# Pulmonary Circuit: Blood Flow Explained The pulmonary circuit is the loop of blood vessels that carries deoxygenated blood from the right ventricle through the pulmonary arteries to the lungs and returns oxygenated blood through the pulmonary veins to the left atrium. It is a low-pressure, low-resistance system, with a mean pulmonary artery pressure near 15 mmHg compared with roughly 90 mmHg in the systemic circuit. That pressure difference matters more than any other single fact about the pulmonary circulation. The lungs are built to accept the entire output of the right ventricle with a fraction of the force the left ventricle generates, and every structural feature of the circuit follows from that design constraint. When the pulmonary circulation fails to stay low-pressure, the right ventricle is the first structure to suffer, and the clinical picture that follows (exercise intolerance, syncope, ascites, jugular distension) is a direct consequence of right heart failure. For veterinary students, the pulmonary circuit is also the clearest place to learn that vessel names do not always predict blood oxygen content, and that the lung vasculature responds to low oxygen in the opposite way to every other vascular bed in the body. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## The Core Contrast: Pulmonary Circulation vs Systemic Circulation The two circuits are arranged in series, not in parallel. The right ventricle ejects into the pulmonary circuit, blood passes through the lungs, returns to the left atrium, enters the left ventricle, and is then ejected into the systemic circuit. Because they are in series, the two ventricles must move the same volume of blood per minute in a healthy animal, even though they generate very different pressures. Three contrasts are worth memorizing in the first week of cardiovascular physiology. First, the direction of oxygenation is reversed relative to the vessel names. Pulmonary arteries are the only arteries in the body that carry deoxygenated blood, and pulmonary veins are the only veins that carry oxygenated blood. Every other artery in the systemic circuit carries oxygenated blood away from the heart, and every other vein carries deoxygenated blood back to it. Second, the pressure and resistance profiles are completely different. The pulmonary circuit is a low-pressure, low-resistance system. The systemic circuit is a high-pressure, high-resistance system. The right ventricle wall is correspondingly thin, roughly a third the thickness of the left ventricular wall in a dog, because it only needs to generate enough pressure to push blood through a vascular bed that offers little resistance. Third, the metabolic response to low oxygen is inverted. In systemic tissues, hypoxia causes local vasodilation so that more blood reaches oxygen-starved tissue. In the lung, hypoxia causes local vasoconstriction, a response called hypoxic pulmonary vasoconstriction. This diverts blood away from poorly ventilated alveoli toward better-ventilated regions, which improves the match between ventilation and perfusion. ### Summary Comparison Table | Feature | Pulmonary circuit | Systemic circuit | |--|--|--| | Blood oxygen content entering | Deoxygenated | Oxygenated | | Blood oxygen content leaving | Oxygenated | Deoxygenated | | Pumping chamber | Right ventricle | Left ventricle | | Mean arterial pressure | About 15 mmHg | About 90 mmHg | | Vascular resistance | Low | High | | Ventricular wall thickness | Thin | Thick | | Artery oxygen content | Deoxygenated (unique) | Oxygenated | | Vein oxygen content | Oxygenated (unique) | Deoxygenated | | Response to local hypoxia | Vasoconstriction | Vasodilation | | Main function | Gas exchange | Tissue delivery | ## Anatomy of the Pulmonary Circuit ### Right Ventricle and Pulmonary Valve The right ventricle receives deoxygenated blood from the right atrium through the tricuspid valve. It ejects through the pulmonary valve into the pulmonary trunk, which is a short, wide vessel that splits into the left and right pulmonary arteries. The pulmonary valve is a three-cusped semilunar valve, and it closes at the start of ventricular diastole to prevent backflow into the right ventricle. ### Pulmonary Arteries The pulmonary trunk divides into the left and right main pulmonary arteries, which enter the hila of the left and right lungs. These arteries branch repeatedly alongside the bronchial tree, becoming lobar, segmental, and then progressively smaller muscular arteries and arterioles. In dogs and cats, the pulmonary arterial tree follows the airways closely, so a radiograph that shows enlarged pulmonary arteries usually also shows the accompanying bronchi. The pulmonary arteries are structurally thinner-walled than systemic arteries of comparable size. They contain less smooth muscle and elastin in the media, which is one reason they can distend to accommodate the entire stroke volume of the right ventricle without a large rise in pressure. ### Pulmonary Capillaries The pulmonary capillaries form a dense network in the alveolar walls. Their endothelium is extremely thin, often less than 0.5 micrometers, and lies directly against the alveolar epithelium, separated only by a shared basement membrane. This arrangement, called the blood-gas barrier, is the site of oxygen and carbon dioxide diffusion. The capillary network is so extensive that at any moment it holds a large volume of blood, and it can recruit additional capillaries when pulmonary blood flow increases, for example during exercise. Recruitment of previously closed capillaries is a major reason pulmonary arterial pressure rises only modestly when cardiac output triples. ### Pulmonary Veins The pulmonary veins collect oxygenated blood from the capillary beds and drain into progressively larger veins that converge at the hilus of each lung. In most domestic mammals, there are several pulmonary veins entering the left atrium rather than a single vein per lung. The pulmonary vein function is to return oxygenated blood to the left atrium at low pressure, and these veins have thin walls with little smooth muscle compared with systemic veins. Because they carry oxygenated blood, pulmonary veins are the only veins in the body with arterial oxygen content. ### Left Atrium The left atrium receives the pulmonary venous return and passes it through the mitral valve into the left ventricle. Left atrial pressure is an important clinical variable because it reflects the downstream pressure against which pulmonary venous blood must drain. When left atrial pressure rises, as in left-sided congestive heart failure, pulmonary capillary pressure rises with it, and fluid can leak into the interstitium and alveoli, producing pulmonary edema. ## Step by Step: How Blood Moves Through the Pulmonary Circuit The flow path can be followed as a sequence. 1. Deoxygenated blood enters the right atrium from the cranial vena cava, caudal vena cava, and, in some species, the azygos vein. 2. The right atrium contracts and passes blood through the tricuspid valve into the right ventricle during diastole. 3. The right ventricle contracts, the tricuspid valve closes, and the pulmonary valve opens. 4. Blood is ejected into the pulmonary trunk and distributed to the left and right main pulmonary arteries. 5. Arterial branches carry blood alongside the airways to the alveolar capillary beds. 6. Gas exchange occurs across the blood-gas barrier. Carbon dioxide diffuses out, oxygen diffuses in. 7. Oxygenated blood collects in pulmonary venules and veins. 8. Pulmonary veins deliver blood to the left atrium. 9. The left atrium passes blood through the mitral valve into the left ventricle. 10. The left ventricle ejects into the systemic circuit. This sequence is the same in every mammal. The differences between species lie mainly in the branching pattern of the pulmonary arteries, the number of pulmonary veins, and the presence or absence of a cardiac shunt. ```mermaid flowchart TD A[Right atrium] --> B[Tricuspid valve] B --> C[Right ventricle] C --> D[Pulmonary valve] D --> E[Pulmonary trunk] E --> F[Left and right pulmonary arteries] F --> G[Pulmonary capillaries] G --> H[Gas exchange] H --> I[Pulmonary veins] I --> J[Left atrium] J --> K[Mitral valve] K --> L[Left ventricle] ``` ## Pressure and Resistance in the Pulmonary Circuit Mean pulmonary artery pressure in healthy mammals is about 15 mmHg. Mean systemic arterial pressure is about 90 mmHg. The right ventricle therefore generates roughly one sixth the pressure of the left ventricle to move the same volume of blood. The reason is resistance. Pulmonary vascular resistance is low because the pulmonary arterioles are wide, numerous, and highly distensible, and because the capillary bed is enormous. The pulmonary circulation also operates at a lower functional residual volume than the systemic circulation, which means the vessels are partially collapsed at rest and can expand when flow increases. Two consequences follow from this design. First, the right ventricle is poorly adapted to sudden increases in afterload. If pulmonary vascular resistance rises acutely, for example from a large pulmonary thromboembolus, the right ventricle can fail quickly. A massive pulmonary embolus in a dog can produce acute right heart failure, syncope, or sudden death. Second, the pulmonary circulation is sensitive to downstream pressure. If left atrial pressure rises, pulmonary venous and capillary pressures rise with it. When pulmonary capillary hydrostatic pressure exceeds the oncotic pressure of plasma, fluid moves into the interstitium and then into the alveoli. This is the mechanism of cardiogenic pulmonary edema in left-sided heart failure. Pulmonary vascular resistance is not fixed. It is modulated by several factors, including alveolar oxygen tension, alveolar carbon dioxide tension, blood pH, and circulating vasoactive mediators. Pulmonary vasodilators reduce pulmonary vascular resistance, and their effects have been studied in both experimental and clinical settings. In a swine model of cardiac arrest, nebulized nitroglycerin was tested as a way to reduce pulmonary vascular resistance during cardiopulmonary resuscitation, on the reasoning that lower pulmonary resistance would improve forward flow during chest compressions [1]. In a computational model of Norwood physiology, where the systemic and pulmonary circulations are supplied in parallel by a single right ventricle, pulmonary vasodilators shifted flow toward the pulmonary circulation and could reduce systemic oxygen delivery, illustrating that lowering pulmonary resistance is not always beneficial when the two circuits are in parallel rather than in series [2]. ## Hypoxic Pulmonary Vasoconstriction Hypoxic pulmonary vasoconstriction is the constriction of pulmonary arterioles in response to low alveolar oxygen tension. It is the opposite of the systemic response to hypoxia, where low oxygen causes vasodilation. The purpose of hypoxic pulmonary vasoconstriction is to match perfusion to ventilation. When a region of lung is poorly ventilated, the air reaching those alveoli has low oxygen content. If blood continued to flow through those capillaries at normal rates, it would leave the lung without picking up oxygen, effectively becoming a right-to-left shunt. By constricting the arterioles supplying poorly ventilated alveoli, the lung diverts blood toward better-ventilated regions. This improves the overall efficiency of gas exchange. The mechanism is intrinsic to the pulmonary vascular smooth muscle. Hypoxia inhibits voltage-gated potassium channels in the smooth muscle cell membrane, which causes membrane depolarization, calcium entry through L-type calcium channels, and smooth muscle contraction. This is a direct effect of oxygen tension on the vessel, not a reflex mediated by the central nervous system, although neural and humoral factors can modulate the response. Hypoxic pulmonary vasoconstriction is beneficial when it is regional and temporary. It becomes harmful when it is global and sustained. In animals living at high altitude, chronic alveolar hypoxia causes widespread pulmonary vasoconstriction, which raises pulmonary vascular resistance and pulmonary artery pressure. Over time, this leads to pulmonary vascular remodeling, right ventricular hypertrophy, and eventually right heart failure, a condition known as high-altitude disease or brisket disease in cattle. The same process occurs in diseases that cause chronic alveolar hypoxia at sea level, including chronic airway disease, pulmonary fibrosis, and conditions that produce chronic hypoventilation. The pulmonary arteries undergo structural changes, including smooth muscle hypertrophy, intimal proliferation, and deposition of extracellular matrix proteins. In a rat model of monocrotaline-induced pulmonary hypertension, re-expression of extra domain A-containing fibronectin increased progressively in lung tissue from day 14 onward, accompanied by rising right ventricular systolic pressure, right ventricular dilation, and reduced tricuspid annular plane systolic excursion [3]. These changes show that sustained pulmonary hypertension is not simply a functional vasoconstriction but a structural remodeling process that progresses over weeks. Pulmonary arterial hypertension can also complicate congenital heart disease in which pulmonary blood flow is chronically increased, such as large left-to-right shunts. A second-hit framework has been proposed in which baseline pulmonary vascular susceptibility interacts over time with sustained hemodynamic or hypoxemic exposure and with superimposed biological stressors [4]. In this framework, the initial insult is the abnormal hemodynamic load, and subsequent genetic, inflammatory, or metabolic factors determine whether the vascular bed remodels. ## How the Pulmonary Circuit Is Assessed in Practice Veterinarians assess the pulmonary circulation through a combination of physical examination, imaging, and, when indicated, direct pressure measurement. Auscultation of the thorax can reveal a split second heart sound, a loud pulmonic component, or a murmur consistent with pulmonary valve stenosis or pulmonary hypertension. Jugular venous distension and ascites suggest elevated right atrial pressure and right heart failure. Thoracic radiography is the most common first-line imaging test. The pulmonary arteries are visible as branching soft tissue opacities alongside the bronchi. Enlarged pulmonary arteries suggest pulmonary hypertension or increased pulmonary blood flow. Pulmonary venous congestion suggests elevated left atrial pressure. In human congenital heart disease, structured radiographic criteria and deep learning models have been developed to assess pulmonary vascularity and estimate the ratio of pulmonary to systemic blood flow, with a deep learning model showing strong agreement with Fick-derived classification [5]. Veterinary radiology relies on similar principles, though species-specific reference ranges apply. Echocardiography allows direct measurement of pulmonary artery flow velocity, estimation of pulmonary artery pressure from tricuspid regurgitation jet velocity, and assessment of right ventricular size and function. In animals with suspected pulmonary hypertension, echocardiography is the primary noninvasive tool. Right heart catheterization provides direct measurement of pulmonary artery pressure and pulmonary vascular resistance. It is used mainly in referral settings and in research. In a study of Fontan circulation patients, higher pulmonary vascular resistance index was associated with worse clinical status and greater systemic congestion [6]. This illustrates the principle that pulmonary vascular resistance is a key determinant of outcome whenever the pulmonary circulation is abnormal. ## Comparative Notes: Birds and Reptiles ### Birds Birds have a four-chambered heart and a complete separation of the pulmonary and systemic circuits, similar to mammals. The right ventricle ejects into the pulmonary arteries, which carry deoxygenated blood to the lungs, and pulmonary veins return oxygenated blood to the left atrium. The avian pulmonary circuit is also a low-pressure system. Two features differ from mammals. First, the avian lung is rigid and does not expand and contract like a mammalian lung. Air flows through the lung in a unidirectional pattern during both inspiration and expiration, driven by air sacs. Second, the pulmonary capillaries in birds are arranged around the parabronchi rather than around alveoli, but the principle of gas exchange across a thin blood-gas barrier is the same. The avian pulmonary artery is relatively muscular compared with the mammalian pulmonary artery, and birds are susceptible to pulmonary hypertension, particularly in species that are bred for rapid growth, such as broiler chickens. The mechanism involves increased pulmonary blood flow and hypoxic pulmonary vasoconstriction, similar to mammals. ### Reptiles Reptiles have a three-chambered heart in most species, with two atria and a single ventricle, although the ventricle is partially divided by a muscular ridge. The pulmonary circuit is not completely separated from the systemic circuit. This allows reptiles to shunt blood between the pulmonary and systemic circuits, which is important for thermoregulation and for diving. In squamate reptiles, the vagus nerve exerts control over the large pulmonary arteries. In a study of rattlesnakes, vagal influence over the pulmonary circulation modulated hemodynamic adjustments to temperature change. Snakes with an intact vagal supply to the pulmonary arteries were able to modulate pulmonary compliance, while left-vagotomized snakes were not, although they compensated for the cardiac shunt by other means to reach the same oxygen uptake [7]. This shows that the pulmonary circuit in reptiles is not a passive conduit but an actively regulated vascular bed. Reptiles also exhibit hypoxic pulmonary vasoconstriction, and the ability to shunt blood away from the lungs is important during periods of apnea, such as when a turtle is submerged. During a right-to-left shunt, deoxygenated blood bypasses the lungs and enters the systemic circulation, which conserves oxygen during diving but reduces arterial oxygen content. ## Clinical Relevance, Limitations and Common Mistakes The pulmonary circuit is clinically relevant in several common conditions. Pulmonary hypertension is defined by elevated pulmonary artery pressure and can be primary or secondary. In dogs, it is most often secondary to left-sided heart disease, chronic airway disease, or pulmonary thromboembolism. In cats, it is most commonly associated with left-sided heart failure or pulmonary disease. Clinical signs include exercise intolerance, syncope, cyanosis, and right-sided congestive heart failure. Pulmonary thromboembolism occurs when a thrombus lodges in the pulmonary arterial tree. It can cause acute right heart failure, hypoxemia, and sudden death. Risk factors in veterinary patients include protein-losing nephropathy, hyperadrenocorticism, immune-mediated hemolytic anemia, and cardiac disease. Pulmonary edema occurs when pulmonary capillary hydrostatic pressure exceeds the reabsorptive capacity of the lung. The most common cause in [veterinary medicine](/blog/careers/veterinary-medicine-careers-from-clinical-practice-to-public-health) is left-sided congestive heart failure. It is treated with diuretics, oxygen, and medications that reduce preload and afterload. Patent ductus arteriosus is a congenital condition in which the ductus arteriosus fails to close after birth. It creates a left-to-right shunt, increasing pulmonary blood flow and eventually causing pulmonary hypertension and right heart failure. In some cases, the shunt reverses and becomes right-to-left, producing differential cyanosis. Common mistakes students make when learning the pulmonary circuit include the following. Assuming that all arteries carry oxygenated blood. Pulmonary arteries carry deoxygenated blood. This is the single most common error in first-year cardiovascular physiology. Assuming that the pulmonary circuit is a high-pressure system like the systemic circuit. It is not. The right ventricle is thin-walled for a reason. Confusing hypoxic pulmonary vasoconstriction with systemic hypoxic vasodilation. The two responses are opposite, and the reason is functional: the lung wants to divert blood away from poorly ventilated regions, while systemic tissues want more blood when they are oxygen-starved. Assuming that the pulmonary and systemic circuits are in parallel. They are in series in mammals and birds. In reptiles and in some congenital heart diseases, they can be in parallel, which changes the hemodynamics substantially. Assuming that pulmonary vascular resistance is fixed. It is dynamic and responds to oxygen, carbon dioxide, pH, and vasoactive mediators. Limitations of this article: individual animals vary in pulmonary vascular anatomy and in their response to disease. A veterinarian should evaluate any animal with suspected pulmonary circulatory disease. ## Quick Review 1. The pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium. 2. Pulmonary arteries are the only arteries carrying deoxygenated blood. Pulmonary veins are the only veins carrying oxygenated blood. 3. Mean pulmonary artery pressure is about 15 mmHg. Mean systemic arterial pressure is about 90 mmHg. 4. The pulmonary circuit is a low-pressure, low-resistance system, and the right ventricular wall is thin. 5. Hypoxic pulmonary vasoconstriction diverts blood away from poorly ventilated alveoli and is the opposite of systemic hypoxic vasodilation. 6. The pulmonary and systemic circuits are in series in mammals and birds, but can be in parallel in reptiles and in some congenital heart diseases. 7. Pulmonary vascular resistance is dynamic and is a key determinant of outcome in pulmonary hypertension and in single-ventricle physiology. ## Frequently Asked Questions ### What is the pulmonary circuit? The pulmonary circuit is the loop of blood vessels that carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium. It is a low-pressure, low-resistance system. ### Why do pulmonary arteries carry deoxygenated blood? Pulmonary arteries carry deoxygenated blood because they transport blood from the right ventricle, which receives venous return from the body, to the lungs for gas exchange. They are the only arteries in the body with deoxygenated blood. ### What is the main difference between pulmonary and systemic circulation? The main difference is pressure and resistance. The pulmonary circuit is a low-pressure, low-resistance system with a mean arterial pressure near 15 mmHg, while the systemic circuit is a high-pressure, high-resistance system with a mean arterial pressure near 90 mmHg. ### What is hypoxic pulmonary vasoconstriction? Hypoxic pulmonary vasoconstriction is the constriction of pulmonary arterioles in response to low alveolar oxygen. It diverts blood away from poorly ventilated alveoli toward better-ventilated regions, improving the match between ventilation and perfusion. ### Do birds have a pulmonary circuit like mammals? Yes. Birds have a four-chambered heart with complete separation of the pulmonary and systemic circuits, and the pulmonary circuit is a low-pressure system. The avian lung is rigid and uses a unidirectional airflow pattern, but the principle of gas exchange is the same. ### Can reptiles shunt blood away from the lungs? Yes. Most reptiles have a three-chambered heart that allows blood to be shunted between the pulmonary and systemic circuits. This is important for thermoregulation and for diving, when blood can bypass the lungs to conserve oxygen. ## Related Articles - [Circulation Research](/blog/guides/circulation-research) - [Pond Water Circulation and Mixing Strategies](/knowledge/animal-farming/aquaculture/pond-water-circulation-mixing-strategies) - [Anesthetic Circuit Disconnection and Leak Detection](/knowledge/veterinary-medicine/anesthesia-analgesia/anesthetic-circuit-disconnection-and-leak-detection) - [Genetic Circuits: Design, Mechanisms, and Applications in Synthetic Biology](/knowledge/molecular-biology/genetic-circuit) - [Feline Systemic Hypertension: Screening and Management](/knowledge/veterinary-medicine/clinical-internal-medicine/feline-systemic-hypertension-screening-management) - [Flow Cytometric Detection of Bovine Leukemia Virus (BLV) in Peripheral Blood Leukocytes: A Diagnostic Approach for Enzootic Bovine Leukosis](/knowledge/diagnostics/serology-immunology/flow-cytometric-detection-bovine-leukemia-virus-peripheral-blood-leukocytes) - [Portal Vein (Vena Portae): Anatomy and Circulation](/blog/guides/portal-vein-vena-portae-anatomy-and-circulation) - [Mesenteric Arteries: Anatomy and Gut Blood Supply](/blog/guides/mesenteric-arteries-anatomy-and-gut-blood-supply) ## Sources 1. [Nebulized Nitroglycerin Improves Carotid Blood Flow During Cardiopulmonary Resuscitation in a Swine Model of Cardiac Arrest.](https://pubmed.ncbi.nlm.nih.gov/40699941/) 2. [Computational Modeling of Oxygen Delivery in Norwood Physiology: Differential Effects of Systemic and Pulmonary Vasodilator Conditions.](https://pubmed.ncbi.nlm.nih.gov/42645818/) 3. [Longitudinal progression of tissue remodeling in experimental pulmonary hypertension is accompanied by a continuously increasing re-expression of extra domain A-containing fibronectin.](https://pubmed.ncbi.nlm.nih.gov/42570086/) 4. [A second-hit conceptual framework for pulmonary arterial hypertension in adult congenital heart disease: genetics, hemodynamics, and treat-and-repair.](https://pubmed.ncbi.nlm.nih.gov/42614538/) 5. [Radiological assessment of pulmonary vascularity in congenital heart disease: standardized clinician assessment versus deep learning-based prediction.](https://pubmed.ncbi.nlm.nih.gov/42625206/) 6. [Ejection Fraction-Related Differences in Left Ventricular and Atrial Strain Indices Among Pediatric Fontan Circulation with Systemic Left Ventricle Morphology.](https://pubmed.ncbi.nlm.nih.gov/41515665/) 7. [Vagal control of pulmonary peripheral circulation in the rattlesnake modulates relevant hemodynamic adjustments to temperature change.](https://pubmed.ncbi.nlm.nih.gov/41571081/)