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Medicine LibreTexts

20.3: The Lungs

  • Page ID
    22401
  • By the end of this section, you will be able to:

    • Describe the overall function of the lungs
    • Outline the anatomy of the blood supply to the lungs
    • Describe the pleurae of the lungs and their function

    A major organ of the respiratory system, each lung houses structures of both the conducting and respiratory zones. The main function of the lungs is to perform the exchange of oxygen and carbon dioxide with air from the atmosphere. To this end, the lungs exchange respiratory gases across a very large epithelial surface area—about 70 square meters—that is highly permeable to gases.

    Gross Anatomy of the Lungs

    The lungs are pyramid-shaped, paired organs that are connected to the trachea by the right and left bronchi; on the inferior surface, the lungs are bordered by the diaphragm. The diaphragm is the flat, dome-shaped muscle located at the base of the lungs and thoracic cavity. The lungs are enclosed by the pleurae, which are attached to the mediastinum. The right lung is shorter and wider than the left lung, and the left lung occupies a smaller volume than the right. The cardiac notch is an indentation on the medial surface of the left lung that allows space for the heart (Figure \(\PageIndex{1}\)). The apex of the lung is the superior region, whereas the base is the opposite region near the diaphragm. The costal surface of the lung borders the ribs. The mediastinal surface faces the midline. The primary bronchi enter the lungs at the hilum, a concave region facing the mediastinum where blood vessels, lymphatic vessels, and nerves also enter the lungs. Collectively this grouping of the bronchi, vessels, and nerves is known as the lung root.

    Diagram of the gross anatomy of the lungs.Figure \(\PageIndex{1}\): Gross Anatomy of the Lungs. The left lung is divided into just two lobes and has less mass due to the cardiac notch while the right lung is divided into three lobes. The right superior lobe is separated from the right middle lobe by the right horizontal fissure. The right middle lobe is separated from the right inferior lobe by the right oblique fissure. The left superior lobe is separated from the right inferior lobe by the left oblique fissure. The main (primary) bronchi deliver air from the trachea into each lung. The lobar (secondary) bronchi deliver air into the lobes of each lung. The segmental (tertiary) bronchi deliver air into the bronchopulmonary segments of each lobe. (Image credit: "Gross Anatomy of the Lungs" by Julie Jenks is licensed under CC BY 4.0 / A derivative from the original work)

    Each lung is composed of smaller units called lobes. Deep grooves called fissures separate these lobes from each other; the lobes are only connected near the mediastinum where connective tissue anchors the lungs in place as well as the major pulmonary arteries and veins routing into each lung. The right lung consists of three lobes: the superior, middle, and inferior lobes. The right superior and middle lobes are separated by the right horizontal fissure, while the right middle and inferior lobes are separated by the right oblique fissure. The left lung consists of two lobes: the superior and inferior lobes, separated by the left oblique fissure. A bronchopulmonary segment is a division of a lobe, and each lobe houses multiple bronchopulmonary segments. Each segment receives air from its own tertiary bronchus and is supplied with blood by its own artery. Some diseases of the lungs typically affect one or more bronchopulmonary segments, and in some cases, the diseased segments can be surgically removed with little influence on neighboring segments. A pulmonary lobule is a subdivision formed as the bronchi branch into bronchioles. Each lobule receives its own large bronchiole that has multiple branches. An interlobular septum is a wall, composed of connective tissue, which separates lobules from one another.

    Blood Supply and Nervous Innervation of the Lungs

    The blood supply of the lungs plays an important role in gas exchange and serves as a transport system for gases throughout the body. In addition, innervation by both the parasympathetic and sympathetic nervous systems provides an important level of control through dilation and constriction of the airways.

    Blood Supply

    The major function of the lungs is to perform gas exchange, which requires blood from the pulmonary circulation. This blood supply contains deoxygenated blood and travels to the lungs where erythrocytes, also known as red blood cells, pick up oxygen to be transported to tissues throughout the body. The pulmonary arteries are arteries that arise from the pulmonary trunk and carry deoxygenated, arterial blood to each lung. Each pulmonary artery branches multiple times as it follows the bronchi, and each branch becomes progressively smaller in diameter. One arteriole and an accompanying venule supply and drain one pulmonary lobule. As they near the alveoli, the pulmonary arterioles become the pulmonary capillary network. The pulmonary capillary network consists of tiny vessels with very thin walls that lack smooth muscle fibers. The capillaries branch and follow the bronchioles and structure of the alveoli. It is at this point that the capillary wall meets the alveolar wall, creating the respiratory membrane. Once the blood is oxygenated, it drains from the alveoli by way of multiple pulmonary veins, which exit the lungs through the hilum.

    Nervous Innervation

    Dilation and constriction of the airway are achieved through nervous control by the parasympathetic and sympathetic nervous systems. The parasympathetic system causes bronchoconstriction, whereas the sympathetic nervous system stimulates bronchodilation. Reflexes such as coughing, and the ability of the lungs to regulate oxygen and carbon dioxide levels, also result from this autonomic nervous system control. Sensory nerve fibers arise from the vagus nerve, and from the second to fifth thoracic ganglia. The pulmonary plexus is a region on the lung root formed by the entrance of the nerves at the hilum. The nerves then follow the bronchi in the lungs and branch to innervate muscle fibers, glands, and blood vessels.

    Pleurae of the Lungs

    Each lung is enclosed within a cavity that is surrounded by the pleurae. The pleura (plural = pleurae) is a serous membrane that surrounds each lung. The right and left pleurae, which enclose the right and left lungs, respectively, are separated by the mediastinum. The pleurae consist of two layers. The visceral pleura is the layer that is superficial to the lungs, and extends into and lines the lung fissures (Figure \(\PageIndex{2}\)). In contrast, the parietal pleura is the outer layer that connects to the thoracic wall, the mediastinum, and the diaphragm. The visceral and parietal pleurae connect to each other at the hilum. The pleural cavity is the space between the visceral and parietal layers.

    The pleurae perform two major functions: They produce pleural fluid (serous fluid) and create cavities that separate the lungs from each other and other thoracic structures. Pleural fluid is secreted by mesothelial cells from both pleural layers and acts to lubricate their surfaces. This lubrication reduces friction between the two layers to prevent trauma during breathing, and creates surface tension that helps maintain the position of the lungs against the thoracic wall. This adhesive characteristic of the pleural fluid causes the lungs to enlarge when the thoracic wall expands during ventilation, allowing the lungs to fill with air. The pleurae also create a division between major organs that prevents interference due to the movement of the organs, while preventing the spread of infection.

    Diagram of the pleurae of the lungs. The parietal pleura lines the inside of the rib cage while the visceral pleura covers the superficial surface of the lobes of the lungs.
    Figure \(\PageIndex{2}\): Parietal and Visceral Pleurae of the Lungs. The pleural sac is formed by the parietal and visceral pleura of the lungs. The parietal pleura is attached to the deep surface of the chest wall, which includes the ribs, sternum, thoracic vertebrae, connective tissue of the mediastinum, internal intercostal muscles, and the superior surface of the diaphragm. The visceral pleura covers the superficial surface of the lobes of the lungs. The pleural cavity is the potential space between the layers that is filled with lubricating serous fluid. (Image credit: "The Lung Pleurae" by OpenStax is licensed under CC BY 3.0)

    DISORDERS OF THE...

    Respiratory System: Pneumothorax

    A pneumothorax is a condition in which a rupture caused by trauma or disease introduces air into the pleural cavity. The air increases pressure on the surface of the lung on the affected side, breaking the surface tension that adheres the visceral pleura to the parietal pleura throughout increases and decreases in the volume of the thoracic cavity during ventilation. With the surface tension between the pleurae broken, the lung collapses due to elastic recoil (Figure \(\PageIndex{3}\)). Treatment may include manual aspiration of the air, along with treatment of the underlying cause of the pneumothorax.

    Pneumothorax on the left, as indicated by air in the pleural cavity and collapse of the left lung.Figure \(\PageIndex{3}\): Pneumothorax. A pneumothorax is caused by air in the pleural cavity, which increases pressure on the lung, causing it to collapse. (Image credit: "Blausen 0742 Pneumothorax" by Bruce Blaus Blausen.com staff, Medical gallery of Blausen Medical is licensed under CC BY 3.0)

    EVERYDAY CONNECTION

    The Effects of Second-Hand Tobacco Smoke

    The burning of a tobacco cigarette creates multiple chemical compounds that are released through mainstream smoke, which is inhaled by the smoker, and through sidestream smoke, which is the smoke that is given off by the burning cigarette. Second-hand smoke, which is a combination of sidestream smoke and the mainstream smoke that is exhaled by the smoker, has been demonstrated by numerous scientific studies to cause disease. At least 40 chemicals in sidestream smoke have been identified that negatively impact human health, leading to the development of cancer or other conditions, such as immune system dysfunction, liver toxicity, cardiac arrhythmias, pulmonary edema, and neurological dysfunction. Furthermore, second-hand smoke has been found to harbor at least 250 compounds that are known to be toxic, carcinogenic, or both. Some major classes of carcinogens in second-hand smoke are polyaromatic hydrocarbons (PAHs), N-nitrosamines, aromatic amines, formaldehyde, and acetaldehyde.

    Tobacco and second-hand smoke are considered to be carcinogenic. Exposure to second-hand smoke can cause lung cancer in individuals who are not tobacco users themselves. It is estimated that the risk of developing lung cancer is increased by up to 30 percent in nonsmokers who live with an individual who smokes in the house, as compared to nonsmokers who are not regularly exposed to second-hand smoke. Children are especially affected by second-hand smoke. Children who live with an individual who smokes inside the home have a larger number of lower respiratory infections, which are associated with hospitalizations, and higher risk of sudden infant death syndrome (SIDS). Second-hand smoke in the home has also been linked to a greater number of ear infections in children, as well as worsening symptoms of asthma.

    Concept Review

    The lungs are the major organs of the respiratory system and are responsible for performing gas exchange. The lungs are paired and separated into lobes; The left lung consists of two lobes, whereas the right lung consists of three lobes. Blood circulation is very important, as blood is required to transport oxygen from the lungs to other tissues throughout the body and deliver carbon dioxide to the lungs for removal. The function of the pulmonary circulation is to aid in gas exchange. The pulmonary artery provides deoxygenated blood to the capillaries that form respiratory membranes with the alveoli, and the pulmonary veins return newly oxygenated blood to the heart for further transport throughout the body. The lungs are innervated by the parasympathetic and sympathetic nervous systems, which coordinate bronchodilation and bronchoconstriction of the airways. The lungs are enclosed by the pleurae, a membrane that is composed of visceral and parietal pleural layers. The space between these two layers is called the pleural cavity. The mesothelial cells of the pleural membrane create pleural (serous) fluid, which serves as both a lubricant (to reduce friction during breathing) and as an adhesive to adhere the lungs to the thoracic wall (to facilitate movement of the lungs during ventilation).

    Review Questions

    Q. Which of the following structures separates the lung into lobes?

    A. mediastinum

    B. fissure

    C. root

    D. pleura

    Answer

    B

    Q. A section of the lung that receives its own tertiary bronchus is called the ________.

    A. bronchopulmonary segment

    B. pulmonary lobule

    C. interpulmonary segment

    D. respiratory segment

    Answer

    A

    Q. The ________ circulation picks up oxygen for cellular use and drops off carbon dioxide for removal from the body.

    A. pulmonary

    B. interlobular

    C. respiratory

    D. bronchial

    Answer

    A

    Q. The pleura that surrounds the lungs consists of two layers, the ________.

    A. visceral and parietal pleurae

    B. mediastinum and parietal pleurae

    C. visceral and mediastinum pleurae

    D. none of the above

    Answer

    A

    Critical Thinking Questions

    Q. Compare and contrast the right and left lungs.

    Answer

    The right and left lungs differ in size and shape to accommodate other organs that encroach on the thoracic region. The right lung consists of three lobes separated by two fissures. The left lung consists of two lobes separated by one fissure and it has a concave region on the mediastinal surface called the cardiac notch that allows space for the heart.

    Q. Why doesn't the change in size of the lungs during normal breathing damage the pleurae?

    Answer

    There is a cavity, called the pleural cavity, between the parietal and visceral layers of the pleura. Mesothelial cells produce and secrete pleural (serous) fluid into the pleural cavity that acts as a lubricant and also creates surface tension that adheres the layers so they stay together as the lungs expand and contract.

    Glossary

    bronchoconstriction
    decrease in the size of the bronchiole due to contraction of the muscular wall
    bronchodilation
    increase in the size of the bronchiole due to contraction of the muscular wall
    bronchopulmonary segment
    division of a lobe of the lung that receives air from a single tertiary (segmental) bronchus
    cardiac notch
    indentation on the surface of the left lung that allows space for the heart
    hilum
    concave structure on the mediastinal surface of the lungs where blood vessels, lymphatic vessels, nerves, and a bronchus enter the lung
    lobe
    large division of lung tissue receiving air from a single secondary (lobar) bronchus
    lung
    organ of the respiratory system that performs gas exchange
    lung root
    collection of the primary bronchus, blood vessels, lymphatic vessels, and nerves that enters each lung at the hilum
    parietal pleura
    outermost layer of the pleura that connects to the thoracic wall, mediastinum, and diaphragm
    pleural cavity
    space between the visceral and parietal pleurae
    pleural fluid
    substance that acts as a lubricant for the visceral and parietal layers of the pleura during the movement of breathing
    pulmonary artery
    artery that arises from the pulmonary trunk and carries deoxygenated, arterial blood to the alveoli
    pulmonary lobule
    subdivision of lung that receives air from a single large bronchiole as it branches from a bronchus
    pulmonary plexus
    network of autonomic nervous system fibers found near the hilum of the lung
    visceral pleura
    innermost layer of the pleura that is superficial to the lungs and extends into the lung fissures

    References

    McKnight CL, Burns B. Pneumothorax. 2020 Nov 16. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan–. PMID: 28722915. Licensed under CC BY 4.0. [Accessed 30 Apr 2021]

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