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Bronchopulmonary segments: want to learn more about it?

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Bronchopulmonary segments

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.

Key facts
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
Vital Capacity
Total Lung Capacity
Mnemonics Right Bronchopulmonary Segments: A PALM Seed Makes Another Little Palm
Left Bronchopulmonary Segments: ASIA ALPS 
D-RIPE for reading plain radiographs of the chest

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.

How are you going to test yourself on the bronchopulmonary segments? Discover the importance of using active recall throughout the learning process. 

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 right – 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

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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.

Lungs in a cadaver: During your dissection sessions, you will need to orientate the lung in an anatomical position outside the thorax. The easiest way is to identify the hilum (medial aspect) and the anterior border (faces anteriorly). The anterior border is easily identifiable due to its sharp, thin aspect.

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.

An easy way to remember all these segments is by using a mnemonic! Just memorise the phrase ' A PALM Seed Makes Another Little Palm' and the terms it stands for will be much easier to recall:

(Superior to inferior)

  • Apical
  • Posterior
  • Anterior
  • Lateral
  • Medial
  • Superior
  • Medial basal
  • Anterior basal
  • Lateral basal
  • Posterior basal

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. 

Although there are fewer segments in the left lung, you might still sometimes struggle to remember them. But worry not, there's a mnemonic to help you out here too! Use the phrase ' ASIA ALPS' to remember the following structures:

  • Apicoposterior
  • Superior lingular
  • Inferior lingular
  • Anterior
  • Anteromedial basal
  • Lateral basal
  • Posterior basal
  • Superior basal

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. 

Lung volumes

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.

Lung capacities

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.

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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.

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.

Bronchiectasis 

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.

Segmental atelectasis

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.

Bronchopulmonary segments: want to learn more about it?

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