Pulmonary Arteries: Function, Location & Anatomy

Every time your heart beats, its right side pushes a full load of blood toward your lungs. The vessels that carry that blood are the pulmonary arteries. If a scan report, an echocardiogram, or a diagnosis like pulmonary hypertension has you wondering what do pulmonary arteries do, here is the short version.

The pulmonary arteries carry oxygen-poor blood from the right side of the heart to the lungs. There, the blood drops off carbon dioxide and picks up fresh oxygen before returning to the left side of the heart through the pulmonary veins. They are the only arteries in the adult body that carry oxygen-poor blood, and they work at much lower pressure than the arteries that feed the rest of the body.

Below, we walk through where these vessels sit in your chest, how they are built, what normal pressure and size look like, and which conditions affect them.

Key Takeaways

• One main pulmonary artery (the pulmonary trunk) leaves the right ventricle and splits into a right and a left pulmonary artery, one for each lung.
• They carry oxygen-poor blood away from the heart. That direction, not the oxygen level, is why they are called arteries.
• The system runs at low pressure. A normal mean pressure is about 14 mmHg; above 20 mmHg at rest meets the current definition of pulmonary hypertension.
• The main pulmonary artery is about 5 cm long and roughly 2.5 cm wide in most adults.
• The two big threats are blood clots (pulmonary embolism) and high pressure (pulmonary hypertension). Sudden shortness of breath or chest pain needs emergency care.

What Do Pulmonary Arteries Do? Four Jobs

Most guides stop at “they take blood to the lungs.” That is the headline, but the vessels do a little more than act as pipes.

1. Deliver blood for gas exchange

Blood returning from your body has given up much of its oxygen and is loaded with carbon dioxide. The pulmonary arteries carry it to the lungs and branch again and again until the vessels are thin enough to wrap around the alveoli, the tiny air sacs at the end of your airways. That is where gas exchange between the alveoli and pulmonary capillaries happens.

2. Handle the heart’s entire output at low pressure

Every drop of blood your heart pumps passes through the lungs, roughly 4 to 8 liters per minute at rest. The pulmonary arteries manage that flood with thin, stretchy walls and wide channels, so pressure stays low. That protects the right ventricle, a thinner, weaker pump than the left, and keeps fluid from being forced out of the capillaries into the air sacs.

3. Send blood where the air is

Here pulmonary arteries behave the opposite way to arteries elsewhere. When a patch of lung is low on oxygen, say from pneumonia or a plugged airway, the small arteries supplying it tighten. Blood is diverted to areas that are breathing well, so less of it passes through the lungs without picking up oxygen. Doctors call this hypoxic pulmonary vasoconstriction. It helps with small, local problems, but when the whole lung is short of oxygen for months, as in severe COPD or at high altitude, this tightening becomes widespread and can raise pressure.

4. Act as a filter

Because the arteries narrow into an enormous net of tiny branches, small clots and debris drifting up from the veins tend to get trapped in the lungs instead of reaching the brain or heart. The body usually breaks down very small clots without symptoms. Larger ones are a different story, which is what a pulmonary embolism is.

A Blood Cell’s Trip Through the Pulmonary Arteries

Following one red blood cell makes the route easier to picture:

1. Right atrium. Oxygen-poor blood from the body enters the upper right chamber of the heart.
2. Right ventricle. It passes into the lower right chamber, the heart chamber that pumps blood into the pulmonary trunk.
3. Pulmonary valve. The ventricle squeezes and pushes the blood through the pulmonary valve, which snaps shut behind it to stop backflow.
4. Pulmonary trunk. The blood rises through the short, wide main pulmonary artery.
5. Right or left pulmonary artery. Under the arch of the aorta, the trunk splits. The cell heads to either the right or left lung.
6. Smaller and smaller branches. Lobar, segmental, and subsegmental arteries follow the airways deeper into the lung.
7. Capillaries. In vessels so narrow that red cells pass in single file, carbon dioxide leaves and oxygen enters, all in under a second.
8. Pulmonary veins. Now oxygen-rich, the blood returns to the left atrium through the pulmonary veins, ready for the left ventricle to send it to the rest of the body.

This loop is called the pulmonary circuit. For how it fits with the larger loop that feeds the rest of your body, see our guide to pulmonary and systemic circulation.

Where Are the Pulmonary Arteries Located?

The pulmonary arteries sit in the middle of your chest, in a space between the lungs called the middle mediastinum, right alongside the heart and the aorta.

• Starting point: the pulmonary trunk begins at the pulmonary valve, at the top front of the right ventricle, just behind the breastbone.
• Course: it runs upward and slightly backward, along the left side of the ascending aorta.
• The split: just below the arch of the aorta, at about the level of the fourth thoracic vertebra (near where the windpipe divides into the two main bronchi), the trunk divides at almost a right angle.
• Into the lungs: each branch enters its lung at the hilum, the “doorway” where the bronchus, vessels, and nerves go in and out.

Anatomy students often use the memory aid RALS: the Right pulmonary artery lies Anterior (in front of) the right main bronchus, while the Left pulmonary artery lies Superior to (above) the left main bronchus.

Pulmonary Artery Anatomy

The pulmonary trunk (main pulmonary artery)

The pulmonary trunk is short and wide, about 5 cm long and 2 to 3 cm across. In a large population study from Framingham, the average diameter on CT was about 25 mm. The study set the upper end of normal at 29 mm for men and 27 mm for women. It is counted among the “great vessels” of the heart, alongside the aorta, the venae cavae, and the pulmonary veins.

The right and left pulmonary arteries

You have one trunk and two main branches. (We explain the counting, including what happens as they divide, in how many pulmonary arteries are there.) The two sides are not mirror images, because the heart and aorta sit slightly to the left.

Feature	Right pulmonary artery	Left pulmonary artery
Length	Longer; it has to cross the midline	Shorter; it is a direct continuation of the trunk
Path	Runs behind the ascending aorta and superior vena cava	Arches over the left main bronchus, passing in front of the descending aorta
Relation to bronchus	In front of the right main bronchus	Above the left main bronchus
Supplies	Right lung (three lobes)	Left lung (two lobes)

The left side has its own quirks, including a link to a fetal blood vessel, which we cover in the function of the left pulmonary artery.

Branches inside the lungs

Once inside each lung, the arteries split into lobar arteries for each lobe, then segmental arteries for each segment, then subsegmental arteries, and so on. They travel alongside the airways the whole way down, which is why doctors describe the artery and bronchus as running in pairs. The final branches become capillaries wrapped around the alveoli.

The artery wall

Like other large arteries, pulmonary arteries have three layers:

• Tunica intima: a smooth inner lining that lets blood glide past and helps control vessel tone.
• Tunica media: a middle layer of elastic fibers and smooth muscle.
• Tunica adventitia: a tough outer layer that anchors the vessel.

The key difference from arteries elsewhere is the middle layer. It is thinner, with less muscle, which makes the vessels stretchy and low-resistance. In pulmonary hypertension, this layer thickens and the small arteries narrow and stiffen. That change is called vascular remodeling.

Why Are They Called Arteries If They Carry Oxygen-Poor Blood?

Many of us learned in school that arteries carry oxygen-rich (“red”) blood and veins carry oxygen-poor (“blue”) blood. That shortcut breaks down in the lungs. The real rule is about direction:

• Arteries carry blood away from the heart.
• Veins carry blood back to the heart.

Pulmonary arteries leave the heart, so they are arteries, even though their blood is low in oxygen. Pulmonary veins return to the heart, so they are veins, even though their blood is the most oxygen-rich in your body. (A related exception: before birth, the umbilical vein is the vessel bringing oxygen-rich blood to the baby.)

Pulmonary Arteries vs Pulmonary Veins vs Aorta

Feature	Pulmonary arteries	Pulmonary veins	Aorta
Leaves or enters	Leaves the right ventricle	Enters the left atrium	Leaves the left ventricle
Blood oxygen	Low (about 65–80% saturated)	High (about 95–100%)	High (about 95–100%)
Destination	Lungs	Heart	Whole body
Typical pressure	About 15–30 / 4–12 mmHg	Very low, under about 12 mmHg	About 120/80 mmHg
How many	1 trunk, 2 main branches	Usually 4	1

If you want the full picture of the return trip, see how many pulmonary veins there are and why the number varies between people.

Normal Pulmonary Artery Pressure

Pressure is the number that matters most when doctors talk about these vessels. The most accurate way to measure it is a right heart catheterization, where a thin tube is threaded through a vein into the heart and out into the pulmonary artery. According to the 2022 ESC/ERS pulmonary hypertension guidelines, normal resting values are:

Measurement	Normal range at rest
Systolic (peak) pressure	15–30 mmHg
Diastolic (resting) pressure	4–12 mmHg
Mean pressure	8–20 mmHg (healthy average about 14)
Pulmonary hypertension threshold	Mean pressure above 20 mmHg

Compare that with a normal arm blood pressure of about 120/80. The pulmonary arteries move the same volume of blood at roughly a fifth of the pressure. When the pressure creeps up, the right ventricle has to push harder, and over time it can thicken, stretch, and fail. Our explainer on high pulmonary artery pressure covers what raises it, and the heart and pulmonary artery relationship explains why the right ventricle is so sensitive to it.

Two Things Most Guides Leave Out

The lungs have a second blood supply

The pulmonary arteries bring blood to the lungs to be refreshed, but they do not really feed the lung tissue itself. That job belongs to the much smaller bronchial arteries, which branch off the aorta and carry oxygen-rich blood to the airway walls and even to the walls of the pulmonary arteries. This double supply is one reason a pulmonary embolism does not always kill the lung tissue beyond it. When a pulmonary artery is blocked for a long time, the bronchial arteries can enlarge to help out, and doctors sometimes spot this on CT scans.

Before birth, they are mostly bypassed

A baby in the womb gets oxygen from the placenta, not the lungs, so there is little point sending much blood there. A short vessel called the ductus arteriosus connects the pulmonary artery to the aorta and lets most blood skip the lungs. After the first breaths, lung vessels relax, pressure in the pulmonary arteries falls, and the ductus normally closes within days, leaving a small fibrous cord. When it stays open, it is called a patent ductus arteriosus, one of several congenital conditions that involve the pulmonary arteries.

Conditions That Affect the Pulmonary Arteries

Pulmonary hypertension

High pressure in the pulmonary arteries. It has many causes, sorted into five groups, from left-sided heart disease and lung disease to old clots and a rare primary disease of the arteries themselves called pulmonary arterial hypertension (PAH). See the five groups of pulmonary hypertension for how they differ. Typical symptoms are breathlessness with exertion, fatigue, chest pressure, lightheadedness, and leg swelling.

Pulmonary embolism (PE)

A blood clot, usually from a leg vein, travels to the lungs and lodges in a pulmonary artery. According to the National Heart, Lung, and Blood Institute, common symptoms include shortness of breath, pain with deep breathing, fast breathing, and a fast heart rate. A large clot sitting across the split of the main pulmonary artery is called a saddle embolism and can be life-threatening. Read about how a pulmonary embolism is treated.

Chronic thromboembolic pulmonary hypertension (CTEPH)

In a small share of people, clots from a past PE do not fully dissolve. They turn into scar tissue that narrows the arteries and raises pressure. It is worth knowing about because it is one of the few forms of pulmonary hypertension that surgery or balloon procedures can sometimes largely fix.

Congenital problems

Babies can be born with a narrowed pulmonary artery (pulmonary artery stenosis), a missing connection (pulmonary atresia), or vessels that are wired to the wrong chamber, as in tetralogy of Fallot or transposition of the great arteries. Many are found before or soon after birth and repaired in childhood.

An enlarged pulmonary artery

A wide pulmonary artery on a chest CT is often the first clue to pulmonary hypertension. A diameter of 30 mm or more, or a pulmonary artery wider than the aorta beside it, prompts a closer look. Rarely, the artery is enlarged with no clear cause.

How Doctors Check Your Pulmonary Arteries

Test	What it shows	Good to know
Echocardiogram	Estimates pressure; shows right heart size and pumping	Usually the first test; no needles or radiation
CT pulmonary angiogram (CTPA)	Clots inside the arteries; artery width	Main test for suspected pulmonary embolism; uses contrast dye
V/Q (ventilation-perfusion) scan	Areas of lung getting air but no blood flow	Preferred screen for old clots (CTEPH)
Right heart catheterization	Exact pressures, blood flow, and resistance	The only way to confirm pulmonary hypertension
Cardiac MRI	Right ventricle size and function; blood flow	No radiation; often used for follow-up
Chest X-ray	Enlarged pulmonary arteries or right heart	A normal X-ray does not rule out disease

For the full workup, see how pulmonary hypertension is diagnosed and how doctors detect a pulmonary embolism.

Keeping Your Pulmonary Arteries Healthy

You cannot control congenital problems or genetics, but many causes of pulmonary artery disease can be prevented or managed:

• Don’t smoke or vape. Smoking drives COPD, a leading cause of pulmonary hypertension.
• Move during long trips and after surgery. Walk, flex your calves, and follow any clot-prevention plan your care team gives you.
• Get sleep apnea and low oxygen checked. Loud snoring, gasping at night, or low oxygen readings deserve testing.
• Treat heart and lung conditions well. Heart failure, valve disease, COPD, and lung fibrosis all raise pulmonary pressure when poorly controlled.
• Mention past clots. If you had a PE and are still short of breath months later, ask whether you should be checked for CTEPH.

When to See a Doctor

Call 911 right away if you have sudden shortness of breath, chest pain that worsens with a deep breath, coughing up blood, fainting, or a racing heartbeat, especially after surgery, a long trip, or a recent leg swelling. These can be signs of a pulmonary embolism.

Make an appointment soon if you notice:

• Shortness of breath with activities that used to be easy
• Unusual tiredness, lightheadedness, or near-fainting during exertion
• Swelling in your ankles, legs, or belly
• Bluish lips or fingertips
• Chest pressure or discomfort with exercise

A pulmonologist or cardiologist can sort out whether the pulmonary arteries are involved. You can search our directory of pulmonologists to find a specialist near you.

Frequently Asked Questions

What do pulmonary arteries do in simple terms?

They are the pipes that take “used” blood from the right side of your heart to your lungs, so it can get rid of carbon dioxide and pick up oxygen. Without them, oxygen-poor blood would have no way to get refreshed.

Do pulmonary arteries carry oxygenated or deoxygenated blood?

Deoxygenated (oxygen-poor) blood. They are the only arteries in the adult body that do. The blood becomes oxygen-rich in the lungs and returns to the heart through the pulmonary veins.

How many pulmonary arteries are there?

There is one main pulmonary artery, the pulmonary trunk, which splits into two: the right pulmonary artery for the right lung and the left pulmonary artery for the left lung. Each then divides into many smaller branches inside the lung.

What is a normal pulmonary artery pressure?

At rest, a normal mean pulmonary artery pressure is about 8 to 20 mmHg, with healthy people averaging around 14. A mean pressure above 20 mmHg, measured by right heart catheterization, meets the definition of pulmonary hypertension.

What happens if a pulmonary artery is blocked?

Blood cannot reach part of the lung, so less oxygen gets into the blood and the right side of the heart has to push against the blockage. A small clot may cause mild breathlessness or no symptoms, while a large one can cause collapse and death. A suspected blockage is an emergency.

Is the right pulmonary artery longer than the left?

Yes. Because the pulmonary trunk sits a little left of center, the right pulmonary artery has to travel farther, crossing behind the aorta to reach the right lung. The left one is shorter and continues almost straight on from the trunk.

Can you live with pulmonary artery disease?

Many people do, for years. Outlook depends on the cause. Most pulmonary emboli are treated successfully with blood thinners, and CTEPH can often be improved with surgery or balloon angioplasty. Pulmonary arterial hypertension has no cure, but modern medicines help many people live longer and feel better. See how long you can live with pulmonary hypertension for more.

References

1. Tucker WD, Weber C, Burns B. Anatomy, Thorax, Heart Pulmonary Arteries. StatPearls. Updated July 24, 2023.
2. Cleveland Clinic. Pulmonary Arteries: What They Are & What They Do. Reviewed October 29, 2022.
3. Humbert M, Kovacs G, Hoeper MM, et al. 2022 ESC/ERS Guidelines for the diagnosis and treatment of pulmonary hypertension. Eur Heart J. 2022;43(38):3618–3731.
4. Truong QA, Massaro JM, Rogers IS, et al. Reference values for normal pulmonary artery dimensions by noncontrast cardiac CT: the Framingham Heart Study. Circ Cardiovasc Imaging. 2012;5(1):147–154.
5. National Heart, Lung, and Blood Institute. Pulmonary Embolism (PE). Updated September 19, 2022.

This article is for general education and is not a substitute for advice from your doctor.