Your heart has two receiving chambers and two pumping chambers.

The right atrium receives oxygen-poor blood returning from body tissues through the major veins. Blood passes into the right ventricle, which pumps it toward the lungs through the pulmonary artery. There it can release carbon dioxide and pick up oxygen across the lung’s exchange surface.

The left atrium receives oxygen-rich blood returning from the lungs through pulmonary veins. It passes blood into the left ventricle, which ejects it into the aorta. From there, arteries distribute blood to organs and tissues throughout the systemic circulation.

Valves between chambers and at the ventricular outlets help maintain forward flow. Their opening and closing depend largely on pressure differences. A wall called the septum separates the right and left sides, normally preventing direct mixing between the two ventricular blood streams.

Atria and ventricles have different workloads. Atria receive returning blood and contribute to ventricular filling. Ventricles generate the pressure that drives blood into an outgoing circulation. Their muscular walls reflect those responsibilities: the left ventricle must produce enough pressure to supply the much larger systemic circuit.

The two sides do not take turns sending unrelated batches of blood around the body. They are linked in series, so blood ejected by the right side passes through the lungs and returns to the left side. Over time, their outputs must remain closely matched for circulation to function normally.

An electrical conduction system coordinates the heartbeat. Activity normally begins in the sinoatrial node, spreads through the atria, and passes through a controlled delay before activating the ventricles. This sequence supports filling before ejection. Electrical coordination is distinct from the valves, which manage flow mechanically rather than issuing the timing signal.

Artery and vein describe direction relative to the heart, not oxygen content. The pulmonary artery carries blood away from the heart despite carrying relatively oxygen-poor blood. Pulmonary veins return relatively oxygen-rich blood. This exception to a common classroom shortcut follows the actual naming rule exactly.

The heart muscle also needs its own blood supply. Coronary arteries branch from the aortic region and deliver oxygen and nutrients to cardiac tissue. Blood inside a chamber is not sufficient by itself to nourish the entire thick wall. A blocked coronary artery therefore threatens heart muscle even though the chambers still contain blood.

During relaxation, the ventricles refill for the next contraction. This filling phase is essential: a strong squeeze cannot eject blood that never entered the chamber. Heart performance consequently depends on both effective contraction and adequate relaxation, not just the force of each beat.

The chambers make sense as parts of a route rather than four isolated compartments. Blood returning from the body enters the right side, travels to the lungs, returns to the left side, and is pumped into the body again. The two sides therefore work in series. Each handles a different destination while contributing to the same continuing circulation.