Lungs

Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. To perform this function, the lungs need to be able to capture as much O2 as possible, a task that is easily achieved owing to their elastic morphology. Each lung is enclosed within the visceral pleura and completely fills 1 of the non-symmetrical pleural cavities, which are situated on the left and right sides of the mediastinum. The lungs encase the bronchial tree and are divided functionally and anatomically into lobes.

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Development

Development of the tracheobronchial tree and lungs occurs in 5 stages. The tracheobronchial tree originates from the foregut of the embryonic gut tube, beginning at week 4 of gestation and ending in childhood.

Table: Development of the tracheobronchial tree and lungs, and clinical relevance
StageDescriptionClinical relevance
Embryonic period
  • Occurs during weeks 4–7
  • Respiratory diverticulum buds off the foregut.
  • Tracheoesophageal groove “pinches off” the bud → trachea and esophagus
  • Trachea bifurcates into right and left bronchial buds.
Defects:
  • Tracheoesophageal fistula
  • Esophageal and/or tracheal atresia
  • Pulmonary sequestration
Pseudoglandular period
  • Occurs during weeks 5–16
  • Bronchial buds → secondary buds → tertiary buds
  • Continued branching → terminal bronchioles
  • Mesoderm → pulmonary vasculature/capillaries
  • Development of type II pneumocyte precursors → produce amniotic fluid
  • Defects:
    • Bronchogenic cyst
    • Bronchial atresia
  • Lung tissue is incapable of gas exchange at this stage.
  • Infants born at this stage cannot survive.
Canalicular period
  • Occurs during weeks 16–26
  • Terminal bronchioles → respiratory bronchioles → alveolar ducts → primitive alveoli
  • Prominent lung capillaries
  • Surfactant production
  • Airway diameter ↑
  • Defects:
    • Pulmonary hypoplasia
    • Respiratory distress syndrome
  • Respiration is possible at 24 weeks.
  • Infants born at the end of this stage can survive with intensive care.
Saccular period
  • Occurs during week 26–birth
  • Alveolar ducts → terminal sacs
  • Gas-exchange surface area of the lungs expands.
  • Surfactant production increases.
  • Blood–air barrier develops fully (type I pneumocytes).
Infants born after 32 weeks or more have a higher survival rate.
Alveolar period
  • Occurs between 32 weeks’ gestation to 8 years of life
  • Mature type II pneumocytes
  • Terminal sacs septate → alveoli
  • Following birth, alveoli ↑ in number:
    • At birth: 50 million
    • By 8 years of age: 300 million
  • In utero: ↑ vascular resistance due to aspiration of amniotic fluid
  • Postpartum: Inspiration of air leads to a drop in pulmonary vascular resistance.

Early stage of lung development:
During the early (embryonic and pseudoglandular) stages of lung development, the lung bud gradually transforms into the trachea and then bifurcates, forming the 2 mainstem bronchi. These bronchi further divide, giving rise to lobar and segmental bronchi.

Image: “Weeks 4–7” by Phil Schatz. License: CC BY 4.0

Gross Anatomy

General characteristics

  • The lungs are paired viscera composed of spongy tissue. 
  • Asymmetrical, with each lung having a slightly different morphology and weight: 
    • Left lung is slightly smaller than the right lung.
    • On average, lungs are heavier in men than in women.
  • Location and spatial relations:
    • Located in the thoracic cavity on either side of the mediastinum
    • Surround the bronchial tree
    • Enclosed within the visceral pleura
    • Attached to the trachea via the main bronchi of the tracheobronchial tree
    • Attached to the heart via pulmonary vessels
  • Basic structure: 
    • Each lung has a base, apex, 2 surfaces, and 3 borders:
      • The base sits on the diaphragm.
      • The apex projects above the 1st rib.
      • 1 surface faces the ribs (costal); the other faces medially (mediastinal) and contains the hilum
      • 3 borders: inferior, posterior, and anterior
  • Hilum:
    • Located between T5 and T7
    • Constitutes the roots of the lungs, a pleura-encased passageway for the pulmonary neurovasculature
    • The following structures pass through the hilum of each lung:
      • Principal bronchus
      • Pulmonary artery
      • 2 pulmonary veins
      • Bronchial vessels
      • Pulmonary autonomic plexus
      • Lymph nodes and vessels
      • Connective tissue

Basic structure of the lung:
The lung is composed of an apex, base, 3 borders (inferior, anterior, and posterior), and 2 surfaces (mediastinal and costal).

Image by Lecturio.

Right versus left lung

The 2 lungs are not symmetrical and vary both in weight and morphology. This difference is due to the size and location of the heart in the left hemithorax. 

Right lung:

  • 3 lobes: superior, middle, and inferior 
  • 2 fissures: oblique and horizontal 
  • Its medial or mediastinal surface is adjacent to:
    • Heart
    • Inferior vena cava
    • Superior vena cava
    • Azygos vein 
    • Esophagus

Left lung:

  • 2 lobes: superior and inferior
  • 1 fissure: oblique
  • Its medial or mediastinal surface is adjacent to:
    • Heart
    • Aortic arch
    • Thoracic aorta
    • Esophagus 
  • The cardiac notch is located on the anteroinferior aspect of the superior lobe of the left lung on the anterior border to make room for the heart.

Lobes and fissures of the lungs

Image by Lecturio.

Bronchopulmonary segments

Beyond the lobes and following the branching of the bronchial tree, the lungs are divided into bronchopulmonary segments, which are the largest functional divisions of the pulmonary lobes.

  • Each segment has its own air and blood supplies.
    • Air supply: tertiary or segmental bronchi
    • Blood supply: branch of the bronchial arteries
  • Asymmetrical distribution of segments between the right and left lungs:
    • Right lung: 10 segments
    • Left lung: 8–10 segments (some may fuse)
Table: Segments of the lungs
Left lungRight lung
Superior lobe:
  • Apico-posterior segment (I + II)
  • Anterior segment (III)
  • Superior lingular segment (IV)
  • Inferior lingular segment (V)
Superior lobe:
  • Apical segment (I)
  • Posterior segment (II)
  • Anterior segment (III)
Middle lobe:
  • Lateral segment (IV)
  • Medial segment (V)
Inferior lobe:
  • Superior segment (VI)
  • Anterior-basal segment (VII + VIII)
  • Lateral-basal segment (IX)
  • Posterior-basal segment (X)
Inferior lobe:
  • Superior segment (VI)
  • Medial-basal segment (VII)
  • Anterior-basal segment (VIII)
  • Lateral-basal segment (IX)
  • Posterior-basal segment (X)

The bronchopulmonary segments of the lungs

Image by Lecturio.

Microscopic Anatomy

The microscopic anatomy of the bronchial tree varies as the airways ramify. In general, the respiratory tract becomes smaller in diameter and has thinner walls with every ramification.

Table: Microscopic anatomy of the lungs
BronchiBronchiolesRespiratory bronchiolesAlveoli
EpitheliumPseudostratified ciliated columnar epitheliumCiliated columnar to cuboidal epitheliumNon-ciliated simple cuboidal epitheliumSimple squamous epithelium
Special cellsGoblet cells (mucus)Club or Clara cells (a component of surfactant)Club or Clara cells (a component of surfactant)Type I and II pneumocytes
Smooth muscleX
CartilageXXX

Alveoli

  • Hollow sacs that serve as the site of gas exchange
  • Found in respiratory bronchioles along the walls ducts, and in the alveolar sacs
  • Separated by interalveolar septa, which is made of elastic fibers and capillaries
  • Lined by type I and II pneumocytes:
    • Type I: comprise 95% of the total alveolar area and form the blood–air barrier
    • Type II: comprise 5% of the total alveolar area and secrete surfactant

Schematic diagram of the structure of the respiratory zone of the lower respiratory tract: Note the composition of the alveolar wall.
1: Respiratory bronchiole
2: Primary septum
3: Alveolar sac
4: Capillaries
5: Type II pneumocyte
6: Type I pneumocyte
7: Alveolar duct

Image by Lecturio.

Neurovasculature

Blood supply

  • The vessels enter the lungs at the hilum.
  • The lung has a dual circulatory system:
    • Pulmonary arteries:
      • Originate as the pulmonary trunk
      • Branch into left and right pulmonary arteries and then into lobar and segmental branches according to the branching of the bronchial tree
      • Carry oxygen-poor blood from the right ventricle to be oxygenated in the lungs
    • Pulmonary veins:
      • Originate from the millions of alveoli in the lungs
      • The tributaries merge following the branching of the bronchial tree.
      • Become the right and left, and superior and inferior pulmonary veins
      • Drain into the left atrium as 4 veins 
      • Carry oxygen-rich blood from the lungs to the left atrium to be distributed throughout the body

Innervation

The lung receives mixed innervation from the pulmonary plexus containing the parasympathetic, sympathetic, and visceral afferent fibers.

  • Parasympathetic fibers: 
    • Presynaptic fibers from the vagus nerve
    • These fibers synapse with parasympathetic ganglion cells.
    • Motor innervation to the smooth muscle of the bronchial tree (constrictor), pulmonary vessels (dilator), and glands of the bronchial tree (secretory-motor)
  • Sympathetic fibers:
    • Postsynaptic fibers with cell bodies in the paravertebral sympathetic ganglia (sympathetic trunks)
    • Innervation to the smooth muscle of the bronchial tree (dilator), pulmonary vessels (constrictor), and type II secretory cells of alveoli (inhibitor)
  • Visceral fibers:
    • Reflexive: conduct subconscious sensations connected to the regulation and control of reflexes (accompany parasympathetic fibers)
    • Nociceptive: pain as a response to injury (accompany sympathetic fibers)

Functions

The main role of the lungs is to oxygenate the body and rid it of CO₂. 

Ventilation:

  • Movement of air between the environment and the lungs via inhalation and exhalation
  • Requires assistance of the intercostal muscles, diaphragm, and accessory muscles of respiration

Respiration or gas exchange:

  • Removal of CO₂ from the blood into the air, and absorption of oxygen from environmental air into the blood
  • Relies on the concentration and structural integrity of hemoglobin

Respiratory regulation:

  • By the chemoreceptors in the lungs and tissues that sense changes in the concentration of oxygen and CO₂
  • By the respiratory center of the brain, with constant feedback from the central and peripheral chemoreceptors

Clinical Relevance

The following conditions of various origins can affect the lungs:

  • Infectious:
    • Pneumonia: acute or chronic inflammation of the lung tissue commonly known as pulmonary inflammation. Causes of pneumonia include bacterial, viral, or fungal infections. In rare cases, pneumonia can also be caused by toxic triggers via inhalation, in immunological processes, or during the course of radiotherapy.
    • Tuberculosis: an infectious disease caused by bacteria of the Mycobacterium tuberculosis complex, which usually attack the lungs, but can also affect other parts of the body. Tuberculosis spreads through air when an individual with an active pulmonary infection coughs or sneezes. Diagnosis is established using the tuberculin skin test, sputum culture, and lung imaging. Management is with antimycobacterial drugs.
    • COVID-19: an infectious disease caused by SARS-CoV-2 that mainly affects the respiratory system, but can also damage other organs. Coronavirus disease 2019 may present asymptomatically as a mild “flu-like” illness, or severely, with shortness of breath and life-threatening complications. Management is based on supportive care and antiviral medications.
  • Malignant:
    • Lung cancer: a condition associated with a relatively high risk of mortality. Lung cancer has become the leading cause of death among men worldwide. This malignant tumor originates from the respiratory epithelium of the bronchi, bronchioles, and alveoli. In 90% of men and 80% of women, bronchial carcinoma appears to be associated with the inhalation of tobacco smoke.
  • Obstructive:
    • Asthma: a chronic inflammatory respiratory condition characterized by bronchial hyperresponsiveness and airflow obstruction. Asthma presents with wheezing, cough, and dyspnea. The diagnosis is confirmed with pulmonary function tests, which show a reversible obstructive pattern. Treatment of asthma is with bronchodilators and inhaled corticosteroids. 
    • Chronic obstructive pulmonary disease (COPD): a lung disease characterized by progressive, irreversible airflow obstruction. Chronic obstructive pulmonary disease presents with progressive dyspnea, chronic cough, prolonged expiration, and wheezing. The diagnosis is based on pulmonary function tests. Management includes smoking cessation, pulmonary rehabilitation, and pharmacotherapy.
    • Emphysema: a condition characterized by dilation of the airways, with decreased elasticity and increased compliance, due to destruction of the alveolar walls. The most common cause of emphysema is smoking. Alpha-1-antitrypsin deficiency may also result in the development of emphysema.
    • Atelectasis: a partial or complete collapse of a part of the lung, usually as a secondary phenomenon from conditions causing bronchial obstruction, external compression, surfactant deficiency, or scarring. Patients are often asymptomatic and the diagnosis is made by imaging. The management of atelectasis includes treatment of the underlying etiology, lung-expansion exercises, and chest physiotherapy.
  • Vascular:
    • Pulmonary embolism: a potentially fatal intraluminal obstruction of the main pulmonary artery or its branches by an embolus. The most common presenting symptom is dyspnea. The diagnosis of pulmonary embolism is by radiographic imaging. The initial management is supportive and focuses on restoring oxygenation and hemodynamic stability.
    • Pulmonary hypertension: a condition associated with elevated pulmonary arterial pressure, which can lead to chronic progressive right heart failure. Pulmonary hypertension presents as progressive dyspnea, and is either primary or occurs due to other medical conditions. Diagnosis is based on echocardiography, ECG, chest X-ray, and pulmonary function tests. Management is aimed at treating the underlying etiology. 
    • Cor pulmonale: right ventricular dysfunction caused by lung disease, which results in pulmonary hypertension. Cor pulmonale is caused by COPD. Dyspnea is the usual presenting symptom, and signs of right-sided heart failure and hypoxemia may be observed. Most patients are diagnosed clinically. Initial treatment is by using oxygen therapy and is targeted at identifying the underlying disease.
  • Interstitial:
    • Idiopathic pulmonary fibrosis: a specific entity of the major idiopathic interstitial pneumonia classification within interstitial lung diseases. Idiopathic pulmonary fibrosis is often characterized by its rapid progression. The average life expectancy is 3–4 years from diagnosis. Lung transplantation is the only curative intervention. 
    • Hypersensitivity pneumonitis: an immunologically induced inflammatory disease caused by repeated inhalation of an inciting agent that triggers a type III and IV hypersensitivity reaction. Hypersensitivity pneumonitis presents as cough, fever, and malaise. Diagnosis is aided by high-resolution CT scans and analysis of the bronchoalveolar lavage fluid. Management is with steroids and by avoiding exposure to the inciting agent.

References

  1. Moore, Keith, L., et al. Clinically Oriented Anatomy. Lippincott Williams & Wilkins, 2017.
  2. Drake, Richard, et al. Gray’s Anatomy for Students E-Book. Elsevier Health Sciences, 2014.
  3. Standring, S. Gray’s anatomy. The Anatomical Basis of Clinical Practice (41st ed.). Edinburgh: Churchill Livingstone/Elsevier, 2016.

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