Video narration
What is the push you feel in your wrist when your heart beats? It's blood moving forward-but not through one simple loop. The heart is a two-sided pump. Its right side receives oxygen-poor blood from your body and sends it to your lungs. There, the blood picks up oxygen and releases carbon dioxide. Then it returns to the heart's left side, which sends it out to the rest of your body. Each side has an upper chamber and a lower chamber. The upper chambers, called atria, fill the lower chambers, called ventricles. Then the ventricles contract, providing the main force that pushes blood onward. Valves act like one-way doors, opening for forward flow and closing to stop blood from slipping backward. So why does your pulse keep repeating? That push comes from one repeating loop: fill, squeeze, refresh blood in the lungs, then send it back to the body.