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Your pulmonary veins are a group of blood vessels that drain oxygenated blood from your lungs and return it to your heart. The word 'pulmonary' comes from the Latin root pulmo, meaning 'of or relating to the lungs.'

In order to understand what the pulmonary veins do, you must have a basic understanding of the circulatory system. Your circulatory system is composed of your heart, which is a four-chambered pump, and a collection of hollow vessels that carry blood away from your heart, deliver it to all of your tissues and organs, and return it to your heart. Your lungs are an important way-station within your circulatory system. This is where your blood becomes oxygenated (i.e., absorbs oxygen).

The blood vessels that carry blood away from your heart are called arteries. In most cases, the blood within your arteries is oxygenated. As your arteries travel away from your heart, they divide into smaller and smaller vessels called arterioles, which themselves divide into tiny, thin-walled vessels called capillaries.

As blood travels through the complex network of capillaries in your body, it becomes progressively deoxygenated as it releases oxygen to your tissues. Gradually, your capillaries join to form larger and larger vessels called venules, which then join into even larger vessels called veins. Finally, your veins return deoxygenated blood to your heart, which pumps it to your lungs to be oxygenated once again.

As you can see, your circulatory system forms a closed loop: your heart pumps blood through your arteries, arterioles, capillaries, venules and veins, only to have that blood return to its original location. The four-chambered design of your heart is critical for keeping your blood moving in the proper direction. Oxygenated blood returning from your lungs enters the left atrium of your heart, where it is channeled through a one-way valve into the heavily muscled left ventricle. When your heart beats, the contracting left ventricle pushes blood through another valve into your aorta, which is the largest artery in your body. With each heartbeat, blood moves farther along on a journey that will eventually bring it back to your heart.

Upon returning to your heart, deoxygenated blood enters the right atrium, which then directs it through yet another valve into the right ventricle. The right ventricle, which is not as muscular as the left ventricle because it only has to pump your blood a short distance, pushes the deoxygenated blood through one more valve into your pulmonary arteries, which carry it to your lungs.

Although the blood vessels that transport blood to and from your lungs are continuous with the remainder of your circulatory system, your heart, pulmonary arteries, lungs, and pulmonary veins are called the 'pulmonary circuit' to distinguish them from the 'systemic circuit,' which serves the rest of your body.

The pulmonary circuit carries deoxygenated blood away from your heart via the pulmonary arteries and returns oxygenated blood to your heart via the pulmonary veins.
Pulmonary Circuit
Anatomy of the Pulmonary Veins
Just like the other veins in your body, your pulmonary veins arise from a network of capillaries. However, the capillaries that give rise to the pulmonary veins differ from capillaries elsewhere in your body. The pulmonary capillaries surround and embrace millions of tiny air sacs, called alveoli, in your lungs. This is where your blood takes up oxygen from the air you inhale.

Pulmonary capillaries draw oxygen from the alveoli.
As they leave the alveoli and course toward your heart, your pulmonary capillaries unite to form progressively larger venules and veins, which gather in the crevices that divide the various divisions, or segments, of your lungs. Eventually, all of the veins within one lung segment unite to form a single vein called a segmental vein.

A segmental vein usually travels alongside the bronchus, or air tube, that serves the same segment. Just prior to exiting your lungs in the center of your chest, the segmental veins from each lung join to form superior and inferior pulmonary veins. The pulmonary veins then enter your heart through the back side of your left atrium.