Pulmonary Trunk: Anatomy, Branches and Function
By Dr. Zubair Khalid, DVM, MS, PhD ·
The pulmonary trunk is the large artery that arises from the right ventricle and carries deoxygenated blood to the lungs. It is guarded at its origin by the pulmonary valve, runs a short intrapericardial course, and bifurcates into the right and left pulmonary arteries.
This article covers the gross anatomy of the pulmonary trunk, its valve and sinuses, its relationships to surrounding structures, its branches, and the functional role it plays in the pulmonary circulation. Comparative notes for dogs, cats, horses, and ruminants are included, along with clinical correlations that matter in veterinary practice.
This article is educational and is not a substitute for veterinary diagnosis or treatment.
What Is the Pulmonary Trunk?
The pulmonary trunk (also called the pulmonary trunk artery or main pulmonary artery) is one of the great vessels of the heart. It begins at the base of the right ventricle, at the pulmonary orifice, and extends craniodorsally before dividing into the right and left pulmonary arteries.
The pulmonary trunk is the only artery in the body that normally carries deoxygenated blood. This is a key point that distinguishes it from every systemic artery. Systemic arteries carry oxygenated blood away from the left side of the heart. The pulmonary trunk carries oxygen-poor, carbon dioxide-rich blood away from the right side of the heart to the lungs, where gas exchange occurs.
In the dog, the pulmonary trunk is relatively short and wide. It emerges from the right ventricular outflow tract and almost immediately bifurcates. The two branches diverge at a distinct angle, and the bifurcation is visible on gross dissection and on imaging studies.
Origin and Course
The pulmonary trunk originates from the conus arteriosus (infundibulum) of the right ventricle. The conus arteriosus is the smooth, funnel-shaped outflow portion of the right ventricle that leads into the pulmonary trunk. This is distinct from the trabeculated inflow portion of the ventricle.
From its origin, the pulmonary trunk passes cranially and slightly to the left, crossing in front of the ascending aorta. Its course is entirely within the pericardial sac, making it an intrapericardial structure. This short intrapericardial course is clinically relevant because it means the pulmonary trunk is accessible during pericardial surgery and is surrounded by pericardial fluid.
The trunk is approximately 2 to 3 cm long in a medium-sized dog before it bifurcates. In cats, it is proportionally similar but smaller in absolute terms. In horses and cattle, the trunk is longer and more robust, reflecting the larger pulmonary circulation.
Relations to Surrounding Structures
The spatial relationships of the pulmonary trunk are important for surgery, imaging interpretation, and understanding how pathology spreads.
- Ascending aorta: The ascending aorta lies to the right of and slightly behind the pulmonary trunk. The pulmonary trunk crosses over the ascending aorta as it travels cranially. This relationship is reversed in transposition of the great arteries, a congenital condition.
- Left atrium: The left atrium lies posterior (dorsal and caudal) to the pulmonary trunk. The close proximity means that left atrial enlargement can compress or displace the pulmonary trunk.
- Left coronary artery: The left coronary artery arises from the left aortic sinus and passes between the pulmonary trunk and the left atrium. This is relevant during surgery involving the pulmonary trunk.
- Pericardium: The pulmonary trunk is enclosed within the pericardial sac along with the aorta and the origins of the coronary arteries.
These relationships matter in congenital heart disease. In conditions such as tetralogy of Fallot, the pulmonary trunk may be narrowed or displaced. In pulmonary artery sling, the left pulmonary artery arises abnormally from the right pulmonary artery and passes between the trachea and esophagus, compressing the airway [1].
The Pulmonary Valve: Cusps, Sinuses and Function
The pulmonary valve (pulmonary semilunar valve) sits at the junction between the right ventricle and the pulmonary trunk. It is a three-cusped valve that prevents backflow of blood from the pulmonary trunk into the right ventricle during diastole.
Each cusp is a pocket-like structure. Behind each cusp is a slight dilation called a sinus (pulmonary sinus). The sinuses are part of the pulmonary trunk wall and help with valve function by allowing the cusps to open and close smoothly.
Table: Pulmonary Valve Cusps, Sinuses and Relations
| Feature | Description | Clinical or Functional Note |
|---|---|---|
| Anterior cusp | Located on the anterior (cranial) aspect of the valve | Faces the right ventricular outflow tract |
| Right cusp | Located on the right side of the valve | Adjacent to the right coronary sinus of the aorta in some species |
| Left cusp | Located on the left side of the valve | Adjacent to the left coronary sinus of the aorta |
| Pulmonary sinuses | Slight dilations behind each cusp | Help cusps close without sticking to the vessel wall |
| Valve function | Opens during systole, closes during diastole | Prevents regurgitation into the right ventricle |
| Relations | Ascending aorta to the right, left atrium posteriorly | Guides surgical and imaging approaches |
The valve opens when right ventricular pressure exceeds pulmonary artery pressure during systole. It closes when right ventricular pressure falls below pulmonary artery pressure during diastole. This cycle repeats with every heartbeat.
In dogs and cats, the pulmonary valve is best evaluated by echocardiography. The right parasternal short-axis view at the level of the heart base shows all three cusps in a classic "Mercedes-Benz" pattern during diastole. This view is standard in veterinary cardiology for assessing valve morphology and detecting stenosis or regurgitation.
Pulmonary stenosis is one of the most common congenital heart defects in dogs, particularly in English Bulldogs, Boxers, and other breeds. When the valve is stenotic, the right ventricle must generate higher pressure to push blood through. Over time, this leads to right ventricular hypertrophy and potentially right heart failure.
Branches of the Pulmonary Trunk
The pulmonary trunk bifurcates into the right and left pulmonary arteries. This bifurcation occurs at the level of the heart base, just cranial to the left atrium.
Right Pulmonary Artery
The right pulmonary artery passes to the right, dorsal to the ascending aorta and cranial to the left atrium. It enters the right lung at the hilus, where it divides into lobar arteries that supply each lung lobe. In the dog, the right lung has four lobes (cranial, middle, caudal, and accessory), and each receives its own lobar artery.
The right pulmonary artery is longer than the left in most species because it must cross behind the aorta to reach the right lung.
Left Pulmonary Artery
The left pulmonary artery passes to the left, dorsal to the pulmonary trunk and cranial to the left atrium. It enters the left lung at the hilus and divides into lobar arteries. The left lung in the dog has three lobes (cranial, middle, and caudal).
The left pulmonary artery is shorter and more direct in its course. In pulmonary artery sling, the left pulmonary artery originates from the right pulmonary artery and courses abnormally between the trachea and esophagus [1].
Intrapulmonary Branches
Within each lung, the pulmonary arteries follow the bronchial tree. Each lobar artery divides into segmental arteries that supply bronchopulmonary segments. These segmental arteries further divide into smaller branches that eventually form the capillary network surrounding the alveoli.
The pulmonary capillary bed is where gas exchange occurs. Deoxygenated blood from the pulmonary arteries releases carbon dioxide and picks up oxygen across the thin alveolar-capillary membrane. Oxygenated blood then returns to the left atrium via the pulmonary veins.
Function of the Pulmonary Trunk
The pulmonary trunk functions as the conduit for deoxygenated blood from the right ventricle to the lungs. It is part of the pulmonary circulation, which is a low-pressure, high-flow system.
Pressure and Flow
Pulmonary artery pressure is much lower than systemic arterial pressure. In healthy dogs, systolic pulmonary artery pressure is typically around 15 to 25 mmHg, compared to systemic systolic pressures of 120 to 160 mmHg. This low pressure reflects the low resistance of the pulmonary vascular bed.
The right ventricle is a thin-walled chamber designed to pump against low resistance. When pulmonary vascular resistance increases, as in pulmonary hypertension, the right ventricle must work harder. Over time, this leads to right ventricular hypertrophy and eventually right heart failure.
Pulmonary hypertension can be caused by many conditions, including chronic airway disease, heartworm infection, left-sided heart failure, and pulmonary thromboembolism. In a case of saddle pulmonary embolism, CT angiography showed right ventricular enlargement with an RV/LV ratio of 1.55 and leftward bowing of the interventricular septum, both signs of right heart strain [2].
Role in Gas Exchange
The pulmonary trunk delivers deoxygenated blood to the lungs, where gas exchange occurs. This is the opposite of what happens in systemic circulation, where arteries deliver oxygenated blood to tissues.
The pulmonary trunk is the only artery carrying deoxygenated blood under normal conditions. This unique feature reflects the dual circulation of the body: systemic circulation (oxygenated blood out, deoxygenated blood back) and pulmonary circulation (deoxygenated blood out, oxygenated blood back).
Comparative Anatomy: Dogs, Cats, Horses and Ruminants
The basic anatomy of the pulmonary trunk is conserved across domestic mammals, but there are species differences in length, branching pattern, and associated structures.
Dogs and Cats
In dogs and cats, the pulmonary trunk is short and wide. It emerges from the right ventricle and bifurcates almost immediately into the right and left pulmonary arteries. The bifurcation is distinct and easily identified on echocardiography and angiography.
The pulmonary valve is tricuspid in both species, with three distinct cusps. The valve is best visualized in the right parasternal short-axis view.
In cats, the pulmonary trunk is proportionally similar to dogs but smaller. Pulmonary hypertension is a recognized complication of many feline diseases, including hypertrophic cardiomyopathy and chronic kidney disease.
Horses
In horses, the pulmonary trunk is longer and more robust than in dogs and cats. The bifurcation into right and left pulmonary arteries is less abrupt, and the branches are larger in diameter.
A key feature in horses is the ligamentum arteriosum, a fibrous remnant of the ductus arteriosus. The ductus arteriosus is a fetal vessel that connects the pulmonary trunk to the aorta, allowing blood to bypass the non-functional fetal lungs. After birth, it closes and becomes the ligamentum arteriosum. In horses, this remnant is well-developed and can be identified during dissection or surgery.
Ruminants
In cattle, sheep, and goats, the pulmonary trunk is also longer than in dogs and cats. The ligamentum arteriosum is present as a fibrous band connecting the pulmonary trunk to the aortic arch.
The pulmonary trunk in ruminants is clinically relevant in conditions such as bovine respiratory disease, which can lead to pulmonary hypertension and right heart failure (cor pulmonale). This is particularly important in feedlot cattle and in animals at high altitude.
Clinical Relevance, Limitations and Common Mistakes
The pulmonary trunk is involved in several clinically important conditions in veterinary medicine.
Pulmonary Stenosis
Pulmonary stenosis is a congenital narrowing of the pulmonary valve or the pulmonary trunk. It is one of the most common congenital heart defects in dogs. Clinical signs include exercise intolerance, syncope, and eventually right heart failure. Diagnosis is by echocardiography, which shows increased flow velocity across the valve and right ventricular hypertrophy.
Pulmonary Hypertension
Pulmonary hypertension is elevated blood pressure in the pulmonary arteries. It can be primary or secondary to other diseases. Common causes in veterinary medicine include heartworm disease, chronic airway disease, and left-sided heart failure. Diagnosis is by echocardiography, which estimates pulmonary artery pressure from tricuspid regurgitation velocity.
Pulmonary Thromboembolism
Pulmonary thromboembolism occurs when a blood clot lodges in the pulmonary arteries. This is a life-threatening emergency. Clinical signs include acute respiratory distress, cyanosis, and collapse. Diagnosis is by CT pulmonary angiography, which can show filling defects in the pulmonary arteries and signs of right heart strain [2].
Congenital Anomalies
Several congenital anomalies affect the pulmonary trunk and its branches. These include:
- Pulmonary atresia: Complete failure of the pulmonary valve to form, requiring surgical intervention [3].
- Pulmonary artery sling: Abnormal origin of the left pulmonary artery from the right pulmonary artery, causing tracheal compression [1].
- Unilateral absence of pulmonary artery: Rare congenital condition where one pulmonary artery is absent [4].
- Anomalous left coronary artery from the pulmonary artery (ALCAPA): Rare congenital condition where the left coronary artery arises from the pulmonary artery instead of the aorta [5].
Common Mistakes in Understanding the Pulmonary Trunk
- Confusing the pulmonary trunk with the aorta: The pulmonary trunk carries deoxygenated blood and arises from the right ventricle. The aorta carries oxygenated blood and arises from the left ventricle.
- Assuming all arteries carry oxygenated blood: The pulmonary trunk is the exception. It is an artery that carries deoxygenated blood.
- Overlooking the ligamentum arteriosum: In horses and ruminants, this remnant is present and can be confused with a pathological structure.
- Misidentifying the bifurcation on imaging: The pulmonary trunk bifurcation is a normal landmark. It should not be confused with a mass or thrombus.
Individual cases require veterinary assessment. The information here is educational and does not replace clinical judgment.
Frequently Asked Questions
What is the pulmonary trunk?
The pulmonary trunk is the great artery that arises from the right ventricle and carries deoxygenated blood to the lungs. It bifurcates into the right and left pulmonary arteries.
Why is the pulmonary trunk the only artery with deoxygenated blood?
The pulmonary trunk is the only artery carrying deoxygenated blood because it is part of the pulmonary circulation, which delivers blood to the lungs for oxygenation. All other arteries carry oxygenated blood to body tissues.
What valve guards the pulmonary trunk?
The pulmonary valve, a three-cusped semilunar valve, guards the opening of the pulmonary trunk at the right ventricle. It prevents backflow of blood into the ventricle during diastole.
What are the branches of the pulmonary trunk?
The pulmonary trunk bifurcates into the right and left pulmonary arteries. These further divide into lobar and segmental arteries within the lungs.
How is the pulmonary trunk different in dogs and cats compared to horses and ruminants?
In dogs and cats, the pulmonary trunk is short with a distinct bifurcation. In horses and ruminants, it is longer, and the ligamentum arteriosum is present as a fibrous remnant of the ductus arteriosus.
What is the clinical significance of the pulmonary trunk?
The pulmonary trunk is involved in conditions such as pulmonary stenosis, pulmonary hypertension, and pulmonary thromboembolism. It is also relevant in congenital heart disease and cardiac surgery.
Can the pulmonary trunk be imaged in veterinary patients?
Yes. Echocardiography is the primary imaging modality for the pulmonary trunk and valve. CT angiography and angiography are also used for detailed assessment.
What is the ligamentum arteriosum?
The ligamentum arteriosum is a fibrous remnant of the ductus arteriosus, a fetal vessel that connected the pulmonary trunk to the aorta. It is present in horses and ruminants and can be identified during dissection.
Related Articles
- Branches of Biology
- Equine Larynx Anatomy and Function in Respiration
- Canine Lymphatic System: Anatomy and Function
- Equine Hoof Anatomy: Structures and Function
- Fish Gills: Anatomy, Function, and Common Health Issues
- Equine Muscular System: Major Muscle Groups and Function
- Vertebral Artery: Course, Branches, and Clinical Relevance
Further Reading
Sources
- Three-dimensional tracheal morphologic changes after left pulmonary artery sling repair with and without tracheoplasty.
- Saddle pulmonary embolism with right heart strain and incidental detection of aortic-origin splenic artery and dual renal arteries on CT angiography.
- Outcomes of staged repair following systemic-to-pulmonary artery shunt in pulmonary atresia with ventricular septal defect.
- [[Congenital unilateral absence of pulmonary artery: a case report].](https://pubmed.ncbi.nlm.nih.gov/42706209/)
- Incidentally diagnosed anomalous left coronary artery from the pulmonary artery in a 76-year-old man undergoing preoperative evaluation: a case report and literature review.