood cells are made.

Figure \(\PageIndex{2}\): Bones are made up of different types of tissues.

Bone Growth

Early in human development, the skeleton consists of only cartilage and other connective tissues. At this point, the skeleton is very flexible. As the fetus develops, hard bone begins to replace the cartilage, and the 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 plates, 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.

Science Friday: Jump in Jerboas

It's a potato on toothpicks! No, it's a hopping noodle! A fuzzy rodent T-rex! Any way you look at them, jerboas are about as cute as they come. These adorable critters bounce about on long, springy legs - appendages that just might help us better understand and perhaps manipulate the growth of human bones.

Summary

• Bones, cartilage, and ligaments make up the skeletal system.
• Functions of the skeletal system include providing support, protecting the soft organs of the body, aiding in movement, and making blood cells.

Explore More

Use the resource below to answer the questions that follow.

• Human Skeleton at http://www.getbodysmart.com/ap/skeletalsystem/skeleton/menu/menu.html

1. What makes up the axial skeleton? What makes up the appendicular skeleton?
2. What is the scapula commonly known as? What other bone helps the scapula function? What do these two bones do?
3. How many thoracic vertebrae do humans have? Where are they located in the spinal column?
4. To which type of vertebrae do ribs attach?
5. What is the atlas bone? What is the role of this bone?

Review

1. Are bones a living organ?
2. What are the three components of the skeletal system?
3. List four functions of the skeletal system.
4. Name and describe the two types of tissue that make up a bone.