Anatomy of the Lungs: Structure, Function, and Importance

The lungs are an essential organ of the respiratory system, responsible for gas exchange between the body and the environment. Each human body contains two lungs that occupy a significant portion of the thoracic cavity, leaving limited space for the heart, which excavates more of the left lung. The two lungs hold the heart tightly between them, providing it with the necessary protection.

To understand the anatomy of the lungs, we must explore their bronchopulmonary segments, lung parenchyma, and lung hilum. Each lung contains ten bronchopulmonary segments, and they are located in the pleural cavity, separated by the mediastinum. The lungs have a spongy texture, and inhaled carbon particles cause them to become mottled black over time. The right lung weighs approximately 700g, making it about 50 to 100g heavier than the left lung.

The left lung’s hilum displays a single bronchus situated posteriorly, bronchial vessels, and the posterior pulmonary plexus. The pulmonary artery lies above the bronchus, and the upper pulmonary vein is anterior to the bronchus, while the lower vein is below it. The mediastinal surface of the left lung has impressions of the left ventricle, ascending aorta, and descending thoracic aorta, with the esophagus leaving an impression in the lower part only.

Exploring the external features of the lungs is one of the essential intrinsic elements of the lungs’ anatomy. By understanding the lungs’ anatomy, we can appreciate their vital role in our respiratory system and the importance of keeping them healthy.

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Features of the lungs

The anatomy of both lungs is conical in shape and shares several key features.

They have an apex at the upper end and a base that rests on the diaphragm.

The lungs also have three borders:

Anterior
Posterior
Inferior

Two surfaces of the lungs exist:

Costal
Medial
- The medial surface further divides into Vertebral and Mediastinal parts.

Apex

The apex of the lungs is located at the highest point of the lung and is blunt in shape. It lies above the level of the anterior end of the first rib, reaching almost 2.5 cm above the medial one-third of the clavicle. It is situated just medial to the supraclavicular fossa, which is a small depression above the clavicle.

The apex of the lungs is covered by the cervical pleura, which is a thin layer of tissue that surrounds the lungs’ uppermost portion. Additionally, the suprapleural membrane covers the apex of the lungs. The suprapleural membrane is a thick layer of connective tissue that separates the lungs from the base of the neck’s soft tissue structures.

On the medial side of the apex of the lungs, the subclavian artery grooves the lung tissue. The subclavian artery is a large blood vessel that supplies blood to the arms and the head. The subclavian artery’s groove is a shallow indentation in the lung tissue, which is created by the pressure of the artery against the lung tissue.

The apex of the lungs has important clinical relevance. It is the site where many respiratory conditions, such as pulmonary embolism, may cause pain or discomfort. Additionally, the apex of the lungs is susceptible to developing tumors, such as lung cancer, due to its proximity to the clavicle and neck structures.

Base

The base of the lungs is semilunar and concave, and it rests upon the diaphragm, which is the primary muscle responsible for breathing. The diaphragm separates the thoracic cavity from the abdominal cavity and plays a crucial role in the mechanics of breathing. It contracts and flattens during inhalation, increasing the volume of the thoracic cavity and causing air to rush into the lungs. Conversely, it relaxes during exhalation, allowing the lungs to deflate and air to be expelled.

In addition to separating the lungs from the liver lobes, fundus of the stomach, and the spleen, the diaphragm is also connected to other structures, such as the ribcage, via its muscular fibers. The coordination of these structures during breathing is essential for the proper functioning of the respiratory system. Any dysfunction or injury to the diaphragm, such as diaphragmatic hernias or paralysis, can lead to breathing difficulties and require medical intervention.

Recent research has also shown that the diaphragm is not just a respiratory muscle but plays a vital role in the immune system as well. It contains immune cells and cytokines that help regulate immune responses in the lungs and other organs. This highlights the importance of the diaphragm beyond its traditional role in breathing and further emphasizes the need to study this muscle to better understand respiratory and immune diseases.

Anterior Border

The anterior border of the lungs is thinner than the posterior border and shorter in length. On the right side, it is vertical and aligns with the anterior or costomediastinal line of pleural reflection. The anterior border of the left lung exhibits a broad cardiac notch below the level of the fourth costal cartilage. The heart and pericardium are not covered by the lung in the region of the notch, leaving them exposed.

It is important to note that the anatomical features of the lungs can vary in different individuals due to a variety of factors, including age, sex, and underlying medical conditions. For instance, individuals with chronic obstructive pulmonary disease (COPD) may have emphysematous changes in their lungs that can alter the shape and position of the lungs, making them more prone to collapse.

Posterior Border

The posterior border of the lungs is thicker and less well-defined than the anterior border. It corresponds to the medial margins of the heads of the ribs and extends from the level of the seventh cervical spine to the tenth thoracic spine. The lungs are attached to the spinal column and the ribcage through muscles and ligaments, and this attachment is essential for the stability and proper function of the respiratory system.

Recent studies have highlighted the importance of the posterior lung segments, which are located along the posterior border of the lungs. These segments are critical for gas exchange, as they have a higher concentration of alveoli, which are the tiny air sacs in the lungs where oxygen and carbon dioxide are exchanged. Additionally, the posterior segments of the lungs are more prone to develop infections, such as pneumonia, due to their location near the mediastinum and the spinal column.

Inferior Border

The inferior border of the lungs separates the base of the lungs from the costal and medial surfaces. The lungs are enclosed within a double-layered membrane called the pleura, which forms a smooth, lubricated surface that allows the lungs to move smoothly against the chest wall during breathing.

The inferior border of the lungs plays a crucial role in maintaining the proper ventilation of the lungs. This is because the diaphragm, a dome-shaped muscle that separates the chest cavity from the abdominal cavity, contracts and relaxes during breathing, causing the lungs to expand and contract. The inferior border of the lungs moves up and down with the diaphragm, creating negative pressure that draws air into the lungs during inhalation.

Recent research has focused on the role of the inferior border of the lungs in the development of pulmonary hypertension, a condition characterized by high blood pressure in the pulmonary arteries that supply blood to the lungs. Studies have shown that changes in the shape and function of the lungs, including alterations in the position of the inferior border, can contribute to the development and progression of pulmonary hypertension.

Costal Surface

The costal surface of the lungs is large, convex, and in contact with the costal pleura and the overlying thoracic wall. This surface of the lungs is divided into upper, middle, and lower lobes by the oblique and horizontal fissures. The upper lobe is the largest and extends from the apex of the lung to the level of the fourth or fifth rib, while the middle lobe is located between the fourth and sixth ribs and the lower lobe extends from the sixth rib to the inferior border of the lung.

The costal surface of the lungs is critical for the proper function of the respiratory system. The lungs expand and contract during breathing, and the costal surface plays a vital role in facilitating this movement. The costal surface is covered by the costal pleura, a serous membrane that forms a smooth, lubricated surface between the lungs and the chest wall. This allows the lungs to move freely against the chest wall during breathing.

Medial Surface

The medial surface of the lungs is divided into a posterior or vertebral part and an anterior or mediastinal part. The vertebral part is related to the structures like vertebral bodies, intervertebral discs, the posterior intercostal vessels, and the splanchnic nerves, which are critical for the proper function of the respiratory system. The vertebral part of the medial surface is also in close proximity to the esophagus, aorta, and thoracic duct.

The mediastinal part of the medial surface is associated with the mediastinal septum and shows a cardiac impression, the hilum, and several other impressions that differ on the two sides. The hilum is a crucial structure that serves as the entry and exit point for the major vessels, bronchi, and lymphatic vessels that supply and drain the lungs. The cardiac impression on the left lung is deeper than the one on the right due to the location of the heart in the mediastinum.

Recent studies have highlighted the importance of the mediastinal part of the medial surface of the lungs in the diagnosis and treatment of various respiratory diseases. For example, in lung cancer, the location and extent of tumor invasion into the mediastinum can significantly impact the choice of treatment and prognosis. Advances in imaging techniques such as endobronchial ultrasound (EBUS) and mediastinoscopy have enabled pulmonologists to obtain precise and accurate diagnoses of mediastinal tumors and lymph nodes.

Fissures and lobes within the anatomy of the lungs

The division of the lung into lobes and fissures is important for functional and diagnostic purposes. The lobes and fissures help to isolate specific regions of the lungs, which can be helpful in determining the extent of diseases such as lung cancer or pneumonia.

The lobes of the lungs are further divided into smaller functional units called bronchopulmonary segments. These segments are separated by connective tissue septa and supplied by their own segmental bronchus, artery, and vein. There are about 10 bronchopulmonary segments in each lung.

The oblique fissure of the lung is important in lung surgery because it allows for the separation of the upper and lower lobes of the lung. The horizontal fissure, which is only present in the right lung, separates the middle lobe from the upper and lower lobes.

In addition, the location and appearance of the fissures and lobes of the lungs can vary among individuals. For example, some individuals may have an incomplete or absent horizontal fissure, or variations in the branching of the bronchial tree may lead to additional lobes or segments in the lungs.

Right Lung:

Divided into three lobes: upper, middle, and lower.
Separated by two fissures: oblique and horizontal.
The oblique fissure passes obliquely downwards and forwards, crossing the posterior border about 6cm below the apex and the inferior border about 5cm from the medial plane.
The horizontal fissure runs horizontally at the level of the fourth costal cartilage and meets the oblique fissure in the midaxillary line.

Left Lung:

Divided into two lobes: upper and lower.
Separated by one fissure: oblique.
The oblique fissure runs almost vertically downwards from the level of the sixth thoracic vertebra, crossing the fifth rib in the midclavicular line and the sixth rib in the midaxillary line.
The lingula, a tongue-shaped projection, can be found below the cardiac notch and is part of the left upper lobe.

The oblique fissure of the lungs separates the inferior lobes from the superior lobes in both lungs. It passes obliquely downwards and forwards, extending from the level of the spinous process of the fourth thoracic vertebra to the sixth costal cartilage anteriorly, where it meets the mediastinal surface. The fissure cuts into the whole thickness of the lung, except at the hilum where structures such as the pulmonary vessels and bronchi enter and exit. The oblique plane of the fissure results in the lower lobe being more posterior and the upper and middle lobes being more anterior. The oblique fissure is visible on chest X-rays and is important for identifying and diagnosing lung diseases.

The horizontal fissure in the right lung, also known as the minor fissure or the transverse fissure, separates the upper lobe from the middle lobe. It begins at the midaxillary line and runs horizontally towards the mediastinum, intersecting with the oblique fissure at the hilum. It is usually located at the level of the fourth rib and extends to the fifth rib. The middle lobe is wedge-shaped and is situated between the horizontal and oblique fissures, while the upper lobe lies superior to the horizontal fissure and anterior to the oblique fissure

The lingula of the left lung is a small, tongue-shaped projection of the upper lobe that lies inferior to the cardiac notch and anterior to the oblique fissure. It is considered to be the homologue of the middle lobe of the right lung, although it is smaller and less well-defined. The lingula is an important anatomical landmark, as it is adjacent to the left pulmonary artery and the left superior pulmonary vein, which are important structures in the circulation of the lung.

During inhalation, the diaphragm and external intercostal muscles contract, increasing the volume of the thoracic cavity and causing the lungs to expand. The maximal expansion of the lungs occurs in the inferior direction due to the maximal movements of the diaphragm and the thoracic wall towards the base of the lung.

The oblique fissure of each lung plays an important role in the expansion and contraction of the lungs. It allows for a more uniform expansion of the lung by separating the lobes and preventing the movement of air from one lobe to another.

Additionally, during expiration, the diaphragm and external intercostal muscles relax, decreasing the volume of the thoracic cavity and causing the lungs to recoil. The elastic recoil of the lungs is due to the elastin fibers in the lung tissue, which allow the lungs to return to their original size after inhalation.

It’s important to note that the lungs can also expand in the superior direction during forced inhalation, which involves the contraction of additional muscles such as the sternocleidomastoid, scalene, and pectoralis minor muscles. This type of inhalation is often seen during exercise or in individuals with respiratory conditions such as asthma.

Bronchopulmonary segments:

Bronchopulmonary segments are defined as the anatomic and functional units of the lung, which are supplied by specific segmental bronchi and arteries. These segments are separated from each other by connective tissue and have independent collateral ventilation. They are considered to be the smallest region of the lung that can be affected by disease or injury.

The right lung has 10 bronchopulmonary segments, while the left lung has 8 to 9. Each segment is named according to its location within the lung, and the segmental bronchus supplying it. For example, the right upper lobe contains three segments: apical, posterior, and anterior, which are supplied by the apical, posterior, and anterior segmental bronchi, respectively. The left upper lobe contains four segments: apicoposterior, anterior, superior lingular, and inferior lingular. The middle lobe of the right lung contains only one segment, while the left lung has no middle lobe. The lower lobe of both lungs contains five segments each.

Knowledge of bronchopulmonary segments is important for diagnostic and therapeutic procedures, such as bronchoscopy, lobectomy, and segmentectomy. It also plays a crucial role in understanding the spread of pulmonary diseases, such as lung cancer, tuberculosis, and pneumonia, as well as in interpreting radiological imaging.

Lung Parenchyma:

Lung parenchyma is the essential functional tissue of the lungs that is responsible for gas exchange. It is made up of the alveoli, which are tiny air sacs, as well as the alveolar ducts and respiratory bronchioles. The alveoli are lined with thin walls of epithelial cells, allowing for the efficient exchange of oxygen and carbon dioxide.

The lung parenchyma is also important in regulating blood flow and airway resistance within the lungs. It contains a network of small blood vessels called capillaries, which allow for the exchange of oxygen and carbon dioxide between the lungs and the rest of the body. The parenchyma also includes smooth muscle and elastic fibers that help to regulate the size and shape of the airways.

In addition to its role in gas exchange, the lung parenchyma is also involved in the immune system. It contains specialized cells that help to defend against infections and foreign substances that may enter the lungs.

It’s worth noting that lung parenchyma can sometimes be damaged, for example, in conditions like emphysema or fibrosis, leading to compromised lung function and difficulties in breathing.

Lung Hilum:

The lung hilum, also known as the pulmonary hilum, is a depression located on the medial aspect of the lung, where the bronchi, pulmonary vessels, and nerves enter and exit the lungs. It is considered the point of entry and exit for the lungs, providing an opening for various structures, including the pulmonary artery, pulmonary vein, and main bronchus.

The pulmonary hilum is covered by a serous membrane called the pulmonary ligament, which allows for movement of the lung during breathing. The lung root, consisting of the bronchi, pulmonary artery and vein, and nerves, attaches to the lung hilum, securing the lung in place.

The lung hilum is also an important site for medical procedures, such as bronchoscopy, where a flexible tube is inserted through the mouth or nose and into the airways to examine the lungs, and pulmonary artery catheterization, where a catheter is inserted through the pulmonary artery to measure blood pressure and obtain blood samples. In addition, abnormalities in the lung hilum, such as enlarged lymph nodes, can be detected through imaging studies like chest X-rays and CT scans, and may be indicative of various lung conditions, including cancer and infections.

The Root of the lungs

The root of the lung is a complex structure that attaches the lung to the mediastinum, which is the central compartment of the thoracic cavity that contains the heart, great vessels, trachea, esophagus, thymus, and other structures. The lung root consists of various structures, including the pulmonary artery, pulmonary veins, bronchi, bronchial arteries and veins, lymphatics, and nerves.

The pulmonary artery carries deoxygenated blood from the right ventricle of the heart to the lungs for oxygenation, while the pulmonary veins carry oxygenated blood from the lungs to the left atrium of the heart. The bronchi are the air passages that carry air from the trachea to the lungs, where they branch into smaller bronchioles that end in the alveoli, which are the tiny air sacs in the lung where gas exchange occurs. The bronchial arteries and veins supply blood to the lung tissue, while the lymphatics drain lymph from the lung tissue and drain into lymph nodes in the mediastinum.

The position of the lung root varies slightly between the left and right lungs. The right lung root is slightly higher than the left and is situated behind the superior vena cava, while the left lung root is situated inferiorly and anteriorly to the aortic arch and descending thoracic aorta. The roots of the lungs lie opposite the bodies of the fifth, sixth, and seventh thoracic vertebrae.

The roots of the lungs are important structures that connect the lungs to the mediastinum. They consist of several structures that enter or leave the lungs at the hilum. Here are some details about the structures that form the roots of the lungs:

Principal bronchus on the left side, and eparterial and hyparterial bronchi on the right side – The left lung has a single main bronchus that arises from the trachea, while the right lung has two main bronchi, the eparterial and hyparterial bronchi, which further divide into lobar bronchi.
One pulmonary artery – The pulmonary artery carries deoxygenated blood from the right ventricle of the heart to the lungs for oxygenation.
Two pulmonary veins, superior and inferior – The oxygenated blood from the lungs is returned to the heart through the two pulmonary veins, superior and inferior.
Bronchial arteries one on the right side and two on the left side – The bronchial arteries supply oxygenated blood to the lung tissue and the supporting structures of the lungs, such as the bronchi and the pulmonary vessels. The right lung has a single bronchial artery, while the left lung has two.
Bronchial veins – The bronchial veins carry deoxygenated blood from the lung tissue and supporting structures back to the systemic circulation.
Anterior and posterior pulmonary plexuses of nerves – The pulmonary plexuses are networks of nerves that provide sensory and autonomic innervation to the lungs. They are located on the surface of the bronchi and vessels in the lung roots.
Lymphatics of the lung – The lymphatic vessels of the lung drain the lymphatic fluid and immune cells from the lung tissue and the bronchopulmonary lymph nodes.
Bronchopulmonary lymph nodes – The bronchopulmonary lymph nodes are located at the hilum of the lung and are responsible for filtering the lymphatic fluid and immune cells from the lungs.
Areolar tissue – The areolar tissue surrounds the structures in the lung root and provides support and protection to the roots of the lungs.

Arterial supply of the anatomy of the lungs

The arterial supply of lungs is one of the main components in the anatomy of the lungs. The bronchial arteries supply nutrition to the bronchial tree and the pulmonary tissues. These are small arteries that vary in number, size, and origin but usually, they follow:

On the right side, there is one bronchial artery that arises from the third right posterior intercostal artery.
On the left side, there are two bronchial arteries, both of which arise from the descending thoracic aorta, the upper opposite fifth thoracic vertebrae and the lower just below the left bronchus.’

Deoxygenated blood is brought to the left lungs by the two pulmonary arteries, and oxygenated blood is returned to the heart by four pulmonary veins.

There are precapillary anatomizes between bronchial and pulmonary arteries. These connections enlarge when any one of them is obstructed in disease.

Venous drainage of the lungs

Bronchial veins carry the venous blood from the first and second divisions of the bronchi. Usually, there are two bronchial veins on each side. The right bronchial veins drain into the azygous vein. The left bronchial veins drain into the hemizygous vein.

The pulmonary veins drain the more significant part of the venous blood from the lungs. Venous drainage of the lungs plays a role in the anatomy of the lungs.

Lymphatics drainage of the lungs

There are two sets of the lymphatics, both of which drain into the bronchopulmonary nodes of the anatomy of the lungs.

Superficial vessels drain the peripheral lung tissue lying beneath the pulmonary pleura. The vessels canyon around the borders of the lung and margins of the fissures to reach the hilum.
Deep lymphatics drain the bronchial tree, the pulmonary vessels, and the connective tissue septa. They amble towards the hilum where they drain into the bronchopulmonary nodes.

In the anatomy of the lungs, the superficial vessels have numerous valves, and the deep vessels have only a few valves or no valves at all. Though there is no free anastomosis between the superficial and deep vessels, some connections exist which can open up, so that lymph can flow from the deep to the superficial lymphatics when the deep vessels are obstructed in the disease in the anatomy of the lungs or of the lymph nodes.

Nerve supply in the anatomy of the lungs

Parasympathetic nerves are derived from the vagus.

These fibers are

Motor to the bronchial muscles, and on stimulation cause bronchospasms.
Secretomotor to the mucous glands of the bronchial tree.
Sensory fibers are responsible for the stretch reflex of the lungs, and the cough reflex.
Sympathetic nerves are derived from the second to fifth sympathetic ganglia. These are inhibitory to the smooth muscle and glands of the bronchial tree. That is how sympathomimetic drugs like adrenaline, cause bronchodilatation, and relieve symptoms of bronchial asthma.

Both parasympathetic and sympathetic nerves first form anterior and posterior pulmonary plexuses situated in front of and behind the lung roots. From the plexuses, nerves are distributed to the lungs, along the blood vessels and bronchi. Here anatomy of the lungs ends with the nerve supply of the lungs.

Takeaway:

In the anatomy of the lungs, the lungs are a pair of respiratory organs situated in the thoracic cavity. Each lung invaginates the corresponding pleural cavity. The mediastinum separates the right and left lungs. The right lung is divided into three lobes (upper, middle, and lower) by two fissures, oblique and horizontal. The left lung is branched into two lobes by the oblique fissure. If the part of the lung that is supplied by specific segment bronchus and arteries then it is called Bronchopulmonary segments. Lung parenchyma is also known as the pulmonary parenchyma which is the portion of the lungs involved in the gaseous transfer.