The heart is the pump of the circulatory system. Blood returns to the heart and enters the right or left atrium.
Blood entering the right atrium is deoxygenated. This means the tissues and organs of the body have used all of the blood's oxygen and nutrients.
And the blood needs to be replenished.
While the right atrium fills with deoxygenated blood, the left atrium is filling with oxygenated blood returning from the lungs.
When the atria are full, the valves separating the atria and the ventricles open while the atria contract. This draws blood into the ventricles. Then, the atrioventricular (AV) valves close. And the ventricles contract, pumping blood out through another set of valves.
Blood leaving the right ventricle passes through the pulmonary valve into the pulmonary trunk. This artery splits into right and left pulmonary arteries, which channel blood into the lungs to receive oxygen before returning to the left atrium.
Blood leaving the left atrium passes through the aortic valve into the biggest artery in the body, the aorta. As the aorta arches over the top of the heart, it gives off major arterial branches to the brain and the arms. Then it's many branches below the heart supply the rest of the body with blood.
As the animation shows, this is a continuous cycle of deoxygenated blood returning to the heart while oxygenated blood is pumped out to tissues and organs.
Click through the rest of the screens to see the cycle broken into four steps.