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The heart may look like one simple pump. But inside, it is more like a carefully designed four-room engine. These four rooms are called chambers. The two upper chambers are the atria. The two lower chambers are the ventricles. The right side of the heart handles blood that is low in oxygen. The left side handles blood that is rich in oxygen. Here is how the journey works. Blood from the body first enters the right atrium. Then it moves into the right ventricle. From there, the heart sends it to the lungs. In the lungs, the blood releases carbon dioxide and picks up fresh oxygen. Then the oxygen-rich blood returns to the left atrium. It moves into the left ventricle. Finally, the left ventricle pushes that blood out to the entire body. This is why the left ventricle has the thickest muscle wall. It has the hardest job: sending blood all the way to the brain, arms, organs, and legs. Between these chambers are valves. Valves work like one-way doors. They open to let blood move forward, then close to stop blood from flowing backward. Without valves, the heart would waste energy pushing blood in the wrong direction. The heart also has large blood vessels connected to it. The vena cava brings blood back from the body. The pulmonary artery carries blood to the lungs. The pulmonary veins bring oxygen-rich blood back from the lungs. The aorta sends oxygen-rich blood out to the body. So the heart is not just a bag of muscle. It is a four-chamber system, a valve system, and a traffic-control center for blood. Every beat follows the same hidden logic: receive, pump, refresh, deliver. That simple rhythm keeps oxygen moving through every part of you. The heart is small enough to fit in your chest. But structurally, it is one of the most elegant engines in the human body.