3.39 Bone Facts

Did you know that babies have more bones than adults? Learn about how bones grow and change as you age, what your bones do, and what they’re made of in this lesson.

Your Bones Are Alive!

Think about dinosaur bones you might have seen in a museum, or maybe even animal bones you have seen in your backyard. Did they remind you of rocks? We sometimes think of bones as nonliving things, but the bones that make up your skeleton are alive, and just like other parts of your body, they grow and change as you age.

Bones Grow

Babies are born with 300 different bones. As you grow up, many of the 300 small bones fuse, or grow together. So by the time you are an adult, your skeleton will have only 206 bones.

Human adults have 206 bones.

When we are about 25 years old our bones stop growing larger, but they keep on building new bone cells to replace old cells. So even though your skeleton stops getting bigger, your bones are constantly changing.

1. Main points:

Bones are living parts of the body that grow and change over time, with babies born with 300 bones that fuse into 206 bones in adulthood, and bone cells continually renewing even after bones stop growing in size around age 25.

2. Questions:

• Content Analysis: How do bones change from infancy to adulthood?
• Contextual Analysis: Why is it important to understand that bones are living and constantly changing?
• Linguistic Analysis: What words in the text convey the idea of growth and change in bones?

3. Further Discussion:

How might the knowledge that bones are constantly renewing affect how we take care of our bodies?

4. Answers:

• Content Analysis: "Babies are born with 300 different bones. As you grow up, many of the 300 small bones fuse… So by the time you are an adult, your skeleton will have only 206 bones."
• Contextual Analysis: Understanding that bones are living and constantly changing helps us realize the importance of bone health and growth.
• Linguistic Analysis: Words like "grow," "change," "fuse," and "building new bone cells" convey the idea of growth and change in bones.

What Do Bones Do?

Bones have three main jobs: they give your body its shape, they work with your muscles to help you move, and they protect your organs and other soft parts.

Bones Give You Shape

Think about the worms you see in your yard. Worms’ bodies are squishy and soft like jelly because worms don’t have bones. Without our bones, humans would be squishy blobs of skin!

Bones Help You Move

Your bones can’t move on their own, but your muscles and bones work together like a team to help you move. Your muscles are attached to your bones, so when you move your muscles, your bones move as well.

When this girl decides to kick the ball, she will contract her leg muscles, causing her whole leg, bones and all, to move.

Bones Protect Other Body Parts

Your bones also act like a shield that protects the softer parts of your body. The bones in your skull protect your brain like a builtin helmet. Your rib cage the bones around your chest and back protect important organs like your heart and lungs.

1. Main points:

Bones provide shape to the body, assist in movement by working with muscles, and protect vital organs like the brain and heart.

2. Questions:

• Content Analysis: What are the three primary functions of bones in the human body?
• Contextual Analysis: How do bones and muscles work together to facilitate movement?
• Linguistic Analysis: What words or phrases in the text illustrate the protective role of bones?

3. Further Discussion:

Why is it important for bones to be both strong and flexible?

4. Answers:

• Content Analysis: "They give your body its shape, they work with your muscles to help you move, and they protect your organs and other soft parts."
• Contextual Analysis: "Your muscles are attached to your bones, so when you move your muscles, your bones move as well."
• Linguistic Analysis: Phrases like "act like a shield," "protects the softer parts," and "builtin helmet" illustrate the protective role of bones.

What Are Bones Made Of?

Bones have to be very strong to protect your organs, but also very light so you can move around. Bones can be both strong and light because they are made of two different types of material. The outer layer of your bones is made of a material called compact bone. Compact bone is dense, smooth and very hard.

Under the compact bone is a lighter, less dense material called cancellous bone, pronounced ‘KANsellus’. Cancellous bone is sometimes called ‘spongy bone’ because like the sponge in your kitchen it has many little holes and air pockets. These holes make cancellous bone very light and are passageways for blood vessels that supply your bones with nutrients so they can live and grow.

The outer layer of a bone is made of smooth, dense compact bone. Under the compact bone is cancellous, or spongy bone, which has many air pockets and passageways.

Even though cancellous bone is lighter than compact bone, it is still very strong, because both types of bone are made of a hard mineral called calcium.

In the very center of your bones is a soft, thick substance that looks like jelly called bone marrow. Bone marrow produces all of your blood cells.

All Shapes and Sizes

The smallest bone in your body, the stirrup, is in your ear, and is about the size of the eraser of a pencil. Your femur, or thigh bone, is the strongest, longest bone in your body. By the time you are an adult it will be over a foot long.

1. Main points:

Bones are composed of compact bone (dense and hard) and cancellous bone (light and spongy), both made of calcium, with bone marrow in the center producing blood cells. Bones vary in size, from the tiny stirrup in the ear to the large femur in the thigh.

2. Questions:

• Content Analysis: What are the different materials that make up a bone and their characteristics?
• Contextual Analysis: Why is it important for bones to be made of both compact and cancellous bone?
• Linguistic Analysis: What words in the text describe the strength and lightness of bones?

3. Further Discussion:

How do the different types of bone contribute to the overall function and health of the skeletal system?

4. Answers:

• Content Analysis: "The outer layer of your bones is made of a material called compact bone… Under the compact bone is a lighter, less dense material called cancellous bone."
• Contextual Analysis: The combination of compact and cancellous bone allows bones to be "very strong to protect your organs, but also very light so you can move around."
• Linguistic Analysis: Words like "dense," "smooth," "hard," "light," and "spongy" describe the strength and lightness of bones.

Lesson Summary

Your bones are alive, and they grow and change as you get older. An adult skeleton has 206 different bones. The three main jobs of your bones are to give your body shape, help your body move, and protect your other body parts.

Bones are made of two types of material: compact bone and cancellous bone. Both compact bone and cancellous bone are made of a hard mineral called calcium. Bone marrow, located inside your bones, makes blood cells.