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 upper and two lower. Cardiac muscle is the special muscle found in the heart. Your heart beats slower when you rest and faster when your body is working harder.

Red blood cells carry oxygen from your lungs to the rest of your body. The liquid part of your blood is called plasma. Arteries carry blood away from the heart, and veins carry blood back to the heart. Capillaries are very tiny blood vessels that connect the smallest arteries to the smallest veins.

When you breathe, your body takes in oxygen and sends out carbon dioxide; when you breathe in, your diaphragm moves down toward your belly.

Glands in your mouth make saliva, a juice that begins to digest your food. After you swallow, waves of muscle squeezing, called peristalsis, push food down to your stomach.

Your skeleton gives your body its shape and support. Tendons connect muscles to bones.

The pupil is the opening in your eye that lets light in. The retina at the back of the eye turns light into nerve signals for the brain.

Skin is the biggest organ of your body; your skin has three layers. The outer layer of skin constantly flakes off and is replaced by new cells from below. Skin helps control your body temperature. Skin is a barrier that helps keep germs out of the body.

A human brain weighs about 3 pounds.

The liver is the largest organ inside your body.

Words to know

• peristalsis: squeezing waves
• plasma: liquid blood part
• retina: back of eye