Your heart is an organ that pumps blood all around your body. The heart has four chambers: two upper and two lower. Heart valves work like one-way doors that keep blood moving in the right direction.

The liquid part of your blood is called plasma. Red blood cells carry oxygen from your lungs to the rest of your body.

Your nose warms and cleans the air you breathe. When you breathe, your body takes in oxygen and sends out carbon dioxide.

Your skeleton gives your body its shape and support. Ligaments connect one bone to another bone, and tendons connect muscles to bones.

Skin is the biggest organ of your body. Your skin has three layers. Skin helps control your body temperature. Skin is a barrier that helps keep germs out of the body.

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.

Your heart beats slower when you rest and faster when your body is working harder.

A human brain weighs about 3 pounds.

The liver is the largest organ inside your body.

Arteries carry blood away from the heart, and veins carry blood back to the heart.

Words to know

• valves: one-way doors
• plasma: blood liquid
• retina: eye layer
• ligaments: bone joiners