Every heartbeat sends blood down two separate roads at the same moment. One road is short and gentle: it runs from the right side of the heart to the lungs and back. The other is long and high-pressure: it runs from the left side of the heart to every organ, muscle and toe, then home again. These are the pulmonary and systemic circulations, and understanding pulmonary vs systemic circulation is the key to making sense of almost every heart and lung condition, from high blood pressure to pulmonary hypertension.
Below you’ll find a clear diagram, a side-by-side comparison table, the step-by-step path blood takes through each loop, and what happens when one of them goes wrong.
Pulmonary vs Systemic Circulation: The Short Answer
Pulmonary circulation carries oxygen-poor blood from the right ventricle to the lungs, where it drops off carbon dioxide and picks up oxygen, then returns that oxygen-rich blood to the left atrium. Systemic circulation carries oxygen-rich blood from the left ventricle through the aorta to the whole body, then brings oxygen-poor blood back to the right atrium through the venae cavae.
Put simply: the pulmonary loop loads oxygen, the systemic loop delivers it. The two loops are connected in series, so blood must pass through one before it can enter the other.
Pulmonary and Systemic Circulation Diagram
A helpful way to read the diagram is as a figure of eight with the heart sitting where the two loops cross. The right side of the heart only ever pumps to the lungs. The left side only ever pumps to the body. A healthy heart keeps the two sides completely separate, so oxygen-rich and oxygen-poor blood never mix.
What Is Pulmonary Circulation?
Pulmonary circulation is the loop that moves blood between the heart and the lungs. Its only job is gas exchange: getting rid of carbon dioxide and topping blood back up with oxygen. For a deeper look at everything this loop does, see our guide to the function of pulmonary circulation.
Pulmonary circulation pathway, step by step
- Right atrium. Oxygen-poor blood arrives from the body through the superior and inferior venae cavae.
- Tricuspid valve to right ventricle. The right atrium pushes blood through the tricuspid valve into the right ventricle, the chamber that pumps blood into the pulmonary trunk.
- Pulmonary valve to pulmonary trunk. The right ventricle contracts and blood passes through the pulmonary valve into the pulmonary trunk.
- Right and left pulmonary arteries. The trunk splits into two branches, one for each lung. (Curious about the exact count further downstream? Read how many pulmonary arteries there are.)
- Pulmonary capillaries. The arteries divide again and again until blood flows in a single-file stream past the alveoli. Here carbon dioxide diffuses out and oxygen diffuses in. This step is explained in detail in our article on gas exchange between alveoli and pulmonary capillaries.
- Pulmonary veins to left atrium. Freshly oxygenated blood drains into the pulmonary veins (usually four of them, see how many pulmonary veins there are) and empties into the left atrium, ready for the systemic trip.
At rest, a red blood cell spends well under a second in a lung capillary. That is enough time because the barrier between air and blood is extraordinarily thin.
What Is Systemic Circulation?
Systemic circulation is the loop that carries oxygen-rich blood from the heart to every tissue in the body and returns oxygen-poor blood to the heart. It feeds the brain, kidneys, gut, muscles and skin, and it even supplies the lungs’ own tissue (more on that surprise below).
Systemic circulation pathway, step by step
- Left atrium. Oxygen-rich blood arrives from the lungs through the pulmonary veins.
- Mitral valve to left ventricle. Blood passes through the mitral (bicuspid) valve into the left ventricle, the heart’s most muscular chamber.
- Aortic valve to aorta. The left ventricle contracts hard enough to push blood through the aortic valve into the aorta, the body’s largest artery.
- Arteries and arterioles. The aorta branches into smaller arteries, then arterioles. Arterioles act like adjustable taps that control how much blood each organ receives and set most of your blood pressure.
- Systemic capillaries. In the tissues, oxygen and nutrients leave the blood while carbon dioxide and other waste products move in.
- Venules, veins and venae cavae. Oxygen-poor blood collects in venules and veins and returns to the right atrium through the superior and inferior venae cavae, and the cycle begins again.
Key Differences Between Pulmonary and Systemic Circulation
Both loops move the same volume of blood every minute, roughly 5 liters at rest in an adult, because the right and left ventricles pump in step. Almost everything else about them is different.
| Feature | Pulmonary circulation | Systemic circulation |
|---|---|---|
| Route | Heart → lungs → heart | Heart → body → heart |
| Pumping chamber | Right ventricle | Left ventricle |
| Starts in / ends in | Right ventricle → left atrium | Left ventricle → right atrium |
| Main outflow vessel | Pulmonary trunk and arteries | Aorta |
| Blood in the arteries | Oxygen-poor | Oxygen-rich |
| Blood in the veins | Oxygen-rich | Oxygen-poor |
| Typical arterial pressure | About 25/8 mmHg (mean around 14 mmHg) | About 120/80 mmHg (mean around 90 mmHg) |
| Resistance | Low (about one-tenth of systemic) | High |
| Distance traveled | Short | Long, reaches every organ |
| Main purpose | Remove CO2, load oxygen | Deliver oxygen and nutrients, collect waste |
| Response to low oxygen | Vessels narrow | Vessels widen |
| Ventricle wall thickness | Thin (roughly 3–5 mm) | Thick (roughly 8–12 mm) |
1. Pressure and resistance
This is the difference that matters most clinically. Pulmonary vessels are short, wide, thin-walled and have relatively little smooth muscle, so the whole lung circuit offers very little resistance. According to StatPearls (NCBI), pulmonary vascular resistance is about one-tenth of systemic resistance. That is why the right ventricle can do its job with a much thinner wall than the left.
The low pressure also protects the lungs. If pressure in the lung capillaries climbed to systemic levels, fluid would be forced out of the vessels and into the air sacs, which is exactly what happens in pulmonary edema. Doctors estimate that back-pressure using the pulmonary capillary wedge pressure (PCWP).
2. Oxygen content: the artery and vein “flip”
In the systemic loop, arteries carry oxygen-rich blood and veins carry oxygen-poor blood. The pulmonary loop is the famous exception: the pulmonary arteries carry oxygen-poor blood and the pulmonary veins carry oxygen-rich blood. The rule to remember is that arteries are defined by direction (away from the heart), not by oxygen content.
3. How each loop reacts to low oxygen
When a body tissue is short of oxygen, its blood vessels widen to bring in more blood. Lung vessels do the opposite. When one area of lung is poorly ventilated, the arteries supplying it tighten and send blood toward better-aerated areas instead. This reflex, called hypoxic pulmonary vasoconstriction, is clever in a small patch of lung, but when the whole lung is short of oxygen for a long time (as in severe COPD or living at high altitude) it can raise pressure throughout the pulmonary circuit.
4. Built-in reserve
The pulmonary circuit has spare capacity. During exercise, extra cardiac output is absorbed by opening up capillaries that were barely in use and stretching the ones already open, so pressure barely rises. That reserve is large: StatPearls notes that around half of the pulmonary circulation can be blocked before a measurable rise in pulmonary pressure appears. The flip side is that by the time pulmonary pressure is high, a lot of damage has often already occurred.
How the Two Circulations Work Together
The two loops are linked in series, so the right and left ventricles must pump almost exactly the same amount of blood with every beat. If the left ventricle pumps even slightly less than the right over time, blood backs up into the lungs. If the right ventricle fails, blood backs up into the body’s veins, causing swollen ankles and a congested liver. The NIH’s National Heart, Lung, and Blood Institute has a simple animated overview of how blood flows through the heart if you’d like to see the sequence in motion.
For a wider view of how the airways, alveoli and vessels fit together, read how the pulmonary system works.
Lesser-Known Facts About the Two Circuits
- The lungs have a second blood supply. Lung tissue itself (the airways and supporting structures) is fed by the bronchial arteries, which branch from the aorta and belong to the systemic circulation. They carry only around 1% of cardiac output, but they explain why a blood clot in a pulmonary artery does not always kill the lung tissue beyond it.
- A tiny natural “shunt”. Some bronchial veins drain into the pulmonary veins, so a small amount of oxygen-poor blood joins oxygen-rich blood on its way to the left heart. This is one reason arterial oxygen saturation is normally 95–100% rather than a perfect 100%.
- Before birth, the lungs are mostly bypassed. A fetus gets oxygen from the placenta, so two shortcuts (the foramen ovale and the ductus arteriosus) route most blood around the lungs. Both normally close soon after birth, when the pulmonary circuit takes over.
- The heart feeds itself through a third route. Coronary circulation is technically a branch of the systemic circuit. We compare it directly in how pulmonary circulation differs from coronary circulation.
- A 13th-century discovery. The Arab physician Ibn al-Nafis described blood passing through the lungs about 350 years before William Harvey published his full account of circulation in 1628.
What Happens When Pulmonary or Systemic Circulation Goes Wrong?
Because the two loops behave so differently, they also fail in different ways.
Problems in the pulmonary circuit
- Pulmonary hypertension. High pressure in the lung arteries, currently defined as a mean pulmonary artery pressure above 20 mmHg at rest in the 2022 ESC/ERS guidelines. The thin-walled right ventricle is not built for that load and can eventually fail. Learn more about high pulmonary artery pressure and how serious pulmonary hypertension is.
- Pulmonary embolism. A clot, usually from a leg vein, travels through the right heart and lodges in a pulmonary artery, blocking blood flow to part of the lung. Know the warning signs of a pulmonary embolism.
- Pulmonary edema. Fluid leaks into the air sacs, most often because a weak left heart lets pressure build up in the pulmonary veins.
Problems in the systemic circuit
- High blood pressure (hypertension). Raised pressure in the body’s arteries strains the left ventricle and damages vessels in the brain, kidneys and eyes. It is a very different disease from pulmonary hypertension; see hypertension vs pulmonary hypertension.
- Atherosclerosis. Fatty plaque narrows systemic arteries and can cause heart attacks, strokes and peripheral artery disease.
- Left-sided heart failure. When the left ventricle can’t keep up, blood backs up into the lungs. This is the most common cause of pulmonary hypertension, known as Group 2 pulmonary hypertension, and a clear example of how a systemic-side problem spills into the pulmonary side.
If you have breathlessness that is getting worse, swelling in your legs, chest pain or fainting episodes, see a doctor promptly. You can find a pulmonologist near you in our directory. Sudden chest pain or severe shortness of breath is an emergency: call 911.
Key Takeaways
- Pulmonary circulation runs from the right ventricle to the lungs to the left atrium; systemic circulation runs from the left ventricle to the body to the right atrium.
- The pulmonary loop loads oxygen; the systemic loop delivers it.
- Pulmonary artery pressure is roughly one-fifth to one-sixth of systemic pressure, and resistance is about one-tenth.
- Pulmonary arteries carry oxygen-poor blood, the reverse of systemic arteries.
- Lung vessels narrow in response to low oxygen, while body vessels widen.
- Both loops pump the same volume per minute, so trouble on one side of the heart eventually affects the other.
Frequently Asked Questions
What is the main difference between pulmonary and systemic circulation?
Pulmonary circulation moves blood between the heart and lungs so it can pick up oxygen and release carbon dioxide. Systemic circulation moves oxygen-rich blood from the heart to the rest of the body and returns oxygen-poor blood to the heart.
Which side of the heart controls pulmonary and systemic circulation?
The right side of the heart (right atrium and right ventricle) drives pulmonary circulation. The left side (left atrium and left ventricle) drives systemic circulation.
Why is pressure lower in pulmonary circulation than in systemic circulation?
The lungs sit right next to the heart and their vessels are short, wide and flexible, so far less force is needed to push blood through them. Keeping pressure low also stops fluid from leaking into the air sacs.
Does the pulmonary artery carry oxygenated or deoxygenated blood?
The pulmonary artery carries deoxygenated (oxygen-poor) blood from the right ventricle to the lungs. It is the only artery in an adult that does so, along with its branches.
Which circulation is longer, pulmonary or systemic?
Systemic circulation is far longer. It reaches every organ and limb, while pulmonary circulation only travels the short distance between the heart and lungs.
Is coronary circulation part of systemic circulation?
Yes. The coronary arteries branch from the base of the aorta, so coronary circulation is considered a specialized part of the systemic circuit that supplies the heart muscle itself.
Can a problem in systemic circulation affect the lungs?
Yes. Left-sided heart failure and long-standing high blood pressure can raise pressure in the pulmonary veins, leading to fluid in the lungs and, over time, pulmonary hypertension.
References
- Jain V, Bordes SJ, Bhardwaj A. Physiology, Pulmonary Circulatory System. StatPearls Publishing, NCBI Bookshelf.
- Physiology, Pulmonary Vascular Resistance. StatPearls Publishing, NCBI Bookshelf.
- Humbert M, et al. 2022 ESC/ERS Guidelines for the diagnosis and treatment of pulmonary hypertension. European Heart Journal. 2022;43(38):3618–3731.
- National Heart, Lung, and Blood Institute. How Blood Flows through the Heart.
This article is for general education and is not a substitute for advice from your doctor.
Zachary is a medical assistant who supports the PulmonaryGuide team with research and helps keep our doctor directory accurate and up to date.
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