SECTION 5.2
Pulmonary and systemic circuits
Trace blood through the pulmonary and systemic circuits, naming the vessels and the state of the blood at each step.
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The circulatory system is not one loop but two, joined at the heart and arranged in series. The pulmonary circuit carries blood from the right side of the heart to the lungs and back to the left side. The systemic circuit carries blood from the left side of the heart to the body and back to the right side. Because the two circuits are in series, the same volume of blood passes through both; the right and left ventricles therefore pump equal outputs. The circuits differ in pressure: the systemic circuit is a high-pressure system that must reach every tissue, while the pulmonary circuit is a low-pressure system because the lungs are close to the heart and their capillaries are delicate. Tracing blood through both circuits, naming each vessel and stating whether the blood is oxygenated or deoxygenated at each step, is the quickest way to master circulatory anatomy.
Key Concepts & Topics
Two circuits in series
The heart is really two pumps side by side. The right side receives blood returning from the body and sends it to the lungs; the left side receives blood returning from the lungs and sends it to the body. The two circuits are arranged in series, which means blood must pass through the pulmonary circuit before it can pass through the systemic circuit, and vice versa. This arrangement has an important consequence: the output of the right ventricle and the output of the left ventricle must be equal over time. If one side pumps more than the other, blood accumulates in the circuit between them. Understanding the series arrangement explains why failure of one side of the heart quickly affects the other and why the lungs can become congested when the left side fails.
For example: If the left ventricle weakens, blood backs up into the pulmonary circuit and the patient becomes breathless, even though the original problem is on the left side of the heart.
The pulmonary circuit
The pulmonary circuit begins in the right atrium, which receives deoxygenated blood from the systemic veins. Blood passes into the right ventricle, which pumps it into the pulmonary trunk. The trunk divides into the right and left pulmonary arteries, which carry deoxygenated blood to the two lungs. In the lungs, the arteries branch into smaller vessels and then into capillary networks around the air sacs, where carbon dioxide leaves the blood and oxygen enters it. The now oxygenated blood is collected by pulmonary veins, which return it to the left atrium. Notice that the pulmonary arteries carry deoxygenated blood and the pulmonary veins carry oxygenated blood, which is the opposite of the usual rule. The circuit is short and operates at low pressure, so its vessel walls are thinner than those of the systemic arteries.
For example: A clot that travels to the lungs lodges in a pulmonary artery, blocking the low-pressure circuit and preventing part of the blood from being oxygenated.
The systemic circuit
The systemic circuit begins in the left atrium, which receives oxygenated blood from the pulmonary veins. Blood passes into the left ventricle, whose thick muscular wall pumps it into the aorta. The aorta branches into progressively smaller arteries that supply every region of the body. These lead into arterioles and then into capillary networks, where oxygen and nutrients leave the blood and carbon dioxide and waste enter it. The now deoxygenated blood is collected by venules and veins, which merge into the superior vena cava (from the head, neck and arms) and the inferior vena cava (from the trunk and legs). These two large veins empty into the right atrium, completing the circuit. The systemic circuit is long, contains most of the blood volume and operates at much higher pressure than the pulmonary circuit.
For example: When you stand up quickly, the systemic veins must return blood from the legs against gravity; the skeletal muscle pump, valves and breathing all help.
Vessels are named by direction, not oxygen
A common source of confusion is the assumption that arteries always carry oxygenated blood and veins always carry deoxygenated blood. The reliable rule is about direction: arteries carry blood away from the heart, and veins carry it towards the heart. In the systemic circuit this happens to mean oxygenated blood in arteries and deoxygenated blood in veins, so the shortcut works. In the pulmonary circuit it fails: the pulmonary arteries carry deoxygenated blood away from the heart to the lungs, and the pulmonary veins carry oxygenated blood back to the heart. Learning the direction rule prevents errors when reading reports, interpreting images or explaining a congenital heart defect in which the normal connections are altered.
Common misconception: The pulmonary artery is the only artery that normally carries deoxygenated blood, and the pulmonary vein is the only vein that normally carries oxygenated blood.
Why the two circuits differ
The two circuits carry the same volume of blood but at very different pressures. The systemic circuit must push blood through a long network of vessels and reach the brain against gravity, so the left ventricle generates a high pressure, around 120 mmHg at peak in a healthy adult. The pulmonary circuit is short and the lungs are close to the heart, so the right ventricle generates much less pressure, around 25 mmHg. The lungs also have delicate capillaries that would be damaged by systemic pressure, and their vessels are correspondingly thin-walled. This is why the left ventricular wall is much thicker than the right, why the two ventricles have different shapes, and why diseases that raise pressure in the pulmonary circuit are serious: the right ventricle is not built to pump against high resistance.
For example: The left ventricle wall is roughly three times thicker than the right, because it must generate systemic pressure while the right serves the low-pressure lungs.
Key facts to remember
- The pulmonary and systemic circuits are arranged in series, so the right and left ventricles pump equal outputs.
- The pulmonary circuit runs right atrium → right ventricle → pulmonary trunk and arteries → lungs → pulmonary veins → left atrium.
- The systemic circuit runs left atrium → left ventricle → aorta → arteries → capillaries → veins → venae cavae → right atrium.
- Arteries and veins are named by direction: arteries carry blood away from the heart, veins towards it, regardless of oxygen content.
- The systemic circuit operates at much higher pressure than the pulmonary circuit, which is why the left ventricular wall is thicker.
Example · Follow one red blood cell. It returns from the leg in the inferior vena cava, enters the right atrium, passes into the right ventricle, and is pumped through the pulmonary trunk into a pulmonary artery. In a lung capillary it releases carbon dioxide and picks up oxygen. It returns in a pulmonary vein to the left atrium, enters the left ventricle, and is pumped into the aorta. It travels through arteries and arterioles to a capillary in the foot, releases oxygen to the tissue, and returns through venules and veins to the inferior vena cava — arriving where it began. Two circuits, one continuous journey.
Read full notes
Read one topic at a time. These notes explain the mechanisms and relationships behind the key facts.
Two circuits in series
Arranging the two circuits in series is efficient because it lets the same blood serve both gas exchange and tissue supply without mixing the two. Blood must pass through the lungs before it can deliver oxygen to the tissues, and through the tissues before it can collect more carbon dioxide for the lungs. The heart sits at the junction, with the right side serving one circuit and the left the other. The series arrangement also imposes a strict requirement: over any period, the two ventricles must eject the same volume, or blood will accumulate on one side. This is why the two ventricles normally have matched outputs and why a mismatch produces the backup that clinicians look for when they assess a patient with heart failure.
The pulmonary circuit
The pulmonary circuit is short and low-pressure, and its design reflects that. The right ventricle has a thinner wall than the left because it does not need to generate high pressure; the pulmonary arteries have thinner walls than systemic arteries for the same reason. The circuit’s purpose is gas exchange, so its capillaries lie immediately beneath the thin walls of the air sacs, giving the shortest possible diffusion path. Because the whole output of the right ventricle passes through the lungs, anything that blocks or narrows pulmonary vessels raises the pressure the right ventricle must work against. This is why pulmonary embolism and chronic lung disease can strain the right side of the heart, and why the low-pressure design, efficient in health, becomes a vulnerability in disease.
The systemic circuit
The systemic circuit is long and high-pressure, and it must serve every tissue, including the brain at the top of the body. The left ventricle therefore has a thick muscular wall, and the aorta and its branches have strong elastic and muscular walls. Because the circuit contains many vessels and much of the blood volume, it also acts as a reservoir: changes in the diameter of systemic arterioles redistribute flow between organs, and changes in venous tone shift blood between the periphery and the heart. The systemic circuit is where blood pressure is measured and regulated, and where most resistance to flow resides. The pulmonary circuit, by contrast, contains relatively little resistance and little blood volume.
Vessels are named by direction
Anatomical naming follows direction, not oxygen content, and this convention is older than the discovery of oxygen. An artery is a vessel that carries blood away from the heart; a vein carries it towards the heart. In the systemic circuit this coincides with oxygen status, which is why the shortcut usually works. In the pulmonary circuit it does not. The same logic applies to congenital heart defects, where connections may be rearranged: a vessel may carry blood of unexpected oxygen content because of where it connects, but it is still named by its direction of flow. Learning the direction rule makes such cases interpretable instead of confusing.
Why the circuits differ in pressure
Pressure in a circuit depends on the resistance to flow and on the volume the pump must move. The systemic circuit is long, branches widely and must overcome gravity to the brain, so resistance is high and the left ventricle must generate high pressure. The pulmonary circuit is short and wide, so resistance is low and the right ventricle needs much less pressure. If the pressures were equal, the delicate pulmonary capillaries would be damaged. The difference in pressure explains the difference in wall thickness, the difference in ventricular shape and the direction in which the heart fails when pressures rise. It also explains a clinical principle: protecting the lungs means keeping left-sided pressures down, and protecting the right ventricle means keeping pulmonary resistance low.
Explore the diagram
Start with the whole view, then select a numbered structure. Compare the additional illustrations to connect what a structure looks like with what it does.
One continuous blood pathway
Reduced motion · use the slider to explore each state.
Venae cavae → right atrium → tricuspid → right ventricle.
Educational sequence · slowed and simplified, not real-time physiology
The aorta and its main branches
Reduced motion · use the slider to explore each state.
Gives brachiocephalic, left common carotid and left subclavian branches.
Educational schematic · selected major pathways, not to scale. Motion shows a slowed trace or comparison, not real-time physiology or disease progression.
Returning to the right atrium
Reduced motion · use the slider to explore each state.
Formed by the two brachiocephalic veins.
Educational schematic · selected major pathways, not to scale. Motion shows a slowed trace or comparison, not real-time physiology or disease progression.
Connect the illustrations to the key ideas
PULMONARY CIRCUIT · right atrium → right ventricle → lungs → left atrium
Carries deoxygenated blood to the lungs and returns it oxygenated, at low pressure.
SYSTEMIC CIRCUIT · left atrium → left ventricle → body → right atrium
Carries oxygenated blood to the tissues and returns it deoxygenated, at high pressure.
ONE SERIES LOOP · both circuits share the same blood
Because they are in series, the two ventricles must eject equal volumes.
Core Concepts Defined
- Pulmonary circuit
- The circuit from the right side of the heart to the lungs and back to the left side.
- Systemic circuit
- The circuit from the left side of the heart to the body and back to the right side.
- Pulmonary trunk
- The large vessel leaving the right ventricle, which divides into the pulmonary arteries.
- Pulmonary artery
- An artery carrying deoxygenated blood from the heart to a lung.
- Pulmonary vein
- A vein carrying oxygenated blood from a lung to the left atrium.
- Aorta
- The largest artery, leaving the left ventricle to begin the systemic circuit.
- Superior vena cava
- The large vein returning blood from the head, neck and arms to the right atrium.
- Inferior vena cava
- The large vein returning blood from the trunk and legs to the right atrium.
- Oxygenated blood
- Blood carrying a high concentration of oxygen, mainly bound to haemoglobin.
- Deoxygenated blood
- Blood with a low oxygen concentration, returning from tissues or travelling to the lungs.
- Series circulation
- Arrangement in which blood passes through both circuits one after the other, so outputs must match.
- Pulmonary hypertension
- Raised pressure in the pulmonary circuit, which strains the right ventricle.
Go deeper · worked example
Tracing one blood cell around both circuits
- Start where the systemic blood returns. Deoxygenated blood from the body enters the right atrium through the superior and inferior venae cavae.
- Enter the pulmonary circuit. The right ventricle pumps the blood into the pulmonary trunk, which divides into the pulmonary arteries carrying it to the lungs.
- Exchange gases in the lungs. In the capillary networks around the air sacs, carbon dioxide leaves the blood and oxygen enters, so the blood becomes oxygenated.
- Enter the systemic circuit. Pulmonary veins return the oxygenated blood to the left atrium, and the left ventricle pumps it into the aorta.
- Reach the tissue and return. The aorta branches into arteries and arterioles, blood exchanges gases and nutrients in the capillaries, and venules and veins return the now deoxygenated blood to the venae cavae — completing the loop.
Explain it yourself · Explain why the blood in the pulmonary artery is deoxygenated while the blood in the pulmonary vein is oxygenated.
Reveal a model explanation
Arteries and veins are named by the direction of flow relative to the heart, not by oxygen content. The pulmonary artery carries blood away from the heart to the lungs, and that blood has just returned from the body, so it is deoxygenated. The pulmonary vein carries blood towards the heart from the lungs, and that blood has just been oxygenated in the lung capillaries. The names describe direction; the oxygen content depends on where the vessel sits in the circuit. This is why the usual shortcut, that arteries carry oxygenated blood, fails in the pulmonary circuit.
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OpenStax, Anatomy and Physiology 2e, 20.5 Circulatory Pathways; Tortora & Derrickson, Principles of Anatomy and Physiology, 12e, ch. 21. Author review only; medical and French-language expert review pending.
Original teaching schematics. English and French wording is author-reviewed for study; anatomy and French-language expert review is pending. Checks are practice only, not a graded examination.
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