Lung Hilum: Anatomy, Structures, and Clinical Notes

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The lung hilum is the region on the mediastinal surface of each lung where the main bronchus, pulmonary artery, pulmonary veins, bronchial vessels, lymphatics, and autonomic nerves enter and leave the organ. The root of the lung is the pedicle of tissue that carries those same structures, wrapped in a sleeve of pleura, from the mediastinum into the hilum.

Those two terms get used interchangeably in clinics and in textbooks, and for most day-to-day purposes that is fine. For a student building a three-dimensional mental model, though, the distinction matters. The hilum is a surface, a doorway. The root is the bundle that passes through it. Get that relationship straight and the rest of hilar anatomy, including the species differences that trip people up in practice, falls into place.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Why the Hilum Matters

Every structure that keeps a lung alive and connected to the body passes through a space roughly the size of a coin. That concentration of anatomy has consequences. It is the site where imaging studies look for hilar masses, lymphadenopathy, and vascular abnormalities. It is the region surgeons must dissect when removing a lobe. And it is the place where comparative anatomy diverges most sharply between humans and domestic species, which is why a veterinary student cannot simply memorize a human anatomy plate and apply it to a dog.

Modern imaging reviews treat the hilum as an important gateway of elementary thoracic structures, and knowledge of its anatomy, signs, and patterns is essential for accurate interpretation of both conventional radiography and cross-sectional imaging [1]. A separate radiology review describes the hilum as a common diagnostic challenge precisely because so many structures converge in one complex region [2].

Defining the Hilum and the Root

Hilum Versus Root: A Working Distinction

The hilum of the lung is the depressed, often somewhat triangular area on the mediastinal (medial) surface of the lung where structures pass in and out. It is a two-dimensional footprint on the lung surface.

The root of the lung is the three-dimensional pedicle. It contains the same structures plus their investing connective tissue and the pleural sleeve that reflects off the mediastinum onto the lung. When you hold a cadaver lung and see a stub of bronchus with vessels dangling from it, you are looking at the root. The ragged surface where those structures were cut is the hilum.

Both terms describe the same functional gateway. The root is what you dissect. The hilum is what you see on the surface and what radiologists describe.

What Passes Through

The contents of the hilum and root are consistent across mammals, though their arrangement is not:

1. Main bronchus (and, in species with separate lobar bronchi at the hilum, the lobar bronchi)
2. Pulmonary artery
3. Pulmonary veins, typically described as superior and inferior on each side
4. Bronchial vessels (bronchial arteries and veins), which supply and drain the airway walls and supporting tissue
5. Lymphatics, including hilar lymph nodes and lymphatic vessels
6. Autonomic nerves, sympathetic and parasympathetic fibers that regulate bronchial smooth muscle and glandular secretion
7. Pleural reflection, which forms the outer sleeve of the root

The bronchial vessels are much smaller than the pulmonary vessels and are easy to overlook in a dissection, but they matter clinically because they are the nutritional supply to the bronchi and they can enlarge in chronic lung disease.

The Usual Arrangement of Hilar Structures

General Pattern

Across species, the general arrangement follows a recognizable pattern. The bronchus sits posteriorly (dorsally) in the hilum. The pulmonary artery lies superior or anterior to the bronchus, depending on the side. The pulmonary veins lie anterior and inferior.

That description is a starting point, not a rule. Hilar morphology is extremely variable. A cadaveric study of 110 lung specimens found hilar arrangement variations in 16.1% of right lungs and 48.2% of left lungs, and the authors concluded that pulmonary hilar morphology is extremely variable [3]. A separate study of 103 cadaveric hila found a classical hilar picture in only 35.19% of left lungs and 40.82% of right lungs [4]. In other words, the textbook pattern is present in fewer than half of specimens.

Right Versus Left Differences

The right and left hila differ in a consistent way that reflects the asymmetry of the bronchial tree and the position of the heart.

On the right side, the eparterial bronchus (the bronchus to the cranial or upper lobe) arises above the level of the pulmonary artery. This means the right pulmonary artery passes behind or below the upper lobe bronchus and in front of the intermediate bronchus. The right hilum sits slightly lower and more anterior on the mediastinal surface than the left [4].

On the left side, the pulmonary artery is typically superior to the main bronchus, and the left hilum is positioned slightly higher and more posterior. The left hilum tends to have a greater anteroposterior length, while the right hilum has a greater vertical length [4].

Variations are more common on the left. In one study, the highest percentage of variable structures on the left side was the bronchus (46.3%), followed by the pulmonary artery (37.31%) and the pulmonary vein (31.48%). On the right side, the pulmonary artery and vein were equally variable (36.73% each), followed by the bronchi (20.41%) [4].

A case report described a rare anomaly in which the left pulmonary artery was posterior to the left mainstem bronchus, a variation not previously reported [5]. Another case described a displaced left upper division bronchus with the pulmonary artery running anterior to the left main bronchus, similar to the arrangement normally seen on the right [6].

Summary Table: Hilar Structures, Position, and Side Differences

Comparative Anatomy: Dogs, Cats, Horses, and Ruminants

Dogs and Cats

Dogs and cats have a lobation pattern that differs from humans. The right lung in the dog has four lobes: cranial, middle, caudal, and accessory. The left lung has two lobes: cranial and caudal. The cranial lobe of the left lung is sometimes subdivided into cranial and caudal parts by a fissure.

The hilar pattern reflects this lobation. In the dog, the right main bronchus gives off the eparterial bronchus to the cranial lobe before continuing as the intermediate bronchus, which supplies the middle, caudal, and accessory lobes. The left main bronchus divides into cranial and caudal lobar bronchi. The pulmonary artery relationship to the bronchus is broadly similar to the human pattern but the smaller size and different lobation mean the hilar footprint is more compact.

Cats have a similar lobation pattern to dogs, with four right lobes and two left lobes (the left cranial lobe often partially divided). The hilar structures are correspondingly smaller and more tightly packed.

Horses

The horse has a right lung with cranial, caudal, and accessory lobes, and a left lung with cranial and caudal lobes. The right lung also has a small intermediate lobe in some descriptions. The horse lung is relatively large and the hilum is correspondingly larger, but the fundamental arrangement of bronchus, pulmonary artery, and pulmonary veins follows the same general pattern.

Ruminants

Ruminants (cattle, sheep, goats) have a more complex lobation. The right lung has cranial, middle, caudal, and accessory lobes. The left lung has cranial and caudal lobes, with the cranial lobe often divided into two parts. The hilar region in cattle is notable for the large caliber of the main bronchus and the presence of well-developed bronchial vessels.

Why Comparative Differences Matter

A veterinary student who learns only the human hilar pattern will be surprised by the number of lobar bronchi that can arise directly at the hilum in domestic species. In the dog, for example, the eparterial bronchus arises at the hilum, not above it as in humans. This means the hilar region in the dog contains more branching structures than the human hilum, and the surgical approach to a lobectomy must account for this.

How the Hilum Is Examined in Practice

Radiography

On a standard thoracic radiograph, the normal hilum appears as a region of increased opacity where the pulmonary vessels and bronchi converge. The two classical radiological signs used to interpret hilar anatomy are the hilum overlay sign and the hilum convergence sign [2]. The hilum overlay sign helps determine whether a mass is located in the hilum or in the adjacent mediastinum. The hilum convergence sign helps determine whether a mass is arising from the hilum itself or from the lung parenchyma.

Computed Tomography

CT provides superior anatomic discrimination of hilar structures [1]. It allows identification of the bronchus, pulmonary artery, and pulmonary veins as separate structures and is the preferred modality for evaluating hilar masses, lymphadenopathy, and vascular anomalies. Automated classification systems have been developed to identify airways, fissures, nodules, and vessels from chest CT images, with vessel classification extending from the hilum to the periphery [7].

Endobronchial Ultrasound

Endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) is used to sample hilar and mediastinal lymph nodes. The technique provides real-time ultrasound feedback of needle placement and proximity to major surrounding structures such as the heart and great vessels [8]. In one reported case, a left hilar mass was successfully sampled by intentionally traversing the pulmonary artery, illustrating both the utility and the anatomic precision required in this region [8].

Gross Dissection

In the anatomy laboratory, the hilum is examined by dissecting the mediastinal pleura and identifying the bronchus, pulmonary artery, and pulmonary veins. The bronchus is identified by its cartilaginous rings. The pulmonary artery is identified by its thicker wall and its position relative to the bronchus. The pulmonary veins are identified by their thinner walls and their anterior and inferior position. The bronchial vessels are much smaller and require careful dissection to identify.

Clinical Relevance, Limitations and Common Mistakes

Clinical Relevance

The hilum is a site of major clinical importance in several contexts.

Hilar masses. A hilar mass can represent primary lung neoplasia, metastatic disease, or lymphadenopathy. NUT midline carcinoma, a rare and lethal malignancy, has been reported presenting as a large hilar mass with mediastinal adenopathy [9]. Extralobar pulmonary sequestration can also present as a mass near the hilar region, covered by independent visceral pleura and with a feeding artery from the pulmonary artery [10].

Hilar invasion by tumor. Local extension of tumor at the hilum is associated with worse long-term survival in stage I non-small cell lung cancer. A study of 213 patients found that the type of operation and tumor size were the most prominent independent prognostic factors, and that bronchial invasion and arterial involvement were closely associated with long-term survival on univariate analysis [11]. Thin-section CT has limited accuracy in detecting hilar or mediastinal invasion, with reported accuracies of 75.0% for pulmonary artery invasion and 66.7% for main bronchus invasion [12].

Surgical planning. The hilar structures are the key landmarks in lobectomy and segmentectomy. A "hilum first, fissure last" approach has been described for robotic segmentectomy in a patient with a displaced left upper division bronchus and fused fissure, allowing fine dissection of hilar structures with minimal bleeding [6]. In thoracoscopic right upper lobectomy, a modified two-port approach has been described in which the posterior mediastinal pleura is dissected to expose the posterior portion of the right upper bronchus and the anterior trunk of the right pulmonary artery [13].

Segmental hilar structures. In segmentectomy, the segmental hilar structures (segmental artery, segmental bronchus, and adjacent parenchyma) are the target of ligation. A retrospective study of 240 patients found that continuous suture ligation of segmental hilar structures was associated with reduced prolonged air leak after pulmonary segmentectomy [14].

Lung transplantation. In experimental lung transplantation models, the hilar anastomosis technique affects operative time and outcomes. A mouse study comparing anterior and posterior hilum anastomosis found that the anterior approach took longer but had similar success rates [15].

Limitations

Hilar anatomy is variable enough that individual cases require imaging and, where relevant, surgical planning by a veterinarian. The statistics cited here describe populations, not individual animals. A normal hilar pattern on one side does not guarantee a normal pattern on the other.

Common Mistakes

Confusing hilum and root. The hilum is the surface region. The root is the pedicle. Using the terms interchangeably is common but imprecise.

Assuming the textbook pattern. The classical hilar picture is present in fewer than half of cadaveric specimens in some studies [4]. Variations are more common on the left [3].

Forgetting species differences. The dog, cat, horse, and ruminant all have different lobation patterns and hilar arrangements compared with humans. The right lung in ruminants and horses has additional lobes.

Overlooking the bronchial vessels. These small vessels are easy to miss but are important in chronic lung disease and in surgical dissection.

Assuming symmetry. The right and left hila differ in position, in the relationship of the pulmonary artery to the bronchus, and in the frequency of variations.

Quick Review

1. The lung hilum is the mediastinal surface where structures enter and leave the lung. The root of the lung is the pedicle that carries those structures, wrapped in pleura.
2. Contents: main bronchus, pulmonary artery, superior and inferior pulmonary veins, bronchial vessels, lymphatics, and autonomic nerves.
3. Usual arrangement: bronchus posterior, pulmonary artery superior or anterior, veins anterior and inferior.
4. Right and left differ. The right hilum is lower and more anterior. The left hilum is higher and more posterior.
5. Hilar morphology is extremely variable. Classical pattern in fewer than half of specimens in some studies.
6. Dogs and cats have four right lobes and two left lobes. Ruminants and horses have additional right lobes.
7. The hilum is a key site for imaging, surgical planning, and staging of lung disease.

Frequently Asked Questions

What is the difference between the hilum and the root of the lung?

The hilum is the surface region on the mediastinal side of the lung where structures enter and exit. The root is the pedicle of structures themselves, wrapped in pleura, that passes through the hilum. The terms are often used interchangeably but are not identical.

What structures pass through the lung hilum?

The main bronchus, pulmonary artery, superior and inferior pulmonary veins, bronchial vessels, lymphatics, and autonomic nerves all pass through the hilum. They are enclosed in a pleural sleeve that forms the root of the lung.

How does the hilar pattern differ between dogs and humans?

Dogs have four right lung lobes and two left lobes, compared with three right and two left in humans. The eparterial bronchus arises at the hilum in dogs rather than above it, so the hilar region contains more branching structures.

Which side has more hilar variations?

The left side has more variations. One study found variations in 48.2% of left lungs and 16.1% of right lungs [3]. Another found the classical hilar picture in only 35.19% of left lungs and 40.82% of right lungs [4].

What is the clinical significance of the lung hilum?

The hilum is where hilar masses, lymphadenopathy, and vascular abnormalities are detected on imaging. It is also the region surgeons dissect during lobectomy and segmentectomy, and tumor extension at the hilum is associated with worse survival in lung cancer [11].

Can the hilar arrangement be predicted from a textbook?

No. Hilar morphology is extremely variable, and the classical pattern is present in fewer than half of specimens in some studies. Imaging is required to determine the arrangement in any individual patient.

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Sources

1. [[Modern imaging of the pulmonary hilum : Anatomy, pathologies, pitfalls].](https://pubmed.ncbi.nlm.nih.gov/36625921/)
2. The hilum of the lung: Two classical radiological signs to decipher it.
3. Anatomical variations of the arrangement of structures at the pulmonary hilum: a cadaveric study.
4. Comprehensive study of pulmonary hilam with its clinical correlation.
5. Hilar Abnormality in the Left Lung: Left Pulmonary Artery Posterior to the Left Mainstem Bronchus.
6. Robotic Segmentectomy in a Patient with a Displaced Left Upper Division Bronchus and Fused Fissure.
7. Automated classification of lung bronchovascular anatomy in CT using AdaBoost.
8. Successful real-time endobronchial ultrasound-guided transbronchial needle aspiration of a hilar lung mass obtained by traversing the pulmonary artery.
9. Cytologic findings of NUT midline carcinoma in the hilum of the lung.
10. Unusual radiological presentation of extralobar pulmonary sequestration: a preoperative diagnostic challenge.
11. Local extension at the hilum region is associated with worse long-term survival in stage I non-small cell lung cancers.
12. Hilar and mediastinal invasion of bronchogenic carcinoma: evaluation by thin-section electron-beam computed tomography.
13. A modified method using a two-port approach for accessing the hilar vasculature without transferring an endostapler from camera port to utility port during thoracoscopic right upper lobectomy.
14. Continuous suture ligation of segmental hilar structures is associated with reduced prolonged air leak after pulmonary segmentectomy.
15. Anterior hilum anastomosis versus posterior hilum anastomosis in a mouse lung transplantation model.