The human heart is sometimes called a double pump because it drives blood along two separate pathways in the circulatory system. With each beat, the heart drives arterial blood through the body, and it draws venous blood back toward itself.
The high-pressure loop of the system is arterial. Blood is pushed through this system when the left ventricle contracts to squeeze fresh, oxygenated blood through the aorta. From there, the blood flows throughout the body to deliver oxygen and essential nutrients to most of the body's cells. With the same ventricular contraction, the smaller right ventricle drives blood through the low-pressure system of deoxygenated blood. The left ventricle sends blood to the lungs to be enriched with oxygen prior to its return to the heart via the left atrium.
Venous blood from around the body flows into the right ventricle via the superior and inferior vena cavae. Between phases of ventricular contraction, valves inside the heart's two atria open to admit blood that's waiting to be sent through the body on the heart's next stroke. This dual system keeps exhausted blood from mingling with freshly oxygenated blood, ensuring that oxygen and nutrients are delivered and carbon dioxide and other cellular wastes are carried away with maximum efficiency.