Video narration
How can one small organ keep your entire body supplied with blood? Think of squeezing a pump bottle: its fluid moves forward because a one-way mechanism stops it flowing back. But the heart is not one pump sending blood everywhere. It is two pumps working in sequence: one serves the lungs, the other the body. Oxygen-poor blood returning from the body enters the right atrium, the receiving chamber, then the right ventricle, which pumps it to the lungs. There it gains oxygen and releases carbon dioxide. It returns to the left atrium, then fills the left ventricle, which pumps oxygen-rich blood to the body. Valves act like doors, keeping blood moving one way. Each heartbeat fills the atria and squeezes the ventricles, repeating both pumps in sequence. Two synchronized pumps: oxygen-poor blood to lungs, oxygen-rich blood around the body.