Video narration
How does the heart keep blood moving forward instead of backward? Think of a hand-operated bicycle pump: release to fill, squeeze to push, repeat. The heart follows that rhythm, but works as two pumps side by side. The right side receives oxygen-poor blood returning from the body and sends it to the lungs. There, blood picks up oxygen. The left side receives it and sends it around the body. First, the atria-the receiving chambers-fill. Blood moves into the ventricles, the stronger pumping chambers. Then the ventricles squeeze and empty, driving blood toward its next destination. Valves act like one-way doors. They open for forward flow and shut behind the blood, so each squeeze moves it onward instead of backward. So: two coordinated pumps fill and squeeze in sequence, sending blood from body to lungs and back to body in one loop.