Lung

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.

Anatomy 

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 lung 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.

Blood supply

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.

Lymphatic drainage 

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.

Check out the following resources to learn more about the lymphatics of the lungs!

Innervation 

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.

Embryology 

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.

Related articles

Articles within this topic:

• Alveoli
• Bronchi
• Bronchial artery
• Bronchial veins
• Bronchiole
• Bronchopulmonary segments
• Carina of trachea
• Clinical case: Incidental pneumonectomy
• Clinical case: Penetration of the pericardium
• Clinical case: Pulmonary embolism
• Common lung diseases
• Costophrenic angle
• Hilum of the lung
• Parietal pleura
• Pleura
• Pulmonary arteries and veins
• Pulmonary trunk
• Trachea