Bronchial Tree

The collective term “bronchial tree” refers to the bronchi and all of their subsequent branches. The bronchi are the airways of the lower respiratory tract. At the level of the 3rd or 4th thoracic vertebra, the trachea bifurcates into the left and right main bronchi. The right main bronchus is shorter and more vertical in direction than the left. Both of these bronchi continue to divide into secondary or lobar bronchi that bifurcate further and further in order to sufficiently spread the respiratory air completely into the left and right pulmonary lobes. The terminal segment of each bronchus contains millions of alveoli, the site of gas exchange.

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Development

The development of the tracheobronchial tree and lungs occurs in 5 stages. The tracheobronchial tree and lungs originate 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 weeks 26–birth
  • Alveolar ducts → terminal sacs
  • Gas-exchange surface area of the lungs expands.
  • Surfactant production increases.
  • Blood–air barrier fully develops (type I pneumocytes).
Infants born ≥ 32 weeks have a higher survival rate.
Alveolar period
  • Occurs during from 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 age 8 years: 300 million
  • In utero: ↑ Vascular resistance due to aspiration of amniotic fluid
  • Postpartum: Inspiration of air leads to a drop in pulmonary vascular resistance.

Gross Anatomy

The bronchial tree begins at the bifurcation of the trachea at the carina, approximately at the level of T5. The trachea bifurcates into the main left and right bronchi, These bronchi continue to branch until they form alveoli, the site of gas exchange. Each bronchial segment progressively becomes smaller in diameter and has a thinner wall.

Main branching structure and components

  • Trachea → carina → main bronchi → lobar bronchi → segmental bronchi → terminal bronchioles → respiratory bronchioles → alveolar duct → alveolar sac → alveoli
  • Bronchi:
    • Main bronchi:
      • The left main bronchus is longer than the right and enters the left lung at the level of T6.
      • The right main bronchus is wider, shorter, and more vertical than the left (most frequent pathway for aspirated foreign objects) and enters the right lung at the level of T5.
    • Lobar or secondary bronchi:
      • 2 left lobar bronchi
      • 3 right lobar bronchi
    • Segmental or tertiary bronchi:
      • Each supplies a bronchopulmonary segment, which is the largest subdivision of a lobe.
      • 10 segments in the right lung
      • 8–10 segments in the left lung
  • Conducting bronchioles:
    • 20–25 generations of branching
    • End as terminal bronchioles
    • Lack cartilage, glands, and alveoli (distinguishing feature from bronchi)
  • Terminal bronchioles give rise to several respiratory bronchioles.
  • Respiratory bronchioles:
    • Characterized by the outpocketing from their lumen: alveoli
    • Each respiratory bronchiole gives rise to 2–11 alveolar ducts.
    • Each alveolar duct gives rise to 5–6 alveolar sacs, the site of gas exchange.

Anterior view of the larynx, trachea, and bronchial tree:
Note the main divisions of the bronchi.

Image by Lecturio.

Neurovasculature

Blood supply:

The bronchial tree is supplied by branches of the left and right bronchial arteries.

  • Left bronchial artery: a direct branch of the thoracic aorta
  • Right bronchial artery: varied origin: branch of the thoracic aorta, an intercostal artery on the right side, or one of the superior branches of the left bronchial artery

Venous drainage: 

  • The bronchial veins are analogous to the bronchial arteries. 
  • The bronchial tree is drained by tributaries of the main left and right bronchial veins:
    • Left bronchial vein drains into the left superior intercostal vein or the accessory hemiazygos vein.
    • Right bronchial vein drains into the azygos vein.

Innervation:

Innervation is supplied by the pulmonary plexus of the vagus nerve.

Microscopic Anatomy

  • Bronchi:
    • Mucosa: pseudostratified ciliated columnar epithelium
    • Contains goblet cells that secrete mucus into the tracheal lumen
    • Submucosa: provides support
    • Smooth muscle
    • Cartilage layer of hyaline cartilage
  • Bronchioles:
    • Ciliated columnar epithelium in larger bronchioles
    • Nonciliated columnar epithelium in smaller bronchioles
    • Does not contain goblet cells
    • Contains club cells, also known as Clara cells, that secrete a component of surfactant
  • Terminal and respiratory bronchioles: 
    • Do not contain glands or cartilage
    • Both present mucosa, submucosa, smooth muscle, and tunica adventitia.
    • Terminal bronchiole: ciliated cuboidal epithelium
    • Respiratory bronchiole: simple cuboidal epithelium and club cells
  • 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 made of elastic fibers and capillaries
    • Lined by types I and II pneumocytes
      • Type I: comprise 95% of total alveolar area and form the blood–air barrier
      • Type II: comprise 5% of total alveolar area and secrete surfactant

Microscopic anatomy of the lower respiratory tract

Image by Lecturio.

Clinical Relevance

The following are conditions that can affect the bronchial tree:

  • Asthma: chronic inflammatory disease of the airways: Inflammation and bronchial hyperreactivity occur. The airways of patients with asthma are more sensitive to various stimuli, leading to paroxysmal and recurrent obstruction of the airways.
  • Emphysema: condition characterized by dilation of the airways, with decreased elasticity and increased compliance, due to alveolar wall destruction: The most common cause of emphysema is smoking. Alpha-1-antitrypsin deficiency may also result in the development of emphysema.
  • Bronchiectasis: disease in which there is permanent enlargement of parts of the airways. In this condition, damage to the airways causes them to widen and become flabby and scarred. Usually, the disorder occurs as the result of preexisting lung disease.

References

  1. Moore, K. L., et al. (2017). Clinically Oriented Anatomy. Lippincott Williams & Wilkins.
  2. Drake, R., 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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