Your heart is an organ that pumps blood all around your body. The heart has four chambers: two upper and two lower. A small patch of tissue in the heart called the sinoatrial (SA) node sends the signal that starts each heartbeat. Heart valves work like one-way doors that keep blood moving in the right direction. Blood makes a trip through the heart. Oxygen-poor blood enters the right atrium, goes to the right ventricle, then through the pulmonary artery to the lungs. Oxygen-rich blood returns by the pulmonary veins to the left atrium, then the left ventricle. Your heart beats slower when you rest and faster when your body is working harder.

The liquid part of your blood is called plasma, and red blood cells carry oxygen from your lungs to the rest of your body. Red blood cells live for about 120 days. Bone marrow, which is found inside some of your bones, makes blood cells, and platelets help your blood clot. 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. Blood travels in two loops, the pulmonary circulation and the systemic circulation. White blood cells are an important part of your immune system.

When you breathe, your body takes in oxygen and sends out carbon dioxide, and your nose warms and cleans the air that you breathe. When you breathe in, your diaphragm moves down toward your belly. At the ends of the smallest airways in your lungs are tiny air sacs called alveoli. Resting adults normally breathe at least 12 times in each minute.

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. After you swallow, waves of muscle squeezing, called peristalsis, push food down to your stomach. Your stomach muscles mix the food that you swallowed with digestive juices, and the food inside your stomach is broken down into a liquid called chyme. The small intestine is where the nutrients from your food pass into your blood. Your large intestine absorbs water, and what is left of your food becomes solid waste. An adult's digestive tract is a long tube at least 25 feet (about 7.6 meters) long.

The pupil is the opening in your eye that lets light in, and the lens of your eye helps to focus light onto the retina. The retina at the back of the eye turns light into nerve signals for the brain.

Sound makes your eardrum move, and the eardrum passes the vibrations to three tiny bones in the middle ear. Nerve signals from the ear travel along the auditory nerve to the brain. The smallest bone in your body is in your ear.

Skin is the biggest organ of your body, and the liver is the largest organ that is inside your body. Your skin has three layers, and the dermis lies beneath the epidermis and holds blood vessels, hair follicles and sweat glands. The outer layer of skin constantly flakes off and is replaced by new cells from below. Collagen is a protein in your skin that helps to keep it strong. Skin is a barrier that helps keep germs out of the body.

Words to know

• valves: one-way doors
• marrow: inside some bones
• peristalsis: squeezing waves