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. Heart valves work like one-way doors. They keep blood moving in the right direction. 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. Arteries carry blood away from the heart, and veins carry blood back to the heart. Bone marrow is found inside some of your bones. It makes blood cells. 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.

Glands in your mouth make saliva, a juice that begins to digest your food. When you swallow, a small flap called the epiglottis covers your windpipe. The small intestine is where nutrients from your food pass into your blood.

Your kidneys filter your blood to take out waste and make urine. Each of your kidneys is about as big as a fist.

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

Sound makes your eardrum move, and the eardrum passes the vibrations to three tiny bones in the middle ear. The smallest bone in your body is in your ear.

Skin is the biggest organ of your body. The liver is the largest organ inside your body. A human brain weighs about 3 pounds.

Your skin has three layers. Skin is a barrier that helps keep germs out of the body.

Muscles help your body move. Humans have three different kinds of muscle.

Words to know

• valves: one-way doors
• marrow: soft bone middle
• diaphragm: breathing helper
• eardrum: ear drum

The short version

Your heart is an organ that pumps blood all around your body. It is about the size of your fist. Skin is the biggest organ of your body, and the liver is the largest organ inside your body. Heart valves work like one-way doors that keep blood moving the right way.

Quick facts

• Your heart is about the size of your fist.
• Skin is the biggest organ of your body.
• Heart valves work like one-way doors.
• Bone marrow makes blood cells.