Airways and lungs

Last updated: March 6, 2023

Summarytoggle arrow icon

The respiratory system consists of a conducting zone (anatomic dead space; i.e., the airways of the mouth, nose, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles) and a respiratory zone (lung parenchyma; i.e., respiratory bronchioles, alveolar ducts, alveolar sacs). The conducting zone is composed of nonrespiratory tissue and provides the passage for ventilation of the respiratory zone, where the O2 and CO2 exchange takes place. The respiratory system is furthermore divided into an upper tract (structures from the larynx upwards) and a lower tract (structures below the larynx). The entire respiratory tract down to the bronchioles is covered in ciliated epithelium, which provides immunologic protection by helping clear the airways of dust and microorganisms. Hyaline cartilage in the form of C-shaped rings (trachea) and plates (bronchi) provides structural protection and integrity. Gas exchange takes place in the alveoli of the lungs. The right lung consists of 3 lobes (upper, middle, lower), while the left lung consists of 2 lobes (upper, lower) and the lingula, a structure that is homologous to the middle lobe of the right lung. The left lung shares its space with the heart, which it accommodates in the cardiac notch. The development of the lungs begins in the embryonic period and continues until approximately 8 years of age.

Gross anatomytoggle arrow icon





Intrapulmonary conducting system

Large airways

Small airways


  • Conduction of air in and out of the respiratory tree
  • Anatomic dead space (no gas exchange)
  • Warms and humidifies air
  • Mucociliary clearance: ciliated epithelium transports mucus, bacteria, and dust towards the throat, where it is either swallowed or expelled through the mouth

Respiratory zone


Left lung Right lung
Lobes and bronchopulmonary segments
  • 2 lobes (upper, lower)
  • Upper lobe includes the lingula, a tongue-shaped projection homologous to the middle lobe of the right lung (the rest of the space occupied by the middle lobe in the right lung is occupied by the heart in the left lung).
  • 8–10 bronchopulmonary segments

Only the right lung has a middle lobe. It can be auscultated in the fourth to six intercostal space anteriorly at the midclavicular line.

The right main bronchus is wider, shorter, and more vertical than the left main bronchus so aspiration of foreign bodies and aspiration pneumonia are more likely in the right lung.

Each bronchopulmonary segment can be surgically removed without affecting the function of the others.

The Lingula is in the Left Lung.



The lungs have a dual blood supply.

Pulmonary circulation

Overview of pulmonary circulation
Vessels Anatomy Characteristics
Pulmonary trunk
  • Carries deoxygenated blood from the right ventricle to the lungs for oxygenation
Left pulmonary artery
Right pulmonary artery
Pulmonary veins

Relationship of pulmonary arteries to the corresponding bronchi (Right: Anterior to the mainstem bronchus; Left: Superior to the mainstem bronchus): RALS.

Bronchial circulation

Overview of bronchial circulation
Vessels Anatomy Characteristics
Bronchial arteries
Bronchial veins
  • Drain deoxygenated blood from hilar structures and conducting zone structures


  • Lymphatic vessels drain the entire respiratory tree (lymphatic vessels are not present in the pulmonary alveoli)
  • Intrapulmonary nodes → bronchopulmonary nodes → tracheobronchial nodes → paratracheal nodes → bronchomediastinal nodes and trunks → thoracic duct on the left and right lymphatic duct on the right

Innervation (pulmonary plexus)

Microscopic anatomytoggle arrow icon

Conducting zone

Respiratory zone [1]

Respiratory bronchioles

Respiratory bronchioles contain simple (non-ciliated) cuboidal and squamous epithelium and smooth muscle.

Pulmonary alveoli

Pulmonary alveoli are separated from each other by the interalveolar septum with elastic fibers and capillaries. Interalveolar pores connect adjacent alveoli.

Pulmonary surfactant produced by type II pneumocytes reduces the surface tension of the thin layer of water that covers the pulmonary epithelium, thereby preventing alveolar collapse at end-expiration, increasing compliance, and reducing the work of breathing!

The presence on microscopy of hemosiderin-laden macrophages of lung tissue indicates alveolar hemorrhage or pulmonary edema.

Deposition and clearance of inhaled particles

Inhaled particles within the respiratory tree are cleared by different means depending on their size.

Particle size Deposit into Clearance via
Small (< 3 μm)


Alveolar macrophages
Medium (3–10 μm) Trachea and/or bronchi Mucociliary clearance
Large (≥ 10 μm) Nasal cavity Nasal hair (vibrissae)

Functiontoggle arrow icon

The main function of the lung is the absorption of oxygen into the blood and the release of carbon dioxide into the air.

Embryologytoggle arrow icon

Overview of embryology [5]
Developmental stage Description Clinical significance

Embryonic period
weeks 4–7)

  • Lung bud at the distal end of the respiratory diverticulum develops into:

Pseudoglandular period

(weeks 5–17)

Canalicular period
(weeks 16–25)

Saccular period
(weeks 26–birth)

  • Development of terminal sacs (thin-walled alveoli separated by primary septa) from alveolar ducts
  • Fetus can breathe outside the uterus from about 24–25 weeks of gestation with intensive care.

Alveolar period
(week 36–8 years)

Referencestoggle arrow icon

  1. 18.1 Phases of Lung Development. Updated: January 1, 2018. Accessed: December 15, 2018.
  2. Reuter S, Moser C, Baack M. Respiratory distress in the newborn. Pediatr Rev. 2014; 35 (10): p.417-429.doi: 10.1542/pir.35-10-417 . | Open in Read by QxMD
  3. Functions of the Respiratory Portion. . Accessed: December 15, 2018.
  4. Reid AT, Sutanto EN, Chander-Veerati P, et al. Ground zero—the airway epithelium. Elsevier ; 2019: p. 61-98
  5. Davis JD, Wypych TP. Cellular and functional heterogeneity of the airway epithelium. Mucosal Immunology. 2021; 14 (5): p.978-990.doi: 10.1038/s41385-020-00370-7 . | Open in Read by QxMD
  6. Basil MC, Cardenas-Diaz FL, Kathiriya JJ, et al. Human distal airways contain a multipotent secretory cell that can regenerate alveoli. Nature. 2022; 604 (7904): p.120-126.doi: 10.1038/s41586-022-04552-0 . | Open in Read by QxMD
  7. Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier Health Sciences ; 2016
  8. Kasper DL, Fauci AS, Hauser SL, Longo DL, Lameson JL, Loscalzo J. Harrison's Principles of Internal Medicine. McGraw-Hill Education ; 2015
  9. Alraiyes AH, Thompson P, Thammasitboon S. Biot's respiration in a chronic opioid user: Improved with adaptive-servo ventilation. Am J Respir Crit Care Med. 2011; 183.doi: 10.1164/ajrccm-conference.2011.183.1_meetingabstracts.a5279 . | Open in Read by QxMD

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