The continuous demands of every cell within the human body and the constant need to remove waste gases like carbon dioxide are met by the lungs. These paired organs assume their role as the gas exchange organs with the first cry of the neonate and continue working until the end of life. While the functional unit is the capillary-alveoli interface, the lung is divided into segments based on the arborization of the bronchi. The bronchopulmonary segments are the largest functional divisions of the anatomical lobes; each receiving their own air and blood supply.
This article will discuss the development and anatomy of the bronchopulmonary segments. It will also review the anatomy of the lungs, and discuss the function of the organs as well. Further clinical discussion involving disorders of the lung, as well as clinical investigation of pulmonary disorders will also be included.
|Total number of bronchopulmonary segments||
10 on the right
8-9 on the left
|Number of segments in the superior lobe||
3 on the right
4 on the left
|Number of segments in the inferior lobe||
5 on the right
4 on the left
|Total lung volume||6000 mL|
|Lung volumes||Tidal Volume (500 mL)
Inspiratory Reserve Volume (3000 mL)
Expiratory Reserve Volume (1100 mL)
Residual Volume (1200 mL)
|Lung capacities||Inspiratory Capacity
Functional Residual Capacity
Total Lung Capacity
Right Bronchopulmonary Segments: A PALM Seed Makes Another Little Palm
Left Bronchopulmonary Segments: ASIA ALPS
D-RIPE for reading plain radiographs of the chest
- Embryology of the Bronchopulmonary Segments
- Review of the Anatomy of the Lungs
- Anatomy of the Bronchopulmonary Segments
- The Function of the Lung
- Pathologies Affecting Bronchopulmonary Segments
- Common Procedures Associated with Pulmonary Disease
- Related diagrams and images
Embryology of the Bronchopulmonary Segments
The primary bronchial buds are two branches arising from the distal end of the respiratory bud during the 4th gestational week. As the primary bronchial buds develop, they each extend laterally into the pericardioperitoneal canals, which differentiate into the pleura of the lungs. The buds continue to branch into secondary and tertiary bronchial buds over time.
By the 5th gestational week, the bronchial buds (which are engulfed by splanchnic mesenchyme) begin to divide further into the bronchi and their respective branches. There is associated dilatation of the tracheobronchial communication that forms the primitive main bronchi. Even in utero, the right main bronchus is more vertical and wider than the left counterpart. This feature persists into extrauterine life and accounts for the increased risk of foreign body aspiration into the right bronchus. As the main bronchi produce secondary bronchi; this subsequently ramifies into the respective lobar, segmental, and intersegmental divisions.
The secondary bronchi supply the lobes of the lung, while the segmental branches will deliver oxygen to the respective segments. Superior and inferior bronchi are located on both sides of the chest. On the left, the superior bronchus supplies the superior lobe, while the inferior bronchus supplies the inferior lobe. However, on the right side, the inferior bronchus bifurcates such that the cranial branch supplies the middle lobe, while the caudal branch supplies the inferior lobe. The right superior bronchus supplies the superior lobe of the right lung (just like its left counterpart). Cumulatively, there are 10 right and 8-9 left segmental bronchi that develop in the 7th gestational week. This development is accompanied by a division of the encompassing mesenchyme; which together with the segmental bronchi, develop into the bronchopulmonary segments.
Review of the Anatomy of the Lungs
The thoracic cavity is generally divided into three parts; namely, the central mediastinum that is bound on either side by the hemithorax. Each hemithorax is bounded anterolaterally and posterolaterally by the ribs, medially by the mediastinum, superiorly by the thoracic inlet and its associated fascia, and inferiorly by the respective hemidiaphragm. Each hemithorax contains a lung, the principal organ of respiration. The lungs are elastic, spongy organs that are structurally unique. They are protected by the surrounding bony structures as they are held in place by their tracheal and cardiac attachments.
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The lungs are encased within a double layered pleural membrane. This serous structure has visceral and parietal layers; the former being attached to the lung and the latter being adherent to the inner thoracic wall and other surrounding areas of the thorax. Between the double-layer is a potential space called the pleural cavity that is filled with serous fluid. This creates a low friction interface that allows smooth movement of the lung against the inner chest wall during breathing.
The lungs are divided into lobes by grooves coursing across its surfaces known as fissures. The right lung has two fissures – the horizontal and oblique fissure. Consequently, there are three lobes on the left – the superior, middle, and inferior lobes. The right oblique fissure separates the inferior lobe from the superior and middle lobes such that the inferior lobe is posterior and inferior to the fissure, while the other two lobes are anterior to the line. On the left-hand side, there is only an oblique fissure. As such, the left lung is only divided into superior and inferior lobes.
The summit of the pyramid-shaped lungs is referred to as its apex, while the lowest point is called its base. The organ has a costal surface that is in contact with the surrounding ribs and a medial surface that is adjacent to the mediastinum and its contents. Each lung also has anterior, posterior, and inferior borders that mark the transition from one surface to another.
Another important feature of each lung is the root of the lung, otherwise called the hilum (pl. hila). This roughly triangular area is located on the medial surface of the organ and marks the point at which many structures enter and leave the lung. It acts as the only point of attachment between the lung and other intrathoracic structures. While both hila contain the same general structures, there are subtle differences in how these structures are arranged. A list of structures found at each hilum is found below (in no particular order):
- Bronchial vessels
- Connective tissue
- Lymph nodes and vessels
- Principal bronchus
- Pulmonary artery
- Pulmonary autonomic plexus
- Two pulmonary veins
The lungs receive dual blood supply from the systemic and pulmonary circulations. The systemic circulation is supplied by the bronchial arteries, while the pulmonary circulation is delivered by the pulmonary arteries. The pulmonary arteries are branches of the pulmonary trunk, arising from the right ventricle of the heart. On the other hand, the bronchial arteries are branches of the intercostobronchial trunk (on the right-hand side) and the descending thoracic aorta (on the left-hand side).
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The venous drainage is also divided into systemic and pulmonary circuits such that bronchial and pulmonary veins drain blood from the lungs. The pulmonary veins return directly to the left atrium of the heart, while the bronchial veins drain to the hemizygous or azygous systems. There is a paradox related to the blood supply of the lungs such that the pulmonary arteries carry deoxygenated blood, while the pulmonary veins carry oxygenated blood. The opposite is seen in the bronchial (and other systemic) circuits where arteries usually carry oxygenated blood, while veins remove deoxygenated blood.
Anatomy of the Bronchopulmonary Segments
The lungs can be further subdivided into bronchopulmonary segments. There are ten such segments located within the right lung, and roughly 8 – 9 on the left side as some of the segments may fuse together. Each bronchopulmonary segment is served by corresponding branches of the bronchial tree, along with their own arterial supply. However, the venous and lymphatic vessels pass through the intervening septae that separate the segments from each other (i.e. within the intersegmental planes). The segments are separated from each other by bands of connective tissue. As a result, each bronchopulmonary segment is functionally separate from the adjacent segments. Aside from the pulmonary fissures, there are no superficial anatomical markings that facilitate identification of the bronchopulmonary segments. That process requires identifying and tracing the tertiary bronchi to their distal ramifications.
While the phrase bronchopulmonary segment was coined by American otolaryngologists Rudolph Kramer and Amael Glass, the concept of segmented parts of the lung supplied by individual branches of the bronchi was put forth by British pathologist William Ewart Gye (Bullock). Efforts were made by numerous surgeons and anatomists to simplify the nomenclature associated with these segments. Eventually, an Ad Hoc International Committee came up with an internationally accepted naming system by merging the nomenclature coined by Chevalier Jackson and John Huber with that of Russell Brock.
Right Bronchopulmonary Segments
The superior lobe of the right lung has three bronchopulmonary segments. The pinnacle of the superior lobe forms the apical segment or segment I (S I). Below and posterior to the apical segment is the posterior segment (S II). When viewed from the costal surface, this segment is limited inferiorly by the posterosuperior part of the right oblique fissure and posterior part of the horizontal fissure. As the name suggests, the anterior segment (S III) is anterior to the posterior segment and anteroinferior to the apical segment. It is limited inferiorly by the horizontal fissure.
The middle lobe of the right lung lies between the horizontal (superiorly) and the anteroinferior part of the oblique fissures (inferiorly). It is subdivided into lateral (S IV) and medial (S V) bronchopulmonary segments. The lateral segment is best represented on the costal surface of the lung, while the superficial boundary of the medial segment wraps around the anterior border of the lung. It tapers off at the hilum and is superiorly related to the oblique fissure.
The inferior lobe of the right lung has five bronchopulmonary segments. The superior segment (S VI) is represented on both the costal and mediastinal surfaces of the right lung; as the segment also includes a portion of the posterior border of the right lung. The medial basal segment (S VII) is best represented on the mediastinal surface of the lung, as it lies below the hilum. It is anteriorly related to the posterior basal segment (S X), which abuts the lateral basal segment (S IX) around the posterior border of the lung. The anterior basal segment (S VIII) is limited anteriorly by the caudal part of the oblique fissure and is juxtaposed with the lateral basal segment posteriorly.
Left Bronchopulmonary Segments
Although there are only two lobes in the left lung, there is some symmetry among the bronchopulmonary segments bilaterally. However, some segments of the left lung merge. Consequently, there are fewer bronchopulmonary segments on the left than there are on the right.
The superior lobe of the left lung contains four bronchopulmonary segments. The apicoposterior segment (S I + II) represents the fusion of the apical and posterior segments. It is limited posteroinferiorly by the superior aspect of the left oblique fissure and is adjacent to the anterior segment (S III) of the superior lobe. Although the lingular lobe of the left lung is considered a part of the superior lobe, it is analogous to the middle lobe of the right lung. Similarly, it is divided into two bronchopulmonary segments, namely the superior (S IV) and inferior (S V) lingular segments. The superior lingular segment is located between the caudal boundary of the anterior segment and the superior boundary of the inferior lingular segment. Both are anterior to the hilum of the left lung, and the inferior segment is limited inferiorly by the inferior half of the oblique fissure.
The bronchopulmonary segments of the left inferior lobe are almost analogous to that of its right counterpart. There are four (instead of five) segments on the inferior lobe. The anteromedial basal segment (S VII + VIII) represents the fusion of the anterior basal and medial basal segments. The other segments (superior [S VI], posterior basal [S X], and lateral basal [S IX]) maintain the same relative positions as observed in the right lung.
The Function of the Lung
In order to fulfill its role of gas exchange, the lungs must be able to facilitate:
- Bi-directional airflow between the atmosphere and alveoli
- Diffusion of carbon dioxide and oxygen between the blood and alveoli
- Carry the gasses to and from body tissues
- Control the ventilation process.
The diaphragm, intercostal muscles, and accessory muscles of respiration (anterior scalene and sternocleidomastoid) all play a unique role in modifying the intrathoracic volume to facilitate inhalation and exhalation. Furthermore, the air is carried to the site of gas exchange by the bronchi and their distal branches. Actual diffusion occurs across the blood-alveoli barrier such that carbon dioxide dissociates from the hemoglobin within the red blood cells and moves into the alveoli, while oxygen crosses into the capillaries and binds to hemoglobin. Oxygenated blood then travels to the heart where it is disseminated throughout the body.
Each lung has a maximum capacity of about 6 liters that can be broken up into smaller lung volumes. Understanding these lung volumes and how they change is important for interpreting the lung function test results. The smallest of these volumes is the tidal volume; it is the amount of air inspired and expired with a normal breath. For the average adult male, this accounts for about 500 milliliters of the total lung volume.
A healthy individual can forcefully inspire approximately 3 liters above the normal tidal volume. This value is referred to as the inspiratory reverse volume. It is conceptually similar to the expiratory reserve volume of 1.1 liters, which is the amount of air that can be forcefully expired at the end of the tidal volume. Of the total lung volume, about 1.2 liters is occupied by air that cannot be expired from the lungs even after maximal expiration. This is known as the residual volume.
The lung volumes can be combined within a pulmonary cycle (i.e. inspiration followed by expiration). If an individual should inhale maximally and then exhale as much air as they possibly can, they should be able to exhale around 4600 mL. This volume is known as their vital capacity. The sum of the vital capacity and the residual volume gives the total lung capacity. This is the maximum amount of air that each lung can accommodate after the maximum inspiratory effort.
At the end of a normal respiratory cycle, the amount of air left in the lung is equal to the residual volume (that can never be exhaled) and the expiratory reserve volumes. These two volumes together form the functional residual capacity. In contrast, the inspiratory capacity is the sum of the inspiratory reserve volume and the tidal volume. This can be achieved by first beginning with normal breathing and at the end of a normal breath, progressively inhale until the lungs are maximally expanded.
Pathologies Affecting Bronchopulmonary Segments
The lung is no stranger to an endless list of acquired disorders as it is in open communication with the external environment. However, the pulmonary defense system makes a valiant effort to stave off pathogens and to mitigate the effects of environmental toxins on the lungs. Nevertheless, the organs still succumb to infections, collagen vascular disorders, and malignancies. In addition to the common disorders of the lungs discussed in another article, here are some additional disorders worth exploring.
Albeit uncommon, bronchiectasis is a post-infectious process characterized by abnormal, irreversible dilatation of the bronchioles. It is classified as a chronic obstructive airway disease characterized by persistent dyspnoea, hemoptysis, coughing, and defective mucociliary activity. If left unattended, some patients may progress to respiratory failure. From a pathological perspective, there is a loss of elasticity in the proximal and medium bronchi resulting from damage to the muscles and elastin in these areas. Scarring, ulceration, edema, and transmural infections are commonly encountered on pathological assessment of airways affected by bronchiectasis. While microbial invasion is the leading cause of this disorder, prolonged bronchial obstruction, aspiration, and cystic fibrosis are also important etiological factors to consider.
These patients often present with a productive cough that has been going on for months to years. They may have experienced hemoptysis, pleuritic chest pain, fever, weight loss, wheezing, and weakness. On the other end of the spectrum, some patients may only have a few symptoms. Clinical examination may be significant for digital clubbing (in severe cases), cyanosis, plethora secondary to hypoxia-induced polycythemia, crackles, and rhonchi, and scattered wheezing. Other constitutional symptoms such as evidence of weight loss and muscle wasting are associated with advanced bronchiectasis. However, if patients present with these features, an adequate work-up should be done as malignant lesions of the lungs can also present in a similar manner.
At birth, it is important for the neonate to take their first breath in order to expand the lungs. If this process is inadequate, or if there is a collapse of an area of lung that was expanded in the past, then there would be regions of airless lung parenchyma. This condition is referred to as atelectasis and it results in a disparity between oxygenation and perfusion to the affected area (i.e. ventilation-perfusion mismatch). Consequently, the patient is susceptible to acquiring atelectasis in the hypoventilated area. In addition to neonatal atelectasis which often results from insufficient surfactant in premature infants, there are three commonly encountered forms of acquired atelectasis:
- Compression atelectasis stems from the mass effect of tumors, a significant amount of liquid, or relatively large amounts of air within the lungs. This type of atelectasis is also associated with a mediastinal shift as a result of the excess tissue or fluid.
- Diseases such as sarcoidosis, lupus pneumonitis, or drugs such as methotrexate (and others) can lead to fibrotic changes in the lungs. The change in the nature of the lung parenchyma from the typically elastic to fibrotic tissue results in reduced lung expansion and subsequent contraction atelectasis.
- Hypersecretion or reduced ciliary motility can result in an excessive buildup of mucus within the airway. Consequently, the airway can become completely obstructed and the free flow of air in and out of the lungs is compromised. As time progresses, the trapped air can be absorbed by the surrounding lung tissue in dependent areas. Since no air is entering those blocked alveoli, they eventually collapse; leading to resorption atelectasis. Chronic diseases such as bronchiectasis, bronchial asthma, or prolonged immobility can predispose patients to developing this form of atelectasis. To mitigate this issue, patients with an anticipated prolonged period of immobility are usually encouraged to perform incentive spirometry and receive chest physiotherapy in order to reduce the risk of developing atelectasis.
Common Procedures Associated with Pulmonary Disease
The work-up for pulmonary disorders often includes hematological, serological, biochemical, and radiological studies. There are also additional procedures that can provide diagnostic information, therapeutic relief, or both. Some of these procedures are discussed below.
Plain Film Radiography (Chest X-Ray)
Plain film radiography relies on the use of x-rays to generate an image of areas of the body and is the most commonly ordered investigation in clinical practice. Recall that x-rays can detect five different densities, namely: air, fat, soft tissue, bone, and metal. Air is the least dense of the structures and it appears black on the final radiograph. On the other end of the spectrum, metals such as prostheses or contrast materials appear white on the radiograph. In between these two extremes (from least dense to most dense) is fat, soft tissue, and bone; which are represented by dark gray, gray, and light gray, respectively. Therefore, plain film radiographs of the chest are able to show most pathologies of the lungs since a normal film should be predominantly black and any variant of this would cause opacification of the affected area. Plain film studies of the chest can reveal the presence of fluid within the thorax, collapse of part or all of a lung, as well as primary and metastatic tumors.
The film is often done using a posteroanterior view. This means that the x-ray beams are emitted from behind the patient and passes anteriorly. Alternatively, anteroposterior, lateral, and apical views can also be requested. The physician can also request films with the patient in the erect, supine, or left lateral decubitus positions.
When reading the plain film, first pay attention to the patient demographics. In addition to ensuring that the film belongs to the patient of interest, information such as age and the date the study was performed is also helpful while interpreting the film. The age of the patient will assist the physician in differentiating between presumed skeletal defects and the growth plate in a skeletally immature individual. Assess whether or not the film is rotated, and whether or not this is a posteroanterior or anteroposterior film.
Rotation can be assessed by looking at the space between the medial head of the clavicle and the spinous process of the vertebra between them. If the film is rotated to the patient’s left, then there will be more space between the spinous process and the left medial clavicular head. The converse is also applicable. Assume that the film is posteroanterior (PA) unless it is otherwise indicated on the film (i.e. the film says AP, meaning anteroposterior). X-rays emitted in an AP direction magnifies the size of the heart, giving a false impression of cardiomegaly. The PA film avoids this magnification, thus the true size of the heart can be assessed. If the widest diameter of the heart is wider than half the diameter of the thorax, then the patient is said to have an enlarged heart. Next, assess the inspiratory effort; it is considered adequate if 7 – 8 ribs can be counted anteriorly and 8 – 10 can be counted posteriorly. The degree of penetration refers to how many x-rays beams were used during the study. If a high quantity of x-ray is used then the film will be over penetrated and the resulting densities will be misinterpreted. Conversely, an underpenetrated film will lead the interpreter to think that there are high-density objects within the lung field. Adequate penetration means that the examiner can count the intervertebral discs from the angle of Louis to the midpoint of the cardiac silhouette. Next, look at the exposure of the film. The interpreter should be able to visualize the thoracic inlet to the diaphragm in an AP or PA film.
Using an outside-in approach, comment on the soft tissue in the periphery and whether or not there is air in the soft tissue space. This subcutaneous emphysema may be the result of trauma and associated pneumothorax or a necrotic pneumonitis with a pneumothorax. Look at the ribs, clavicles, and vertebrae. Fractures of these bones may cause lung punctures or result in flail segment. Comment on the lung fields, any opacification, and its distribution, blood vessels, and lung fissures. Go on to mention whether or not the trachea is central, based on its relationship to the medial heads of each clavicle, and discuss the cardiac silhouette as described earlier. Also look at the junction between the heart and the diaphragm (cardiophrenic angle) and the diaphragm and the ribs (costophrenic angle) as fluid from a pleural effusion or hemothorax may settle here. The final impression should be combined with findings from the history and clinical examination to make an adequate diagnosis.
Computed tomography and magnetic resonance imagining of the chest provides more detailed information that may be missed with plain x-ray. These options can be employed if there is incongruence between the clinical picture and the investigation findings.
There are cases in which direct visualization of the bronchial tree is necessary to either diagnose or treat a patient. Bronchoscopy allows the clinician to see the airway and possibly implement a therapeutic measure to relieve the patient. There are two types of bronchoscopy that are commonly performed. The older technique is the rigid bronchoscopy that usually requires the patient to be placed under general anesthesia. The rigid bronchoscope is a metallic device with a flanged distal tip with ports for passing biopsy or suction tools as well as a light source. Proximally, the device has an irrigation port, attachment for the light source, and a port for observing the pathway. During this procedure, the patient is placed in the supine position with the neck hyperextended. It is useful in retrieving foreign bodies and aspirating thick fluids from the airway. It can also be used to introduce a stent into the airway or to perform an intraluminal resection of a midline mass.
Alternatively, a flexible fiber-optic scope (i.e. flexible bronchoscope) can be introduced into the patient's airway while they are awake with local anesthesia to the oropharynx. It can be introduced nasally or orally, passed between the vocal folds, into the trachea and then the bronchial tree. Unlike the rigid scope procedure, the flexible bronchoscope provides the added option of visualizing the segmental bronchioles, where hard to reach tissue or sputum can be collected for diagnostic analysis.
Patients who are found to have relatively small lung tumors may be candidates for removal of only the affected portion of the lung. This procedure – known as a segmentectomy and wedge resection – is dependent on the number of segments involved and the status of the contralateral lung. The segmentectomy refers specifically to the removal of an anatomical segment of the lung, while the wedge resection is the removal of a non-anatomical portion of lung parenchyma. The opposite lung should be able to compensate for the decreased lung volume. Segmentectomies can be performed by video-assisted thoracoscopic surgeries or with the traditional thoracotomy.