Pulmonary Hilar Anatomy: 3D Relationships

1. Conceptual Overview

The pulmonary hilum is not a flat doorway but a three-dimensional crossing point where the airway, pulmonary arteries, pulmonary veins, lymphatics, bronchial vessels, and autonomic nerves enter or leave the lung. A two-dimensional anterior diagram can make the relationships appear deceptively simple. In practice, the surgeon, bronchoscopist, interventionalist, and radiologist must understand the hilum as a layered spatial structure: bronchi are generally posterior, pulmonary veins are generally anterior and inferior, and pulmonary arteries occupy a side-specific relationship to the bronchi.

The supplied animation builds this anatomy in sequence: the trachea and right and left main bronchi are introduced first, followed by the pulmonary trunk, pulmonary veins, and the right and left lungs. It then links the central mediastinal structures to the right and left hilar surfaces, labeling the right and left pulmonary arteries and the four pulmonary veins. Pulmonary Hilar Anatomy from th… Pulmonary Hilar Anatomy from th… This sequence is pedagogically useful because the hilum is best understood by adding one system at a time rather than memorizing a static list.

2. The Airway as the Posterior Framework

The trachea bifurcates into the right and left main bronchi, which form the most durable posterior landmarks of the hilum. The right main bronchus is shorter, wider, and more vertical than the left. Soon after entering the lung, it gives rise to the right upper-lobe bronchus, which is eparterial—that is, it arises above the level of the right pulmonary artery. This feature is a defining right-sided asymmetry.

The left main bronchus has a longer mediastinal course and passes beneath the aortic arch and left pulmonary artery before entering the left hilum. There is no eparterial bronchus on the left. The left pulmonary artery crosses superiorly over the left main bronchus before curving posteriorly, creating the classic relationship in which the artery “arches” over the bronchus.

This bronchial framework explains why hilar anatomy differs between sides. On the right, the right upper-lobe bronchus appears high and posterior-superior, while the bronchus intermedius continues inferiorly toward the middle and lower lobes. On the left, the pulmonary artery occupies the superior hilar position, while the main bronchus remains more posterior and central.

3. Pulmonary Arteries: Parallel to Bronchi, but Not Identical

The pulmonary arteries carry deoxygenated blood from the right ventricle to the lungs. At the hilar and segmental levels, they generally travel with the bronchi, but their branching pattern is more variable than the bronchial tree. This distinction is important: bronchi provide a relatively stable map of the lung, whereas arterial branches often show clinically relevant variation.

The right pulmonary artery passes behind the ascending aorta and superior vena cava, then enters the right hilum. It lies anterior and slightly inferior to the right main bronchus, gives branches to the upper lobe, and then continues as the interlobar pulmonary artery along the bronchus intermedius. In the animation, the RPA is marked in yellow and projects to the superior portion of the right hilar surface, emphasizing its relationship to the upper airway and superior pulmonary vein. Pulmonary Hilar Anatomy from th…

The left pulmonary artery is longer outside the pericardium and passes beneath the aortic arch. It arches over and posterior to the left main bronchus and then descends toward the left lower-lobe arterial tree. This relationship is clinically relevant during left hilar dissection, ductal surgery, pulmonary artery reconstruction, lung transplantation, and interpretation of cross-sectional imaging.

A practical rule is that the pulmonary arteries follow the bronchi more closely than the pulmonary veins do. Segmental and subsegmental pulmonary arteries generally accompany the corresponding bronchi and are named according to the bronchopulmonary segments they supply [1]. However, arterial branching is especially variable in the upper lobes, and operative planning should rely on patient-specific imaging rather than a generic schema [1,2].

4. Pulmonary Veins: Anterior, Inferior, and Intersegmental

The pulmonary veins return oxygenated blood to the left atrium. In contrast to the arteries, their intrapulmonary branches do not simply parallel the bronchi. Pulmonary veins course largely in intersegmental planes and converge centrally toward the superior and inferior pulmonary venous trunks. This is why venous anatomy is critical for segmentectomy, lobectomy, atrial fibrillation ablation planning, pulmonary venous intervention, and congenital pulmonary venous anomaly repair.

The typical arrangement is four pulmonary veins: right superior, right inferior, left superior, and left inferior. The superior pulmonary veins are the most anterior hilar structures, while the inferior pulmonary veins are the most inferior and relatively posterior-medial at their atrial entry [1]. In the animation, the pulmonary veins are marked in red and are connected from the hilar surfaces to the left atrium, emphasizing that the veins exit the lungs and enter the posterior left atrium rather than accompanying the pulmonary arteries to the right ventricle.

The right superior pulmonary vein drains the right upper and middle lobes. The right inferior pulmonary vein drains the right lower lobe. The left superior pulmonary vein drains the left upper lobe and lingula. The left inferior pulmonary vein drains the left lower lobe [1]. Variants are common, particularly accessory right pulmonary veins and common left pulmonary venous trunks, and anomalous pulmonary venous return must be recognized when venous drainage does not connect normally to the left atrium [1].

5. Right Hilum: Eparterial Bronchus and Anterior Vein

The right hilum is vertically oriented and anatomically more complex because the right lung has three lobes, an eparterial bronchus, and the bronchus intermedius.

From superior to inferior, the classic right hilar sequence is:

1. Right upper-lobe bronchus
2. Right pulmonary artery
3. Bronchus intermedius
4. Pulmonary veins, especially the superior pulmonary vein anteriorly and the inferior pulmonary vein inferiorly

From anterior to posterior, the practical relationship is:

Superior pulmonary vein → pulmonary artery → bronchus

This anterior-to-posterior arrangement is essential during right hilar dissection. The superior pulmonary vein may be encountered first from an anterior approach. The pulmonary artery lies deeper and more posterior than the vein, while the bronchus forms the posterior boundary. The right pulmonary artery becomes the interlobar artery after giving upper-lobe branches and descends along the bronchus intermedius. This close relationship explains why airway and vascular injuries may occur together during difficult hilar dissection.

For congenital and pediatric cardiac surgeons, the right hilum is encountered less commonly than by thoracic surgeons, but its relationships matter during pulmonary artery reconstruction, pulmonary vein evaluation, lung transplantation, airway compression assessment, and operations involving the posterior left atrium or pulmonary venous confluence.

6. Left Hilum: Pulmonary Artery Arches Over the Bronchus

The left hilum has a different signature relationship: the left pulmonary artery lies superior to the left main bronchus. This is the classic “RALS” teaching point: Right pulmonary artery Anterior to the bronchus; Left pulmonary artery Superior to the bronchus. Although oversimplified, the concept remains useful.

The left pulmonary artery exits the pericardium beneath the aortic arch, crosses above the left main bronchus, and curves posteriorly around the left upper-lobe bronchus. The left superior pulmonary vein lies anteriorly, and the left inferior pulmonary vein lies inferiorly and more posterior-medially as it approaches the left atrium.

This anatomy is particularly relevant for procedures near the left atrial roof, pulmonary venous ostia, left pulmonary artery, and left main bronchus. Computed tomographic studies of the left atrium, pulmonary veins, bronchi, and pulmonary arteries demonstrate that the superior pulmonary veins can lie in very close contact with adjacent bronchi and pulmonary arteries, which has implications for catheter ablation and for avoiding collateral injury [3].

7. Relationship to the Left Atrium

The pulmonary veins are not simply hilar structures; they are also atrial structures. All four typical pulmonary veins enter the posterior wall of the left atrium. The right pulmonary veins enter near the right atrial–left atrial groove, and the left pulmonary veins enter more laterally beneath the left pulmonary artery and anterior to the descending thoracic aorta.

This relationship explains several clinical observations. First, pulmonary venous obstruction may occur at the venoatrial junction, within the extraparenchymal vein, or more peripherally within intrapulmonary veins. Second, posterior left atrial operations require awareness of the pulmonary venous ostia and their relation to the bronchi and pulmonary arteries. Third, anomalous venous drainage, such as partial anomalous pulmonary venous return, is best understood as a failure of one or more pulmonary venous channels to connect appropriately to the left atrium.

In the animation, the four pulmonary veins are traced from each hilar surface to the posterior left atrium, making the central point clear: the veins leave the lung anterior-inferiorly at the hilum but ultimately enter the left atrium posteriorly. This transition from hilar anatomy to atrial anatomy is where many congenital and acquired pulmonary venous pathologies become surgically relevant.

8. Cross-Sectional Imaging Implications

CT and MRI require the same three-dimensional mental model. Webb and colleagues emphasized that interpretation of hilar abnormalities on CT depends on detailed knowledge of cross-sectional hilar anatomy [4]. Modern multidetector CT extends this principle by allowing reconstruction of patient-specific bronchovascular anatomy.

A systematic approach to CT interpretation should identify:

1. The tracheal bifurcation and main bronchi
2. The right and left pulmonary arteries relative to the bronchi
3. The superior and inferior pulmonary veins relative to the left atrium
4. Segmental arterial branches, especially upper-lobe variants
5. Pulmonary venous variants, including accessory veins and common trunks
6. Hilar lymph nodes or masses that distort the expected relationships

Contrast timing matters. An early pulmonary arterial phase may show arteries well but incompletely opacify pulmonary veins. A delayed or venous phase may be necessary when pulmonary venous anatomy is the main diagnostic question, particularly in suspected anomalous pulmonary venous return or pulmonary vein stenosis.

9. Surgical and Procedural Relevance

The pulmonary hilum is a high-risk anatomical region because airway, artery, vein, lymphatic tissue, and autonomic nerves converge in a compact space. Surgical safety depends on recognizing not only the named structures but also their depth relationships.

For thoracic surgery, hilar dissection follows the principle that veins are usually anterior, arteries are intermediate, and bronchi are posterior. Segmentectomy requires understanding that pulmonary veins often define intersegmental planes, whereas arteries and bronchi define the central axis of a segment [2,5].

For congenital cardiac surgery, pulmonary hilar anatomy becomes relevant during pulmonary artery reconstruction, pulmonary venous repair, lung transplantation exposure, posterior mediastinal dissection, and operations involving anomalous pulmonary venous return. Misinterpreting the pulmonary venous course can lead to residual obstruction, missed anomalous drainage, or an inadequately oriented baffle.

For catheter procedures, the proximity of pulmonary veins to bronchi and pulmonary arteries matters during pulmonary vein isolation, pulmonary vein intervention, and pulmonary artery angioplasty. Thermal, mechanical, or stent-related injury can occur when adjacent structures are not appreciated in three dimensions [3].

10. Key Anatomical Principles

• The bronchus is the posterior anchor. In both hila, the main bronchus provides the most consistent posterior landmark.
• The pulmonary vein is anterior and inferior. The superior pulmonary vein is typically the most anterior hilar structure; the inferior pulmonary vein is the most inferior.
• The pulmonary artery is side-specific. On the right, it is anterior to the bronchus; on the left, it arches superiorly over the bronchus.
• Arteries follow bronchi more closely than veins do. Segmental arteries usually accompany bronchi, whereas veins run in intersegmental planes.
• The left atrium is part of pulmonary venous anatomy. The veins must be followed all the way to their atrial connections.
• CT phase determines what you see. Arterial-phase imaging may miss or underdefine pulmonary venous anatomy.
• Variants are common. Patient-specific imaging should guide operative or interventional planning.

References

1. Kandathil A, Chamarthy M. Pulmonary vascular anatomy & anatomical variants. Cardiovasc Diagn Ther. 2018;8(3):201-207. doi:10.21037/cdt.2018.01.04. PMID: 30057869. PubMed Central (PMC)
2. Onuki T, Kanzaki M, Kikkawa T, Isaka T, Sakamoto K, Oyama K, Murasugi M. New findings on the three-dimensional anatomical relations between the bronchi and pulmonary blood vessels at the pulmonary hilum. Clin Anat. 2015;28(4):506-511. doi:10.1002/ca.22486. PMID: 25546314. PubMed
3. Li YG, Yang M, Li Y, Wang Q, Yu L, Sun J. Spatial relationship between left atrial roof or superior pulmonary veins and bronchi or pulmonary arteries by dual-source computed tomography: implication for preventing injury of bronchi and pulmonary arteries during atrial fibrillation ablation. Europace. 2011;13(6):809-814. doi:10.1093/europace/eur034. PMID: 21345923.
4. Webb WR, Glazer GM, Gamsu G. Computed tomography of the normal pulmonary hilum. J Comput Assist Tomogr. 1981;5(4):476-484. doi:10.1097/00004728-198108000-00003. PMID: 7263985. PubMed
5. Van der Spuy JC. The surgical anatomy of the pulmonary vessels. Thorax. 1953;8(3):189-200. doi:10.1136/thx.8.3.189. PMID: 13102416.