When blood is first pumped out of the human heart, it first enters a certain organ, then it returns to the heart a second time before continuing its trip through the body. Which organ is this?

The human circulatory system is often called a "double-loop" system, because blood essentially makes two circuits. The heart acts as a powerful dual-sided pump at the center of it all. When oxygen-depleted blood returns from its journey around the body, it enters the right side of the heart. This side's only job is to give that blood a push on its first, shorter loop, known as the pulmonary circuit.

This initial trip takes the blood directly to the lungs. Here, in a vast network (Review) of tiny capillaries surrounding air sacs, a crucial exchange occurs. The blood releases its waste product, carbon dioxide, which you then exhale. At the same time, it absorbs a fresh supply of oxygen from the air you have just inhaled. This process changes the blood from a dark red to a vibrant, bright red color.

Now rich with oxygen, the blood is ready to service the rest of the body, but it first needs another powerful push. It travels back to the left side of the heart, which is stronger and more muscular. The left side then pumps the newly oxygenated blood out on its second, much longer loop—the systemic circuit—delivering vital oxygen to every organ and tissue from your brain to your toes before the cycle begins again.