Your heart is an organ that pumps blood all around your body. It is about the size of your fist. The heart has four chambers. Two of them are upper and two of them are lower.

Arteries carry blood away from the heart. Veins carry blood back to the heart. Red blood cells carry oxygen from your lungs. They take it to the rest of your body. White blood cells are an important part of your immune system.

Your nose warms and cleans the air you breathe. When you breathe in, your diaphragm moves down toward your belly.

Skin is the biggest organ of your body. Your skin has three layers.

The liver is the largest organ that is inside your body. Each of your kidneys is about as big as a fist. A human brain weighs about 3 pounds.

The pupil is the opening in your eye that lets light in. Your eyes turn light into signals. The signals go to your brain.

Sound makes your eardrum move. The eardrum passes the vibrations to three tiny bones in the middle ear.

Words to know

• arteries: blood tubes
• veins: blood tubes
• organ: body part
• pupil: eye opening