One pump, two loops
The circulatory system is a closed plumbing network, and the heart is the pump that drives it. But it is really two pumps in one organ, sitting side by side. The right side pushes blood to the lungs; the left side pushes blood to everything else. Because the two loops are connected end to end, every drop of blood passes through both in turn.
This is the heart of systemic versus pulmonary circulation. The pulmonary circuit carries oxygen-poor blood to the lungs to pick up oxygen and drop off carbon dioxide. The systemic circuit carries oxygen-rich blood out to the brain, muscles, gut and kidneys, then collects the spent blood and brings it back. The lungs are the only organ that gets the entire output of the heart on every pass.
Trace one drop around
The clearest way to learn the layout is to follow a single drop of blood as it makes a full lap. Start where tired, oxygen-poor blood returns to the heart, and notice how the circulation never stops moving forward in one direction — valves and pressure keep it from sloshing back.
- Oxygen-poor blood from the body empties into the right atrium, then drops into the right ventricle.
- The right ventricle pumps it through the pulmonary artery to the lungs — the pulmonary circuit — where it gains oxygen.
- Oxygen-rich blood returns via the pulmonary veins to the left atrium, then the left ventricle.
- The powerful left ventricle ejects it into the aorta and out through the systemic circuit to the whole body.
- Tissues take the oxygen; the now oxygen-poor blood collects in veins and heads back to the right atrium to begin again.
Why the design matters
Putting the two loops in series guarantees that blood is freshly oxygenated just before it is sent to the body. The whole point of the circulation is delivering oxygen (and fuel) to cells and carrying away waste. Keeping the lung loop and the body loop separate but linked lets the heart act as one coordinated pump while never mixing fresh and spent blood in a healthy adult.