Video narration
After you take a breath, oxygen reaches your lungs-but how does the blood get back to your heart? Around the air sacs, a web of tiny capillaries lets oxygen pass from the air into the blood, while carbon dioxide leaves the blood to be breathed out. That oxygen-rich blood flows out of the capillaries and into the pulmonary veins-the vessels that carry it from the lungs toward the heart. Then it enters the left atrium, the heart's upper chamber on its left side, where blood returning from the lungs arrives. The word pulmonary points to the lungs, not the direction blood travels. Here's the surprising part: this blood is oxygen-rich, yet its vessels are still called veins. Veins are named by direction: they carry blood toward the heart. So it gets back through the pulmonary veins, entering the heart at the left atrium.