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Why does your heart keep beating, even when you’re asleep? Picture a soap dispenser: press, and liquid moves forward; let go, and more fills the pump. Your body’s cells need a steady delivery of oxygen. If blood stops flowing, they can’t keep working.

Your heart works like that pump. When its muscle relaxes, blood flows in. When it squeezes, blood moves out. Tiny one-way doors, called valves, stop blood slipping backward. The right side sends blood to the lungs for oxygen; the left sends it through your body.

Feel your wrist for a moment. That pulse is a wave from each squeeze of your heart, pushing blood into your arteries. Blood then returns to the heart, ready to go again. What keeps blood moving forward? The heart squeezes, and valves prevent backflow.