Pulmonary artery and vein names cause more confusion than almost any other pair of blood vessels in the body. Here is the short answer: the pulmonary veins carry oxygenated blood, and the pulmonary artery carries deoxygenated blood. Oxygen-poor blood leaves the right ventricle through the pulmonary artery, picks up oxygen in the lungs, and returns to the left atrium through the pulmonary veins, ready to be pumped to the rest of the body.
That is the reverse of what most of us learned in school, where “arteries carry oxygen-rich blood and veins carry oxygen-poor blood.” The rule we were taught is actually a shortcut. Doctors name blood vessels by the direction of flow, not by how much oxygen they carry. Arteries carry blood away from the heart. Veins carry blood toward it. In the lung circuit, that definition flips the usual oxygen pattern.
So if you are wondering whether to pick “pulmonary vein or artery” on an exam, remember this: the pulmonary vein is the only pulmonary vessel that carries oxygenated blood back to the heart. The rest of this guide explains why, what the normal numbers look like, and why the difference matters when something goes wrong.
Pulmonary Artery and Vein at a Glance
Before getting into the details, here is a pulmonary artery vs pulmonary vein comparison you can come back to.
| Feature | Pulmonary artery | Pulmonary veins |
|---|---|---|
| Direction of flow | Heart → lungs | Lungs → heart |
| Starts at | Right ventricle (via the pulmonary valve) | Lung tissue (capillaries around the alveoli) |
| Ends at | Capillaries around the alveoli | Left atrium |
| Oxygen content | Low (deoxygenated) | High (oxygenated) |
| Typical oxygen saturation | About 65–80% | About 95–100% |
| Typical PO₂ | About 40 mmHg | About 100 mmHg |
| Number | One trunk that splits into right and left branches | Usually four (two from each lung) |
| Wall | Thicker, more muscular | Thinner |
| Common problems | Pulmonary embolism, pulmonary hypertension | Pulmonary vein stenosis, atrial fibrillation triggers, anomalous drainage |
The oxygen saturation ranges above match the normal mixed venous and arterial values listed in the 2022 ESC/ERS pulmonary hypertension guidelines. Blood sampled from the pulmonary artery is, in fact, the reference “mixed venous” sample doctors use during a right heart catheterization.
Does the Pulmonary Artery Carry Oxygenated Blood?
No. In a healthy adult, the pulmonary artery carries oxygen-poor blood. It is the only artery in the body that does so after birth.
Here is the route. Blood returning from your organs and muscles has already handed over much of its oxygen. It flows into the right atrium, then down into the right ventricle. The right ventricle is the chamber that pumps blood into the pulmonary trunk, pushing it through the pulmonary valve, a one-way door that stops blood slipping back, and into the main pulmonary artery.
The pulmonary trunk is short, about 5 cm long, and then splits beneath the aortic arch into the right and left pulmonary arteries. Cleveland Clinic notes that the main pulmonary artery averages about 2.7 cm across in women and 2.9 cm in men. Each branch keeps dividing inside its lung until it becomes a mesh of tiny capillaries wrapped around the air sacs. If you want to follow that branching in detail, our guide to how many pulmonary arteries there are walks through each level.
Because this blood has not yet reached the lungs, its oxygen pressure is low (around 40 mmHg) and its carbon dioxide is relatively high. That is exactly what the lungs are designed to fix. For more on the job these vessels do, see our full guide to the function of the pulmonary arteries.
Do Pulmonary Veins Carry Oxygenated Blood?
Yes. The pulmonary veins carry the most oxygen-rich blood in your body.
Once blood passes through the capillaries in the lung, it has swapped its carbon dioxide for fresh oxygen. Those capillaries merge into small venules, then larger veins, and finally into the main pulmonary veins that exit each lung at the hilum. A StatPearls review confirms that oxygenated blood from the lung capillaries returns to the left atrium through four pulmonary veins.
Most people have two veins from each lung:
- Right superior pulmonary vein – drains the right upper and middle lobes
- Right inferior pulmonary vein – drains the right lower lobe
- Left superior pulmonary vein – drains the left upper lobe and lingula
- Left inferior pulmonary vein – drains the left lower lobe
About 60–70% of people have this classic four-vein pattern. The rest have three or five, usually because two veins join before entering the heart or an extra right-sided vein is present. These variations are normally harmless. Our article on how many pulmonary veins there are covers the variants in more depth, and you can read more about what the pulmonary veins do day to day.
From the left atrium, the oxygenated blood drops into the left ventricle, which pumps it into the aorta and out to the whole body.
Why the Pulmonary Artery and Vein Break the Usual Rule
The “arteries = red, veins = blue” idea works for the systemic circulation, which is the big loop that serves your brain, gut, muscles and skin. The lungs run a second, separate loop. Once you see the two loops side by side, the naming stops feeling strange.
- Systemic loop: left ventricle → aorta → arteries → body tissues → veins → right atrium. Arteries carry oxygen-rich blood to the tissues; veins bring oxygen-poor blood back.
- Pulmonary loop: right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium. The pulmonary artery carries oxygen-poor blood to the lungs; the pulmonary veins bring oxygen-rich blood back.
In both loops, arteries leave the heart and veins return to it. The only thing that changes is where oxygen gets loaded. In the body, tissues use oxygen. In the lungs, blood gains it. For a fuller picture of how these loops connect, see our explainer on pulmonary and systemic circulation.
A useful memory trick: A for Away, V for Visit. Arteries carry blood away from the heart; veins bring blood back to visit it. Oxygen content has nothing to do with the name.
Why textbook diagrams color them “backwards”
Anatomy diagrams usually color the pulmonary arteries blue and the pulmonary veins red. That is not an error. The colors show oxygen content, not vessel type. Real blood is never actually blue, though. Deoxygenated blood is a darker red, and the bluish look of veins under the skin is a trick of light passing through tissue.
What Happens in Between: Gas Exchange in the Lungs
The switch from oxygen-poor to oxygen-rich blood happens in roughly 300 million alveoli, the tiny air sacs at the end of your airways. Each one is wrapped in capillaries so thin that red blood cells pass almost single file.
Oxygen moves from the air in the alveolus into the blood because its pressure is higher in the air sac (about 100 mmHg) than in the incoming pulmonary artery blood (about 40 mmHg). Carbon dioxide moves the other way for the same reason. The process takes less than a second at rest. Our article on gas exchange between the alveoli and pulmonary capillaries explains the physics step by step.
The lungs also do something clever to keep this efficient. If a patch of lung is poorly ventilated, the small arteries supplying it tighten, sending blood toward better-aired regions. This reflex is called hypoxic pulmonary vasoconstriction, and it is a big reason chronic low oxygen can eventually raise pressure in the pulmonary arteries.
Where they sit inside the lung
The two vessel types also travel differently through the lung. In lung anatomy, each pulmonary artery pulmonary vein pair takes its own path: the arteries run alongside the bronchi, branching as the airways branch, while the veins run in the connective tissue between lung segments. Radiologists use this pattern to tell pulmonary arteries and veins apart on a CT scan.
Normal Pressures and Sizes
The pulmonary circuit is a low-pressure system. The right ventricle only needs to push blood a short distance through a very large, soft network of vessels, so pressures are a fraction of those in the aorta. That is also why both the pulmonary veins and arteries have thinner, more stretchy walls than their counterparts elsewhere in the body.
| Measurement | Normal range |
|---|---|
| Pulmonary artery systolic pressure | 15–30 mmHg |
| Pulmonary artery diastolic pressure | 4–12 mmHg |
| Mean pulmonary artery pressure (mPAP) | 8–20 mmHg |
| Pulmonary arterial wedge pressure (reflects pulmonary vein/left atrial pressure) | ≤15 mmHg |
| Pulmonary vascular resistance | 0.3–2.0 Wood units |
| Main pulmonary artery diameter | About 2.7 cm (women), 2.9 cm (men) |
| Pulmonary vein diameter | About 9–13 mm |
Pressure values are from the 2022 ESC/ERS guidelines; vessel sizes are from Cleveland Clinic.
Thresholds that suggest a problem
| Finding | What it may suggest |
|---|---|
| Mean pulmonary artery pressure >20 mmHg at rest | Pulmonary hypertension |
| mPAP >20 mmHg + wedge pressure ≤15 mmHg + resistance >2 Wood units | Pre-capillary PH (problem in the arteries or lungs) |
| mPAP >20 mmHg + wedge pressure >15 mmHg | Post-capillary PH (back-pressure from the left heart and pulmonary veins) |
| Pulmonary artery ≥30 mm on CT, or wider than the aorta | Possible pulmonary hypertension; needs further testing |
The current definition lowered the old cut-off of 25 mmHg to 20 mmHg. If you have been told your pulmonary pressure is raised, our guide to high pulmonary artery pressure explains what the numbers mean and how they are measured.
Pulmonary Artery vs Vein: Why the Difference Matters Clinically
Knowing which vessel is which isn’t just exam trivia. The two sides fail in different ways, and the symptoms often point to which side is involved.
When the pulmonary arteries are the problem
- Pulmonary embolism (PE). A blood clot, usually from a leg vein, travels through the right heart and lodges in the pulmonary arteries. Blood can no longer reach part of the lung, so oxygen levels fall. Typical warning signs include sudden breathlessness, sharp chest pain and a fast heartbeat. Learn how to tell if you might have a pulmonary embolism.
- Pulmonary arterial hypertension (PAH). The small arteries thicken and narrow, forcing the right ventricle to work harder until it eventually tires.
- Congenital narrowing. Conditions such as pulmonary artery stenosis or pulmonary atresia limit blood flow to the lungs from birth.
When the pulmonary veins are the problem
- Left-sided heart disease. When the left ventricle or mitral valve struggles, pressure backs up into the pulmonary veins and then into the lungs. This is group 2 pulmonary hypertension, the most common type worldwide.
- Atrial fibrillation. Small sleeves of heart muscle extend a short way into the pulmonary veins, and these are a frequent source of the stray electrical signals that trigger AF. Catheter ablation, called pulmonary vein isolation, targets exactly this spot.
- Pulmonary vein stenosis. Narrowing of one or more veins, seen in some infants with heart defects and occasionally in adults after AF ablation.
- Anomalous pulmonary venous return. In TAPVR or PAPVR, some or all pulmonary veins drain into the right side of the heart instead of the left, so oxygenated blood goes back to the lungs instead of out to the body.
Cleveland Clinic’s overview of the pulmonary veins is a good patient-friendly summary of these conditions.
A Note on Fetal Circulation
Before birth, the rules change again. A baby in the womb gets oxygen from the placenta, not the lungs, which are filled with fluid. Most blood leaving the right ventricle bypasses the lungs through a small channel called the ductus arteriosus. The oxygen-rich blood actually arrives through the umbilical vein, while the umbilical arteries carry oxygen-poor blood back to the placenta.
So in the fetus, the umbilical vessels are the “odd ones out,” and the pulmonary artery carries a mix of blood rather than truly oxygenated blood. Within the first breaths after birth, the lungs inflate, pressure in the pulmonary arteries drops, the ductus begins to close, and the adult pattern takes over.
One More Exception: The Lungs Feed Themselves Separately
The pulmonary arteries bring blood to the lungs to be oxygenated, but they do not nourish the lung’s own airway walls. That job belongs to the bronchial arteries, small branches of the systemic circulation that carry oxygen-rich blood from the aorta. Part of this bronchial blood drains into the pulmonary veins after use. That small amount of mixing is one reason your arterial oxygen saturation sits at 95–100% rather than a perfect 100%.
Frequently Asked Questions
What is the main difference between the pulmonary artery and vein?
The pulmonary artery carries oxygen-poor blood from the right ventricle to the lungs. The pulmonary veins carry oxygen-rich blood from the lungs back to the left atrium. Both are named by direction of flow: arteries leave the heart and veins return to it.
Does the pulmonary artery carry oxygenated blood?
No. After birth, the pulmonary artery carries deoxygenated blood with an oxygen saturation of about 65–80%. It is the only artery in the adult body that carries oxygen-poor blood.
Does pulmonary artery carry oxygenated blood in a fetus?
Not really. Before birth, the lungs are not used for breathing, and most blood in the pulmonary artery is diverted through the ductus arteriosus into the aorta. The vessel carrying the most oxygen in a fetus is the umbilical vein.
Do pulmonary veins carry oxygenated blood?
Yes. The pulmonary veins carry freshly oxygenated blood from the lungs to the left atrium, with a saturation of roughly 95–100%. They are the only veins in the body that carry oxygen-rich blood.
Which pulmonary vessel carries oxygenated blood to the heart?
The pulmonary veins. Most people have four of them, two from each lung, which empty into the left atrium.
Why is the pulmonary artery called an artery if it carries deoxygenated blood?
Because the word “artery” describes direction, not oxygen. Any vessel that carries blood away from the heart is an artery, and the pulmonary artery carries blood away from the right ventricle toward the lungs.
How many pulmonary arteries and veins are there?
There is one main pulmonary artery (the pulmonary trunk), which splits into right and left pulmonary arteries. Most people have four pulmonary veins, though three or five is a normal variation in about a third of people.
What happens if the pulmonary artery or veins are blocked?
A blocked pulmonary artery, usually from a blood clot, is a pulmonary embolism and needs urgent care. Narrowed or blocked pulmonary veins cause blood to back up into the lungs, leading to breathlessness, fluid in the lungs and raised pulmonary pressure.
Bottom Line
When it comes to the pulmonary artery and vein, the answer is the opposite of what the old classroom rule suggests. The pulmonary artery carries oxygen-poor blood from the right ventricle to the lungs, and the pulmonary veins carry oxygen-rich blood back to the left atrium. Both names make sense once you remember that arteries leave the heart and veins return to it. Knowing which side is which also helps make sense of real conditions, from pulmonary embolism and pulmonary hypertension on the arterial side to atrial fibrillation and venous congestion on the venous side. For a broader look at the whole loop, see our guide to how pulmonary circulation works.
Medical disclaimer: This article is for general educational purposes only and is not a substitute for professional medical advice, diagnosis or treatment. If you have symptoms such as sudden shortness of breath, chest pain or coughing up blood, seek emergency care. Always talk to a qualified healthcare provider about your own health.
Shirley Setia is a medical student who writes and updates our guides to pulmonary embolism and pulmonary hypertension. Her articles are reviewed by a doctor on our team before publication.
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