Pulmonary Hilar Anatomy #1: Airways, Arteries, and Veins

The pulmonary hilum is a compact three-dimensional interface in which the main bronchi, pulmonary arteries, pulmonary veins, bronchial vessels, lymphatics, and nerves enter or leave the lung. For congenital and cardiothoracic surgeons, its anatomy is most useful when understood not as isolated structures but as a set of reproducible spatial relationships. A practical reconstruction begins with the tracheobronchial tree, adds the pulmonary arterial tree, and finally superimposes the pulmonary veins. This layered approach corresponds to the supplied anatomical illustrations, which sequentially depict the bronchi and segmental branches, pulmonary arteries, and four major pulmonary veins.

The central principle is asymmetry: the right and left pulmonary hila are not mirror images. The right pulmonary artery is predominantly anterior to the right-sided airway at the hilum, whereas the left pulmonary artery crosses superior to the left main bronchus before passing posteriorly around the upper-lobe bronchus. The pulmonary veins occupy a more anterior and inferior plane, with the superior pulmonary vein generally representing the most anterior major vascular structure and the inferior pulmonary vein lying inferiorly and relatively posterior-medially. These relationships provide a useful framework for interpreting cross-sectional imaging and for surgical dissection around the pulmonary arteries, veins, and bronchi.[1–3]

1. Tracheobronchial Framework

The trachea bifurcates at the carina into the right and left main bronchi. From this point, the bronchial tree establishes the basic segmental framework of each lung. The modern concept of bronchopulmonary segmentation developed from detailed anatomical studies demonstrating that a segmental bronchus and accompanying pulmonary arterial branch occupy the central portion of a bronchopulmonary segment, whereas pulmonary veins tend to course in intersegmental planes.[1,2]

On the right, the main bronchus gives rise first to the right upper-lobe bronchus. Because this bronchus arises above the level at which the right pulmonary artery crosses the bronchial tree, it is termed the eparterial bronchus. The continuation distal to the upper-lobe bronchus is the bronchus intermedius, which subsequently divides into the middle- and lower-lobe bronchi.

The conventional right-sided segmental bronchial nomenclature is:

• upper lobe: B1 apical, B2 posterior, B3 anterior;
• middle lobe: B4 lateral and B5 medial;
• lower lobe: B6 superior, followed by basal bronchi B7 medial, B8 anterior, B9 lateral, and B10 posterior.

This pattern provides a useful standard but is not invariant. Three-dimensional CT studies demonstrate substantial variation in branching configuration, particularly in the right upper and lower lobes. For example, upper-lobe bronchi may arise independently or in combined trunks such as B1+2 or B2+3.[4] Therefore, segment numbers should be regarded as a nomenclatural framework rather than an assumption that every patient has the same branching geometry.

The left bronchial tree differs substantially. The left main bronchus courses beneath the aortic arch and is generally longer than the right main bronchus before dividing into upper- and lower-lobe bronchi. The left upper lobe commonly has a combined apicoposterior segment, designated B1+2, followed by B3 anterior. The lingula is supplied by B4 superior lingular and B5 inferior lingular bronchi. The lower lobe contains B6 and basal branches B8, B9, and B10; a separate B7 is frequently absent or incorporated into an anteromedial basal configuration. Variation in these patterns is common and is particularly relevant when correlating bronchoscopy, CT, and operative anatomy.[1,4]

2. Pulmonary Arterial Relationships at the Hilum

The main pulmonary artery arises from the right ventricular outflow tract and bifurcates into the right and left pulmonary arteries. From the surgical perspective, the relationship between each pulmonary artery and its corresponding main bronchus is one of the most useful hilar orientation landmarks.

A commonly used mnemonic is RALS: Right Anterior, Left Superior. It refers specifically to the relationship between the pulmonary artery and the main bronchus.

Right pulmonary artery

After arising from the main pulmonary artery, the right pulmonary artery (RPA) courses horizontally toward the right hilum. In the mediastinum it passes posterior to the ascending aorta and superior vena cava and anterior to the airway. At the hilum, the RPA therefore lies predominantly anterior to the right main bronchus/bronchus intermedius.[3]

The early origin of the right upper-lobe bronchus produces the characteristic eparterial relationship: the upper-lobe bronchus is superior to the pulmonary artery, whereas the bronchus intermedius lies posterior to it. Thus, a craniocaudal description of the right hilum differs from the simple anterior-posterior relationship. Classically, the right hilar structures are organized from superior to inferior as the eparterial bronchus, pulmonary arterial structures, hyparterial bronchus, and inferior pulmonary vein.

Distally, the RPA gives variable upper-lobe branches before continuing as the interlobar pulmonary artery. The interlobar artery descends anterior and then lateral to the bronchus intermedius as it supplies the middle and lower lobes. Pulmonary arterial branching is more variable than the bronchial tree; even when a segmental artery accompanies a corresponding bronchus distally, its proximal origin may differ substantially among individuals.[2,5]

Left pulmonary artery

The left pulmonary artery (LPA) has a distinctly different relationship. It passes toward the left hilum and courses superior to the left main bronchus. It then arches over the bronchus and continues posteriorly around the left upper-lobe bronchial complex before descending as the interlobar pulmonary artery.

This superior arterial relationship explains the absence of a true eparterial bronchus on the left. The LPA itself occupies the upper portion of the left hilar complex, whereas the main bronchus lies inferior and posterior to it. Classical hilar descriptions therefore place the LPA superior to the left main bronchus, contrasting directly with the predominantly anterior relationship of the RPA to the right airway.[3]

These relationships are especially important when interpreting CT or approaching the pulmonary arteries surgically. A vessel encountered anterior to the right-sided airway is likely to be pulmonary arterial or venous depending on its precise level, whereas on the left the proximal pulmonary artery should be anticipated above the bronchus.

3. Pulmonary Veins

The usual pulmonary venous anatomy consists of four major veins entering the posterior left atrium: right superior, right inferior, left superior, and left inferior pulmonary veins.[6–8]

The pulmonary veins differ fundamentally from the pulmonary arteries in their intrapulmonary organization. Segmental pulmonary arteries accompany bronchi centrally within bronchopulmonary segments, whereas pulmonary veins tend to run along intersegmental planes, collecting blood from adjacent segments before converging toward the hilum.[2,5] Consequently, veins frequently define the boundaries between segments rather than reproducing the bronchial branching pattern.

Right pulmonary veins

The right superior pulmonary vein (RSPV) usually drains the right upper and middle lobes. At the hilum it occupies an anterior position relative to the pulmonary artery and airway. The right inferior pulmonary vein (RIPV) drains predominantly the lower lobe and approaches the left atrium at a lower and more posterior-medial level.

The right-sided venous anatomy demonstrates considerable variability. Separate middle-lobe venous drainage, accessory right pulmonary veins, or alternative connections of middle-lobe veins to superior or inferior pulmonary venous systems are well described.[6,7] In a CT series of 176 patients, 82% had two right pulmonary venous ostia, whereas approximately 17% had three, illustrating that additional right-sided venous ostia are not unusual.[6]

Left pulmonary veins

The left superior pulmonary vein (LSPV) drains the upper lobe, including the lingula, and passes anteriorly toward the left atrium. The left inferior pulmonary vein (LIPV) drains the lower lobe and enters the atrium inferiorly.

A common left-sided variant is fusion of the superior and inferior veins into a common venous trunk or common ostium. CT series consistently demonstrate clinically important variation in pulmonary venous number and drainage configuration.[6–8] Accordingly, the visual model of four discrete pulmonary veins is the appropriate baseline anatomy but should not be assumed without imaging in patients undergoing congenital cardiac reconstruction or procedures near the posterior left atrium.

4. Three-Dimensional Hilar Relationships

The most clinically useful way to integrate these structures is to consider both the anterior-posterior and superior-inferior axes.

From anterior to posterior, a simplified hilar relationship on both sides is:

pulmonary vein → pulmonary artery → bronchus.

This principle is particularly useful at the central hilum, where the superior pulmonary vein is generally the most anterior major structure. However, three-dimensional relationships change as vessels divide, and individual segmental branches may cross one another; therefore, this sequence should not be applied rigidly to distal hilar or intrapulmonary anatomy.[3,5]

The superior-inferior relationship is more asymmetric. On the right, the upper-lobe bronchus is eparterial and lies above the RPA. On the left, the LPA itself lies above the main bronchus. The inferior pulmonary veins occupy the lowest portion of both hilar complexes.

Three-dimensional CT studies further demonstrate that bronchi and corresponding pulmonary arteries do not necessarily remain immediately adjacent at the central hilum. Onuki and colleagues reconstructed bronchial and vascular anatomy in 158 patients and showed separation between segmental bronchi and arterial branches, particularly in the upper lobes, while pulmonary veins generally followed different intersegmental trajectories.[5] This reinforces an important surgical principle: hilar anatomy is better understood as intersecting three-dimensional trees than as a fixed linear sequence.

5. Relationship to the Esophagus and Mediastinum

The esophagus lies posterior to the trachea and continues posterior to the left atrium. At the hilar level it therefore occupies a posterior mediastinal plane relative to the major pulmonary venous and arterial structures. This relationship becomes particularly relevant during posterior mediastinal dissection, pulmonary venous mobilization, repair of anomalous pulmonary venous connections, and operations involving the posterior left atrial wall.

The left main bronchus passes inferior to the aortic arch, while the LPA occupies the interval around its superior aspect. On the right, the RPA traverses behind the ascending aorta and superior vena cava before reaching the lung. Thus, central pulmonary arterial mobilization requires awareness not only of the bronchi but also of the surrounding systemic great vessels.

6. Surgical and Imaging Relevance

For congenital heart surgery, pulmonary hilar anatomy becomes especially important during branch pulmonary artery reconstruction, pulmonary artery sling repair, pulmonary venous surgery, lung or hilar mobilization, complex redo operations, and procedures involving airway compression.

Cross-sectional imaging should be interpreted by tracing each structure continuously rather than identifying isolated axial profiles. Modern CT permits simultaneous three-dimensional reconstruction of the airway, pulmonary arteries, and pulmonary veins, which is particularly valuable when congenital anomalies distort normal relationships.[5,8]

The same principle applies intraoperatively. Identification of an apparent vessel at the hilum should incorporate its direction of travel, relationship to the bronchus, and destination. Pulmonary arteries accompany the bronchial tree toward the center of bronchopulmonary segments, whereas pulmonary veins more commonly occupy intersegmental planes. Unexpected venous branches should not be divided solely because they appear anatomically minor, because venous drainage patterns may cross conventional lobar boundaries.[2]

Key Surgical and Clinical Principles

• Right pulmonary artery: anterior to the right-sided bronchial tree at the central hilum.
• Left pulmonary artery: superior to the left main bronchus, then arching posteriorly around the upper-lobe bronchial complex.
• The right upper-lobe bronchus is eparterial; there is no equivalent eparterial bronchus on the left.
• The superior pulmonary vein is generally the most anterior major hilar vessel.
• The inferior pulmonary vein is the most inferior major hilar vascular structure and approaches the left atrium in a relatively posterior-medial plane.
• Segmental arteries accompany bronchi centrally; pulmonary veins predominantly follow intersegmental planes.
• Bronchial, arterial, and especially venous branching patterns vary considerably; preoperative CT or MRI should supersede assumptions based solely on textbook anatomy when operative strategy depends on individual hilar geometry.

References

1. Rigler LG. Segmental anatomy of the lung. Radiology. 1949;52(4):582–583. doi:10.1148/52.4.582. PMID: 18116760.
2. Van Der Spuy JC. The surgical anatomy of the pulmonary vessels. Thorax. 1953;8(3):189–194. doi:10.1136/thx.8.3.189. PMID: 13102416.
3. Webb WR, Glazer G, 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.
4. Huang M, Wang T, Wang X, Zhao X. An anatomical study of the right bronchial tree using multi-detector computed tomography. Surg Radiol Anat. 2019;41(3):335–338. doi:10.1007/s00276-019-02199-7. PMID: 30725216.
5. 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.
6. Thorning C, Hamady M, Liaw JVP, et al. CT evaluation of pulmonary venous anatomy variation in patients undergoing catheter ablation for atrial fibrillation. Clin Imaging. 2011;35(1):1–9. doi:10.1016/j.clinimag.2009.11.005. PMID: 21237413.
7. Tekbaş G, Gümüş H, Önder H, et al. Evaluation of pulmonary vein variations and anomalies with 64 slice multi detector computed tomography. Wien Klin Wochenschr. 2012;124(1–2):3–10. doi:10.1007/s00508-011-0086-9. PMID: 22183816.
8. Lyen S, Wijesuriya S, Ngan-Soo E, Mathias H, Yeong M, Hamilton M, Manghat N. Anomalous pulmonary venous drainage: a pictorial essay with a CT focus. J Congenit Cardiol. 2017;1:7. doi:10.1186/s40949-017-0008-4.