The pulmonary veins carry oxygenated blood to the heart. There are four of them, two from each lung, and they all empty directly into the left atrium, the upper left chamber of the heart. From there, the freshly oxygenated blood moves through the rest of the heart and out to the body.
How the Pulmonary Veins Work
After you breathe in, your lungs transfer oxygen into tiny blood vessels called capillaries. These capillaries feed into progressively larger vessels that eventually form the four pulmonary veins. Each vein connects to the left atrium through its own opening. The blood they deliver typically has an oxygen saturation between 95% and 100%.
This makes pulmonary veins unusual. In the rest of the body, veins carry oxygen-poor blood back toward the heart, while arteries carry oxygen-rich blood away from it. The pulmonary system flips this pattern. Pulmonary arteries carry oxygen-poor blood from the heart to the lungs, and pulmonary veins carry oxygen-rich blood from the lungs back to the heart. The naming follows the direction of flow (toward or away from the heart), not the oxygen content.
Where the Blood Goes Next
Once oxygenated blood enters the left atrium, it passes through a one-way valve called the mitral valve into the left ventricle, the heart’s most powerful pumping chamber. The left ventricle then contracts and pushes blood through the aortic valve into the aorta, the body’s largest artery. From the aorta, blood branches out through smaller and smaller arteries to deliver oxygen to every organ and tissue.
After cells use that oxygen and produce carbon dioxide as waste, the now oxygen-depleted blood returns through veins to the right side of the heart. The right ventricle pumps it into the pulmonary arteries and back to the lungs, where it picks up a fresh load of oxygen and releases carbon dioxide. Then the cycle starts again through the pulmonary veins.
Fetal Circulation Is Different
Before birth, a baby’s lungs aren’t yet breathing air, so the pulmonary veins don’t play their usual role. Instead, the fetus gets oxygenated blood from the placenta through the umbilical vein. This blood travels through a shortcut called the ductus venosus, which passes it through the liver and into a large vein (the inferior vena cava) that delivers it to the right atrium. At birth, the lungs inflate, blood flow shifts to the pulmonary circuit, and the pulmonary veins take over as the primary oxygen delivery route to the heart.
What Happens When These Veins Don’t Connect Properly
A rare birth defect called total anomalous pulmonary venous return (TAPVR) occurs when the pulmonary veins connect to the wrong part of the heart instead of the left atrium. Oxygen-rich blood ends up on the right side of the heart, where it mixes with oxygen-poor blood. The result is that less oxygen reaches the body than normal.
Babies born with TAPVR usually survive initially because they also have a small hole between the two upper chambers of the heart. This hole allows some of the mixed blood to cross over to the left side so it can be pumped out to the body. Even so, affected infants typically show signs at birth or shortly after: bluish skin, difficulty breathing, poor feeding, and a weak pulse. TAPVR is classified as a critical congenital heart defect, and surgery is needed to reroute the pulmonary veins to their correct destination. Even after successful repair, some people experience lifelong complications.