Pulmonary circulation is the loop of blood flow that carries oxygen-poor blood from the right side of the heart to the lungs and returns oxygen-rich blood to the left side of the heart, where it can be pumped to the rest of the body. Unlike the high-pressure systemic circulation that feeds your organs and muscles, the pulmonary circuit operates at roughly one-fifth the pressure, and for good reason: its thin, delicate vessels sit wrapped around air sacs where gas exchange happens in fractions of a second. That low-pressure design makes the pulmonary circulation remarkably efficient under normal conditions, but also uniquely vulnerable when things go wrong.
A Low-Pressure System by Design
The pulmonary circulation is built around three connected compartments: an arterial tree that fans out from the right ventricle, an enormous capillary bed threaded through the lung tissue, and a venular tree that collects freshly oxygenated blood and funnels it back to the left atrium.1PubMed Central. The Pulmonary Vasculature The walls of these vessels are thinner and more compliant than their systemic counterparts. Mean pulmonary artery pressure in a healthy person at rest is only about 15 mmHg, compared with roughly 90 mmHg on the systemic side. That pressure difference is not a flaw. The right ventricle is a thinner, more crescent-shaped chamber than the muscular left ventricle, and it does not need to generate much force because the pulmonary vessels offer so little resistance.
The lung is also the only organ served by two separate blood supplies. The pulmonary circulation handles gas exchange, while a smaller systemic supply called the bronchial circulation delivers oxygenated blood to the walls of the airways and larger pulmonary vessels themselves.2Comprehensive Physiology. Lung Circulation Most people never hear about the bronchial circulation, but it matters clinically: in diseases that destroy pulmonary vessels, bronchial arteries can enlarge to partially compensate, and they are the source of bleeding in some cases of severe lung disease.
How the Lungs Handle a Surge in Blood Flow
When you start exercising, your heart pumps more blood per minute, and all of it has to pass through the lungs. If the pulmonary vessels were rigid pipes, pressure would spike dangerously with every increase in output. Instead, the lung vasculature has two built-in shock absorbers: recruitment and distension. Recruitment means opening capillaries that were not carrying much blood at rest. Distension means stretching already-open capillaries wider to accept more flow.3PubMed Central. Pulmonary capillary recruitment and distention in mammalian lungs: species similarities
During everyday activity, recruitment does most of the work. As exercise intensity climbs, distension kicks in to handle the extra volume. A study that measured capillary surface area in exercising people found that most subjects relied on recruitment at moderate effort, while one highly trained marathon runner, who already had high resting cardiac output, appeared to rely more on distension from the start of exercise.4PubMed. Pulmonary capillary surface area in supine exercising humans: demonstration of vascular recruitment Together, these mechanisms keep pulmonary artery pressure remarkably stable across a wide range of physical demands.5PubMed Central. The Pulmonary Vasculature
Gas Exchange at the Capillary Level
The entire point of pulmonary circulation is to bring blood close enough to inhaled air for oxygen and carbon dioxide to swap places. That exchange happens across the alveolar-capillary membrane, a tissue barrier so thin that gases cross it by simple diffusion. The lung contains hundreds of millions of alveoli, and the capillary network woven around them creates a combined surface area roughly the size of a tennis court. Blood passing through a pulmonary capillary spends less than a second in contact with the alveolar wall, yet that is enough time for nearly complete gas equilibration under normal conditions.
When disease damages either the membrane itself or reduces the volume of blood the capillary bed can hold, gas transfer suffers. In pulmonary arterial hypertension, for example, both the membrane’s ability to pass gas and the volume of blood in the capillaries decline, and those losses explain much of the breathlessness patients experience.6PubMed Central. Loss of alveolar membrane diffusing capacity and pulmonary capillary blood volume in pulmonary arterial hypertension The damage is not purely about blood flow; the membrane itself deteriorates over time in these patients, compounding the problem.
Hypoxic Pulmonary Vasoconstriction
The pulmonary circulation has a reflex that is the exact opposite of what happens in the rest of the body. In systemic tissues, when oxygen drops, blood vessels dilate to bring more blood and more oxygen to the starving area. In the lungs, low oxygen causes the nearby pulmonary arteries to constrict. This seems counterintuitive until you think about what the lungs are trying to do. If a region of lung is poorly ventilated, perhaps because of mucus plugging or a collapsed segment, there is no point sending blood there. The blood would pass through without picking up oxygen and return to the left heart still deoxygenated. Instead, the local vessels squeeze down, redirecting blood to healthier, better-ventilated regions where gas exchange can actually happen.7PubMed Central. Hypoxic Pulmonary Vasoconstriction: From Molecular Mechanisms to Medicine
The mechanism lives inside the smooth muscle cells of pulmonary arteries themselves. When oxygen drops, these cells undergo changes in calcium signaling that trigger contraction.8PubMed Central. Revisiting the mechanism of hypoxic pulmonary vasoconstriction using isolated perfused/ventilated mouse lung The response is fast enough to redirect blood within seconds of a ventilation change. That speed is what makes it so useful during moment-to-moment breathing, but it becomes a problem when the low oxygen is not localized but global, as happens at high altitude or in severe lung disease. When every part of the lung constricts at once, the result is a sustained rise in pulmonary artery pressure that can strain the right ventricle.9PubMed Central. Hypoxic Pulmonary Vasoconstriction: From Molecular Mechanisms to Medicine
Chemical Signals That Keep the Vessels Relaxed
The low resting tone of pulmonary vessels is not passive. It is actively maintained by chemical signals produced by the endothelial cells that line every vessel. The most important of these is nitric oxide, a gas molecule that the endothelium constantly releases in small amounts. Nitric oxide relaxes the smooth muscle in vessel walls, keeping them open and resistance low.10PubMed. Nitric oxide in the pulmonary vasculature Another key vasodilator is prostacyclin, which works through a different signaling pathway but has a similar relaxing effect on the vessel wall.11PubMed. Endothelial modulation of pulmonary vascular tone
On the other side of the balance, the endothelium also produces vasoconstrictors like endothelin-1 and thromboxane A2. In a healthy lung, the vasodilator signals dominate, and the net effect is a wide-open, low-resistance circuit. When the endothelium is damaged or dysfunctional, the balance tips toward constriction and vessel wall thickening. That imbalance is central to how pulmonary hypertension develops, and it is also why the three major classes of pulmonary hypertension drugs each target one of these pathways: prostacyclin analogs, drugs that boost nitric oxide signaling, and endothelin receptor blockers.12PubMed. Medical therapies for pulmonary arterial hypertension
The autonomic nervous system also plays a role, though a smaller one than in the systemic circulation. Both sympathetic and parasympathetic nerves reach the pulmonary vessels and can shift vascular tone and stiffness, but the endothelial chemical signals generally overshadow neural input under normal conditions.13PubMed Central. Autonomic control of the pulmonary circulation: Implications for pulmonary hypertension
The Dramatic Switch at Birth
Before birth, the lungs are filled with fluid, not air, and there is no reason to send much blood through them. The fetus gets its oxygen from the placenta, so the pulmonary circulation is kept in a high-resistance, low-flow state. Only about 10 to 15 percent of cardiac output passes through the fetal lungs; the rest is shunted past them through two bypasses: the foramen ovale between the atria and the ductus arteriosus connecting the pulmonary artery to the aorta.
At the moment of a baby’s first breath, this entire arrangement has to reverse. The lungs inflate, oxygen floods the alveoli, and pulmonary vascular resistance plummets. Blood flow to the lungs surges. Nitric oxide plays a starring role in this transition: animal studies show that blocking nitric oxide production at the time of delivery roughly halves the expected rise in pulmonary blood flow.14PubMed. Role of endothelium-derived relaxing factor during transition of pulmonary circulation at birth The nitric oxide pathway is considered essential for normal pulmonary vasodilation during the transition from fetal to newborn life.15PubMed. Type I nitric oxide synthase is decreased in the fetal pulmonary circulation of hypertensive lambs When this transition fails, persistent pulmonary hypertension of the newborn results, a condition treated with inhaled nitric oxide among other interventions.
Pulmonary Hypertension and Vascular Remodeling
Pulmonary hypertension is a condition in which the pressure in the pulmonary arteries rises above normal levels and stays elevated. The causes range widely: it can stem from left heart disease pushing pressure backward into the lungs, from chronic lung diseases that destroy capillary beds, from blood clots that never fully dissolve, or from a progressive thickening and stiffening of the small pulmonary arteries themselves. That last category, called pulmonary arterial hypertension, involves structural changes across all layers of the vessel wall, including proliferation of smooth muscle and endothelial cells and infiltration of inflammatory cells.16PubMed Central. Pulmonary vascular remodeling in pulmonary hypertension
The right ventricle is the part of the heart that suffers first. Built for a low-resistance circuit, it copes poorly with sustained increases in afterload. It can compensate for a while by thickening its walls and increasing its contractile force, but once the pressure exceeds what it can generate, it begins to dilate and fail. That is why pulmonary hypertension, regardless of its cause, ultimately becomes a disease of the right heart.
Pulmonary Embolism and the Right Ventricle
A pulmonary embolism occurs when a blood clot, usually originating in the deep veins of the legs, breaks free and lodges in a pulmonary artery. The clot physically blocks blood flow, raising pulmonary artery pressure and forcing the right ventricle to work harder against the obstruction.17PubMed Central. Pulmonary Embolism and Right Ventricular Dysfunction: Mechanism and Management Roughly 45 percent of people with an acute pulmonary embolism develop some degree of right ventricular failure, and a small percentage go on to develop chronic thromboembolic pulmonary hypertension, in which organized clot material permanently narrows the pulmonary arteries.18PubMed Central. Pathophysiology of right ventricular failure in acute pulmonary embolism and chronic thromboembolic pulmonary hypertension: a pictorial essay for the interventional radiologist
What makes a large pulmonary embolism so dangerous is the speed of the hemodynamic collapse. The right ventricle, unlike the left, cannot rapidly adapt to a sudden doubling or tripling of resistance. If the clot burden is large enough, the right ventricle dilates, its output drops, and blood pressure crashes.19European Journal of Heart Failure. Contemporary Management of Acute Right Ventricular Failure: A Statement from the Heart Failure Association and the Working Group on Pulmonary Circulation and Right Ventricular Function of the European Society of Cardiology Treatment aims to relieve the obstruction, through anticoagulation, clot-dissolving drugs, or catheter-based or surgical removal, before the right heart gives out.
When the Capillary Barrier Breaks Down
The alveolar-capillary membrane is not just a surface for gas exchange; it is also a barrier that keeps blood fluid inside the vessels and out of the air spaces. In acute lung injury and its more severe form, acute respiratory distress syndrome (ARDS), inflammation damages the endothelial lining of pulmonary capillaries, causing them to leak protein-rich fluid into the surrounding tissue and eventually into the alveoli themselves.20PubMed Central. Mechanisms of pulmonary endothelial barrier dysfunction in acute lung injury and acute respiratory distress syndrome Once the alveoli fill with fluid, oxygen cannot cross the membrane efficiently, and the patient becomes severely hypoxic.
Under normal circumstances, a small amount of fluid does seep out of pulmonary capillaries, but the lung’s lymphatic system drains it away continuously. The interstitial space around the capillaries can absorb a surprising amount of extra fluid, up to about 40 percent above its normal volume, before edema becomes apparent.21PubMed Central. The Acute Respiratory Distress Syndrome: Mechanisms and Perspective Therapeutic Approaches But in ARDS, the damage is too extensive and too fast for these safety mechanisms to keep up. The tight junctions between epithelial cells break down, and large proteins and even red blood cells flood into the air spaces.22PubMed Central. New insights into the mechanisms of pulmonary edema in acute lung injury
High Altitude and Pulmonary Edema
High-altitude pulmonary edema (HAPE) is one of the clearest examples of what happens when hypoxic pulmonary vasoconstriction goes too far. As you ascend above about 2,500 meters, the drop in oxygen pressure triggers vasoconstriction throughout the pulmonary vascular bed. In most people, the response is moderate and manageable. In susceptible individuals, the vasoconstriction is exaggerated, pulmonary artery pressure rises sharply, and the resulting high capillary pressure physically forces fluid and proteins across the capillary wall in a process sometimes described as “stress failure.”23PubMed Central. High altitude pulmonary edema-clinical features, pathophysiology, prevention and treatment
What makes HAPE-susceptible individuals different? The evidence points toward reduced nitric oxide availability during hypoxia. Measurements of exhaled nitric oxide and related markers in the blood consistently show lower levels in people prone to HAPE.24PubMed. Physiological aspects of high-altitude pulmonary edema Without enough nitric oxide to counterbalance the vasoconstriction, pulmonary artery pressure climbs unchecked. Drugs that lower pulmonary artery pressure, such as nifedipine and tadalafil, are effective at preventing HAPE, which confirms that the pressure rise is the central problem. Interestingly, studies using catheterization and bronchoalveolar lavage in the earliest stages of HAPE show that the edema is not caused by inflammation. It is a pure pressure-driven leak, distinguishing it from the inflammatory edema seen in ARDS.25PubMed. Physiological aspects of high-altitude pulmonary edema
How Breathing Itself Affects Pulmonary Blood Flow
The pulmonary vessels sit inside the chest, surrounded by pressure changes with every breath, and this creates a mechanical interaction between ventilation and blood flow that matters most when someone is on a ventilator. As the lungs inflate, the expanding air sacs squeeze the capillaries running through them. If lung volumes get high enough, the pressure difference between the airway and the space around the lung can actually exceed pulmonary artery pressure, collapsing the vessels and sharply increasing resistance to blood flow.26PubMed Central. Heart-lung interactions during mechanical ventilation: the basics
This is why ventilator settings matter so much for patients with right heart problems. Excessive tidal volumes or high airway pressures can overdistend the lungs, raising pulmonary vascular resistance and impeding right ventricular ejection at each breath.27PubMed Central. Cardiopulmonary interactions during mechanical ventilation in critically ill patients Clinicians managing ventilated patients with pulmonary hypertension or right ventricular failure walk a constant tightrope: delivering enough ventilation to maintain gas exchange without pushing lung pressures so high that they choke off the very blood flow the lungs need.
Why a Four-Chambered Heart Makes It Possible
The complete separation of pulmonary and systemic circulation that mammals enjoy depends on having a four-chambered heart with no mixing between oxygenated and deoxygenated blood. This was not always the case in the vertebrate lineage. Fish have a two-chambered heart that pumps blood through gills and then directly to the body in a single loop. Amphibians developed a three-chambered heart with two atria but a single ventricle, allowing some separation but also some mixing. A fully divided four-chambered heart, with separate right and left ventricles, evolved independently in crocodilians, birds, and mammals.28PubMed Central. The vertebrate heart: an evolutionary perspective
The advantage of full separation is that the pulmonary and systemic circuits can operate at different pressures. The right ventricle can stay thin-walled and generate just enough pressure for the low-resistance lung circuit, while the left ventricle can build the high pressure needed to push blood through the entire body. In animals with incomplete separation, the systemic pressure partially reaches the lungs, which limits how thin and efficient the pulmonary capillaries can be. The four-chambered design allowed the evolution of a thinner, more compliant pulmonary vasculature and, with it, more efficient gas exchange, which is one of the reasons mammals and birds can sustain the high metabolic rates needed for endothermic life.
Imaging the Pulmonary Circulation
Assessing how well blood is flowing through the lungs has always been a challenge. Traditional imaging like chest X-rays can show gross abnormalities, and CT angiography is excellent at spotting pulmonary emboli, but neither tells you much about regional perfusion, meaning which areas of the lung are receiving good blood flow and which are not. Newer techniques are starting to fill that gap. A recent study validated a low-dose, two-image dynamic CT method that measures regional lung perfusion and found it matched established multivolume CT measurements with strong agreement.29PubMed Central. Pulmonary regional blood flow: validation of low-dose two-volume dynamic CT perfusion imaging in a swine model If techniques like this become widely available, clinicians could map perfusion deficits in conditions like chronic thromboembolic disease or vascular remodeling with lower radiation doses than current methods require.
Dual-energy CT and contrast-enhanced MRI are also being explored for similar purposes. The goal across all of these approaches is the same: to move beyond simply seeing whether vessels are open or blocked and instead to quantify how well each region of lung is actually being perfused. For a system where the distribution of blood flow is influenced by gravity, posture, local hypoxic vasoconstriction, and mechanical forces from breathing, that kind of detailed mapping could transform how pulmonary vascular diseases are diagnosed and monitored.