The majority of biochemical reactions that occur within the body are dependent on energy. In order to create these energy moieties, the cellular organelles utilize oxygen to drive these chemical reactions. The human lungs are a pair of spongy organs within the thoracic cavity that facilitate gaseous exchange. They are a part of the respiratory system, which also includes the nose, nasal sinuses, mouth, pharynx, larynx, and trachea. At the level of the lungs, much-needed oxygen is absorbed into the blood, while waste gases are excreted and exhaled.
Although they are the primary site of gaseous exchange, the lungs work in conjunction with the musculoskeletal system of the thorax (ribs, sternum, diaphragm, and other accessory muscles) to facilitate inhalation and exhalation.
|Surfaces||Costal, medial (mediastinal and vertebral), diaphragmatic|
|Borders||Anterior, posterior, and inferior|
|Parts of the lung||Apex and base|
Left lung - superior and inferior
Right lung - superior, middle, and inferior
Left lung - oblique
Right lung - oblique and horizontal
Pulmonary trunk -> pulmonary arteries (right and left) -> superior and inferior pulmonary arteries (right and left)
Intercostobronchial trunk and thoracic aorta -> bronchial arteries (right and left)
|Innervation||Pulmonary plexus (vagus nerve, cervical cardiac nerves)|
|Embryology||Four stages of development - pseudoglandular, canalicular, terminal sac, alveolar stages|
The goal of this article is to explore the embryology and anatomy of the human lungs. It will also include a brief review of the respiratory tract. Additional information about the histology of the lungs, bronchopulmonary segments, bronchi, and trachea can be found in the respective articles.
- Review of the respiratory tract
- Blood supply
- Lymphatic drainage
Review of the respiratory tract
The respiratory system is a collection of organs that are responsible for carrying and exchanging essential gases. The system starts with the nose and mouth, which both serve as entry points for air to access the respiratory tract. The nose is the primary entry point, however, air can also pass through the mouth in the advent that the nasal passage is obstructed. The nostrils and nasal sinuses are designed to warm, humidify, and filter air as it passes through the upper airway. The remainder of the upper airway is comprised of the pharynx, larynx, and trachea. The carina – which is found at the bifurcation of the trachea – marks the transition from the upper respiratory tract to the lower respiratory tract.
The lungs are spongy, expandable organs that occupy the thoracic cavity. Although they occur in pairs, they are morphologically different. Each lung occupies the respective hemithorax, within the mediastinum and its contents located between them. They are suspended freely within the pleural membrane and are only attached to the trachea and heart by the main bronchioles and pulmonary vessels, respectively. The morphological difference between the left and right lungs is also reflected in the weight of the organs; as the left (565 g) weighs less than the right (625 g). They also tend to be heavier in men than they are in women; although this feature is dependent on the height of the individuals. Furthermore, the adult lung has a dark, mottled appearance that is reflective of the filtration of carbon-based moieties from the airway. This is significantly different from the lungs of a new-born, which are light pink.
The pleura is a serous membrane that envelopes each lung. The membrane is made up of a visceral layer that is adherent to the lung, and a parietal layer that is fixed to the inner thoracic wall, lower cervical vertebra, costovertebral area, mediastinum, and diaphragm. There is a potential space between the parietal and visceral layers known as the pleural cavity. The cavity is filled with serous fluid that allows the parietal and visceral layers to freely glide over each other during breathing; thus, reducing the impedance to the breathing mechanism that would arise from frictional forces exerted between the surfaces.
At their most basic level, each long is characterized by its apex and base; the apex projects towards the superior thoracic aperture, while the base is placed on the diaphragm. Alternatively, we can describe the lung as having three surfaces (costal, medial and diaphragmatic) which are divided by three borders (anterior, posterior and inferior).
The organs are roughly conical in shape and are divided by fissures into lobes. The left lung has two lobes and one fissure; while the right lung has three lobes and two fissures. The lobes are then further subdivided into bronchopulmonary segments; such that the left lung has 9 – 10 segments, while the right lung has 10. Between lobes are interlobar surfaces of lungs which are separated by fissures of the lungs.
The apex is the highest point of the lung, extending into the thoracic inlet. It is a dome-shaped part of the lung that protrudes above the first costal cartilage and the medial third of the clavicle. Both lungs are separated from the ipsilateral scalenus anterior muscles by the intervening subclavian artery. As the vessel takes its superior course toward the first rib, it passes along the anterior surface of the lung and over the suprapleural membrane.
The apex of each lung is anteriorly related to the anterior ramus of spinal nerve T1, the stellate (cervicothoracic sympathetic) ganglion, and the superior intercostal artery found on the same side. The medial surface of the apex of the left lung is laterally related to the left brachiocephalic vein and left subclavian artery. On the other hand, the medial surface of the apex of the right lung is laterally related to the trachea, brachiocephalic trunk, and right brachiocephalic vein. Both apices are medially related to the ipsilateral scalenus medius muscles.
The medial surface of the lung is the part that faces the mediastinum and consists of the mediastinal and vertebral surfaces. Like the other lung surfaces, the medial surface has numerous indentations left by the adjacent structures that make an impression on the surface. The anterior aspect of the medial surface is referred to as the anterior mediastinal part, while the dorsal half is known as the posterior vertebral part. The anterior mediastinal part can be identified not only by its relationship to the mediastinum but also by the concavity that accommodates the cardiac impression. The posterior vertebral part can be found next to the thoracic vertebra and their associated intervertebral discs.
There is a rounded posterior border that separates the vertebral part of the medial surface from the costal part. Unlike other lung borders, the posterior border is an imaginary line that coincides with the heads of the adjacent ribs. Anteriorly, the costal and medial surfaces of the lung meet at the anterior border. It is a sharp and thin border that overlays the pericardium that reaches the area where the parietal pleura reflects at the junction of the costal cartilages and the mediastinum (i.e. the costomediastinal line of pleural reflection). While this border is almost vertical on the right-hand side, on the left, it has a variable course. It continues to the 4th costal cartilage but becomes irregular at the cardiac notch.
However, what makes the medial surface unique is the presence of the hilum (pl. hila). Located between the 5th and 7th thoracic vertebrae, the hila are the roots of the lungs through which the neurovascular and airway structures enter and leave the lung parenchyma. It also provides a point of attachment between the lung and the medially related heart and trachea. The structures found at the hilum of both lungs are the same, but their relationship to each other is slightly different. All these structures are encased in the pleural membrane as well. The following structures are found at each hilum:
- Principal bronchus
- Pulmonary artery
- Two pulmonary veins
- Bronchial vessels
- Pulmonary autonomic plexus
- Lymph nodes and vessels
- Connective tissue
There are several other structures that are closely related to each hilum that is of surgical importance. Anteriorly, the pericardiophrenic vessels, phrenic nerve, and anterior pulmonary plexus travel toward their respective target organs. Inferiorly is the pulmonary ligament, and posteriorly are the posterior pulmonary plexus and vagus nerve.
The base, which is also the diaphragmatic surface of the lungs, rests on the thoracic part of the diaphragm. On the left-hand side, the diaphragm separates the base of the lung from the spleen and stomach, while on the right it separates the lung from the liver. The right lung has a deeper basal concavity when compared to the left lung, and is, therefore, shorter than its counterpart. This is because the right hemidiaphragm is slightly higher than the left hemidiaphragm in order to accommodate the liver on the right-hand side.
The base of the lung is separated from the costal surface by the inferior border. Medially at its origin, the border appears round where it intervenes between the two surfaces. However, it becomes thinner and sharper as it runs all the way to the costodiaphragmatic recess. The costal surface of the lung is less remarkable than the medial surface. It is marked by the impressions of the overlying ribs and the grooves of the respective fissures.
The left lung
In order to accommodate the left-sidedness of the heart, the left lung has less tissue in the anteromedial region of the organ. Consequently, the left lung appears morphologically different from the lung such that it has one less fissure and one less lobe. The oblique fissure of the left lung divides the organ into the superior and inferior lobes.
Fissure of the left lung
The differences between the oblique fissure of the left lung and that of the contralateral side are quite subtle. The left oblique fissure has a similar course to that of the right oblique fissure (described below) One of the differences is in relation to the clock face method used to describe the point of origin and termination of the fissure with respect to the hilum of the lung. On the left, the oblique fissure arises from the 10 o’clock position and ends at the 5 o’clock position. The other difference is that the left oblique fissure is slightly more vertical than the right.
Lobes of the left lung
The inferior lobe is posteroinferior with respect to the oblique fissure. On the other hand, the superior lobe is anterosuperior to the fissure. The inferior lobe is much larger than the superior lobe; it contains most of the left lung base, a majority of the posterior border, and the lower posterior part of the medial and lateral surfaces. In contrast, the superior lobe includes the apex, and most of the costal and medial surfaces, and the entire anterior border. The superior lobe also includes the cardiac notch and the associated lingula found in that area.
Hilum of the left lung
The left hilum is inferior and anterior to the aortic arch and thoracic aorta, respectively. The pulmonary artery is the most superior structure within the left hilum. Immediately below is the principal bronchus, followed by the lower pulmonary veins. The upper pulmonary vein is anteroinferior to the pulmonary artery and anterior to the principal bronchus.
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The right lung
The right lung is slightly shorter and wider than the left lung. These differences result from the presence of the liver under the left hemidiaphragm and the left-sided placement of the heart. Overall the right lung is larger than its left counterpart. Furthermore, it is divided by two grooves into three lobes. The oblique fissure is one of two grooves found on the surface of the right lung; the other is the horizontal fissure. Both fissures divide the lung into the superior, middle, and inferior lobes.
Fissures of the right lung
On the medial surface of the lung, the oblique fissure can be seen at the 7 o’clock position of the hilum of the right lung. It travels anteriorly and downwards to traverse the anterior aspect of the base of the lung. Shortly thereafter, it passes over the inferior border and emerges on the costal surface about 7 cm away from the anterior border. It then travels superolaterally around the lung in the 5th intercostal space. As it continues towards the posterior border, it forms a junction with the horizontal fissure in the 4th intercostal space at the mid-axillary line. At the posterior border, the oblique fissure can be found either at or slightly below the level of the spinous process of the fourth vertebra. It then crosses over to the medial surface where it meets the posterosuperior part of the hilum of the right lung at the 1 o’clock position.
The horizontal fissure has a much shorter course than the oblique fissure. It starts at the mid-axillary line within the 4th intercostal space. It courses anteriorly in a horizontal plane towards the anterior border of the lung. At the anterior border, the horizontal plane can be found at the 4th costochondral junction. It then turns abruptly on the medial surface of the lung and continues toward the hilum at the 9 o’clock position.
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Lobes of the right lung
The superior lobe of the right lung is located above the horizontal fissure and anterosuperior to the oblique fissure. It contains the apex, majority of the upper part of the costal surface, and the upper part of the medial surface as well.
The middle lobe is the smallest of the three right lung lobes. It is caudally related to the horizontal fissure and anterior to the oblique fissure. It is cuneiform in shape and involves the anterior basal aspect of the lung, the anteroinferior part of the costal surface, and the inferior region of the anterior border.
The inferior lobe accounts for the majority of the lung surface volume. It is situated behind the oblique fissure. This lobe includes the posteroinferior parts of the costal and medial surfaces and the entire lung base that is posterior to the oblique fissure.
Hilum of the right lung
The right hilum is caudally related to the terminal azygos vein and posteriorly related to the right atrium and superior vena cava. The structures within the right hilum are arranged such that the principal bronchus is posteriorly related to the pulmonary artery. The upper pulmonary vein is anterior and inferior to the pulmonary artery and anterior and superior to the lower pulmonary vein, which is the lowest structure within the left hilum.
The lungs have dual, parallel blood supply referred to as pulmonary and systemic circuits. The pulmonary circuit arises from the heart and brings deoxygenated blood to the lung for gas exchange. This circuit also takes oxygenated blood back to the heart to be redistributed to the rest of the body. The systemic circuit takes oxygenated blood from the heart to the lung parenchyma that cannot be supplied by simple diffusion. The venous arm of this circuit also takes deoxygenated blood back to the heart to be processed.
The right ventricle pumps deoxygenated blood to the lungs through the pulmonary arteries. These vessels arise from the pulmonary trunk as it bifurcates behind the superior vena cava to give one vessel to each lung. Each pulmonary artery also undergoes a second division to produce superior and inferior branches. The inferior right pulmonary artery is larger than the ipsilateral superior artery. It passes between the superior pulmonary vein and the intermediate bronchus before supplying the middle and inferior lobes. In addition to supplying these lobes, it also sends a recurrent branch to augment the blood supply to the superior lobe.
The right superior pulmonary artery bifurcates as it supplies the superior lobe of that lung. At the point of bifurcation, the left pulmonary artery passes under the aortic arch to enter the left oblique fissure. The subsequent divisions of this vessel can vary. The first branch of the left pulmonary artery is the largest of all the subsequent divisions. It is responsible for supplying the anterior bronchopulmonary segment of the left superior lobe. There are additional branches from this vessel; however, the pattern of arborization varies significantly when compared to the right counterpart. The left pulmonary artery also gives off a lingular branch within the oblique fissure that supplies the lingula. There are many other branches arising from the left pulmonary artery that extends throughout the left lung to supply the rest of the organ.
While the pulmonary arteries deliver low-pressured deoxygenated blood to the lungs, the bronchial arteries deliver oxygenated blood at high pressures to the organ and its supporting elements. The majority of individuals have two vessels on the left, and one on the right. The right bronchial artery arises from the intercostobronchial trunk alongside the posterior intercostal artery. The vessels work as a vasa vasorum (to the pulmonary vessels and aorta) and vasa nervorum for the vagus nerve to supply the middle third of the esophagus, and other structures.
On the left-hand side, the bronchial arteries are direct branches of the thoracic aorta. There is a superior left bronchial artery that supplies part of the aortic arch and an inferior left bronchial artery that supplies the bronchi and connective tissue of the hilum.
The capillary networks within the alveolar walls coalesce distally to form the pulmonary venous circulation. Numerous intra-segmental vessels within each bronchopulmonary segment unite to form segmental veins. The vessels between segments (i.e. inter-segmental vessels) then merge to form larger solitary veins in each lobe. Therefore, the right side has three lobar veins, while the left side only has two. The superior and middle lobar veins ultimately fuse; therefore, only 2 lobar veins merge to form the pulmonary veins. These low-pressure vessels take oxygenated blood back to the left atrium. It is subsequently distributed to the rest of the body.
While the pulmonary veins drain the lung parenchyma, the hila are drained by bronchial veins. There are superficial and deep bronchial veins that serve as tributaries for the bronchial vein. Each hilum is served by a single bronchial vein. The intra-bronchial venous plexuses travel adjacent to the bronchi in the hilum before merging to form the deep bronchial veins. They have been noted to drain either into the pulmonary vein or directly into the left atrium.
The subpleural venous channels are the primary tributaries to the superficial bronchial veins. These veins are fed by the vasa vasorum of the pulmonary vessels, hilar lymph nodes, and extrapulmonary bronchi. The superficial bronchial veins drain to the brachiocephalic, accessory hemiazygos, and left superior intercostal veins on the left-hand side, and the azygos vein on the right-hand side.
The lymphatic vessels that drain the lungs originate below the pleura and are referred to as the subpleural plexus. This group of vessels is further subdivided into superficial and deep groups; of which the latter group follows the pulmonary artery and vein, as well as the bronchi. All the superficial lymphatic vessels meet at the hilum where they drain to the ipsilateral bronchopulmonary lymph nodes.
Peripherally, there is communication between the superficial and deep lymphatic pathway. The communicating vessels are capable of dilating to redirect the flow of lymphatic fluid from one system to another in the presence of lung disease that obstructs the lymphatics. Traditionally, the inferior lobes of the lungs drain their lymphatic fluid to the inferior tracheobronchial nodes. On the other hand, superior tracheobronchial nodes drain lymphatic fluid from the upper lobes.
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The autonomic nervous system and the vagus nerve share the responsibility for innervating the lungs. The autonomic nervous system is responsible for dilating and constricting the airway and for regulating bronchial secretions. Branches from the vagus nerve and sympathetic branches from the cervical cardiac nerves unite to form the pulmonary plexus. This plexus is further subdivided into anterior and posterior divisions according to its relationship with the hilum of the lung. In addition to supplying the bronchi, it also innervates the visceral pleura.
During intrauterine life, the developing fetus is not only dependent on the expectant mother for nourishment, but also to get rid of its waste matter. Even though the fetal lungs will not be participating in gas exchange throughout the pregnancy, its development is necessary to support the fetus as soon as it emerges from the womb. There are three fundamental features that the lungs must develop in order to facilitate the transition from placental gas exchange to pulmonary gas exchange. The lungs must first increase the amount of surfactant that is being produced. This compound functions like a lubricant which helps to reduce the surface tension in the alveoli and allows them to expand with inhalation. Secondly, the lungs must acquire a gas exchange capacity. This occurs once the alveolar units begin to develop in late pregnancy. Thirdly, there needs to be a parallel dual circulation that allows blood to be oxygenated, and the lung parenchyma to be perfused simultaneously.Lung maturation ensues throughout the embryonic period and continues into childhood (up to the age of 8). Embryonic lung maturation occurs in four stages that are regulated by intricately arranged biochemical cascades. These stages are known as the pseudoglandular, canalicular, terminal sac, and alveolar stages. Cells of the endodermal lining of the foregut are exposed to transcription factors such as thyroid transcription factor 1, homeobox proteins, hepatocyte nuclear factor, and other compounds. They induce the differentiation of foregut endoderm into committed respiratory cell lineage.The development of the lung parenchyma occurs in relation to the development of the bronchopulmonary tree. By the 5th gestational week, the splanchnic mesenchyme that surrounds the bronchial buds (terminal branches of the laryngotracheal diverticulum) begins to expand. The splanchnic mesenchyme also releases many signaling proteins (including fibroblast growth factor 10), which promotes the growth of the respiratory buds. Therefore the growth of the splanchnic mesenchyme occurs concurrently with the expansion and ramification of the bronchial buds. While the bronchial buds give rise to the bronchi and bronchioles, the splanchnic mesenchyme forms the lung parenchyma.
By the 6th gestational week, lung development enters the pseudoglandular stage. Histologically, the tissue is arranged in a tubuloacinar pattern; much like that observed in exocrine glands. These features are dominant throughout the early stage of pregnancy, up to around the 16th gestational week. At this stage, the maturing lung contains the conductive entities of the lung. However, the gas exchange components are not yet formed. Therefore, should a pregnant individual give birth at this gestational age, the infant is unlikely to survive since the lungs are immature.Within the lung, there is a disparity in the rate of development of the lung such that the lung apices mature faster than the lung bases. Consequently, while the lung bases may still be in the pseudoglandular phase, the apices would have transitioned into the canalicular phase. This period, which lasts from the 16th to the 26th gestational week, is characterized by the formation of the primordial alveolar ducts. The formation of these tubules is preceded by dilation of the bronchial lumen (and terminal bronchioles as well). Around the 24th gestational week, the terminal bronchioles divide to give at least two respiratory bronchioles. Each respiratory bronchiole subsequently arborizes to give up to six primordial alveolar ducts. There are terminal sacculations at the end of the primordial alveolar ducts known as primordial alveoli. The walls of these sacs are thin enough to facilitate gas exchange where the lungs acquire significant vascular trees at this point as well. Therefore, an infant born within this period has a chance at surviving. However, they are likely to face other challenges with other poorly developed organ systems.Towards the end of the 26th gestational week, more primordial alveoli, with notably thinner walls develop. In this terminal sac phase of lung development, the walls of the sacs are lined by endoderm-derived squamous epithelium. These cells are specifically referred to as type I pneumocytes and they facilitate gaseous exchange. Dispersed among the other squamous cells are the type II pneumocytes. These cells are round secretory cells that produce surfactant. Even though the lungs began producing and secreting surfactant as early as 20 to 22 weeks, the quantity significantly increases during the last few months of pregnancy. It should be noted that the terminal sac phase continues from the 26th gestational week through to delivery of the infant. Although there is an increase in the amount of surfactant being produced, it isn’t sufficient to efficiently support life prior to the 32nd gestational week. Therefore, infants born prior to 32 weeks have a fighting chance of surviving with medical intervention.
The final maturation of the primordial alveoli begins from the 32nd gestational week and may continue until the 8th year of extrauterine life. The stage begins with clusters of alveolar sacs at the end of the respiratory bronchioles. The sacs subsequently differentiate into alveolar ducts. Additionally, the vascular beds can be seen bulging into the alveolar walls as the sacs become progressively thinner. Eventually, the walls of the capillary beds and the type I pneumocytes become so closely related that they form the alveolocapillary membrane. This interface is the also known as the respiratory membrane or the pulmonary diffusion barrier. As the name suggests, it acts as the point of gas exchange between the alveoli and the surrounding capillaries. At this stage, the lungs are capable of supporting life; therefore, an infant born at this point and beyond is more likely to survive. Even though these significant events occur in utero, the majority of alveolar maturation occurs during extrauterine life. There is a significant increase in the number of primordial alveoli and respiratory bronchioles, as well as distension of the primordial alveoli after birth. Overall, both factors account for the increase in the size of the lungs; although the increase in the quantity of the respiratory entities plays a bigger role than does expansion of the primordial alveoli.
There are other events that occur within the uterus that promotes lung maturation. The fetal breathing movements and the amount of amniotic fluid present in the amniotic sac both influence the fetal lung development. Fetal breathing movements refer to practice motions that result in aspiration of some amniotic fluid into the lungs. They occur occasionally but become more frequent later on during the pregnancy.
Each lung occupies the respective hemithorax, with the mediastinum and its contents located between them. They are suspended freely within the pleural membrane and are only attached to the trachea and heart by the main bronchioles and pulmonary vessels, respectively.
The pleura is a serous membrane that envelopes each lung. The membrane is made up of a visceral layer that is adherent to the lung, and a parietal layer that is fixed to the inner thoracic wall, lower cervical vertebra, costovertebral area, mediastinum, and diaphragm. There is a potential space between the parietal and visceral layers known as the pleural cavity.
Each lung is characterized by an apex, a base, costal, medial and diaphragmatic surfaces, and anterior, posterior and inferior borders. The organs are roughly conical in shape and are divided by fissures into lobes. The left lung has two lobes and one fissure; while the right lung has three lobes and two fissures. The lobes are then further subdivided into bronchopulmonary segments; such that the left lung has 9 – 10 segments, while the right lung has 10. Between lobes are interlobar surfaces of lungs which are separated by fissures of the lungs.
The apex is the highest point of the lung, extending into the thoracic inlet. It is a dome-shaped part of the lung that protrudes above the first costal cartilage and the medial third of the clavicle. The medial surface of the lung is the part that faces the mediastinum and which has several parts, features, and impressions like the anterior and posterior mediastinal parts together with the cardiac impression and the hilum.
The base, which is also the diaphragmatic surface of the lungs, rests on the thoracic part of the diaphragm. On the left-hand side, the diaphragm separates the base of the lung from the spleen and stomach, while on the right it separates the lung from the liver. The costal surface of the lung is less remarkable than the medial surface. It is marked by the impressions of the overlying ribs and the grooves of the respective fissures.
The right ventricle pumps deoxygenated blood to the lungs through the pulmonary arteries. These vessels arise from the pulmonary trunk as it bifurcates behind the superior vena cava to give one vessel to each lung. Each pulmonary artery also undergoes a second division to produce superior and inferior branches. While the pulmonary arteries deliver low-pressured deoxygenated blood to the lungs, the bronchial arteries deliver oxygenated blood at high pressures to the organ and its supporting elements. The majority of individuals have two vessels on the left, and one on the right.
The lymphatic vessels that drain the lungs originate below the pleura and are referred to as the subpleural plexus. This group of vessels is further subdivided into superficial and deep groups.
The autonomic nervous system is responsible for dilating and constricting the airway and for regulating bronchial secretions. Branches from the vagus nerve and sympathetic branches from the cervical cardiac nerves unite to form the pulmonary plexus.
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