Airways and lungs

Summary

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, e.g., 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 anatomy

Overview

Conducting zone

Large airways

Small airways

Airway resistance is lowest in the small airways due to the large number of parallel bronchioles, while the highest airway resistance is in the larger airways (trachea, bronchi).

Function

Respiratory zone

Lungs

Left lung Right lung
Lobes and bronchopulmonary segments
Bronchi
Notches

Function

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.

Vasculature, lymphatics, and innervation

Vasculature

The lungs have a dual blood supply

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

Bronchial circulation

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


Lymphatics

  • 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
    • Branches accompany the blood vessels and bronchi into the lung.
    • Parasympathetic fibers from the vagus nerve (M3 receptors) innervate smooth muscle and glands (bronchoconstriction, increase secretion)
    • Sympathetic fibers (act on ß2-receptors): innervate blood vessels, smooth muscle, and glands (bronchodilation, vasoconstriction, decrease secretion)

References:[1][2][3]

Microscopic anatomy

Conducting zone

Respiratory zone

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!

Hemosiderin-laden macrophages are present in the context of 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)

Alveoli

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

References:[5][6][7][8]

Function

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

Embryology

Developmental stage Description Clinical significance

Embryonic period

Weeks 4–7

Pseudoglandular period

Weeks 5–17

Canalicular period

Weeks 16–25

Saccular period

Weeks 26–birth

  • Development of: alveolar ducts and thin-walled alveoli (separated by primary septa)
  • Fetus is able to survive and breathe outside the uterus from about 24–25 weeks of gestation with intensive care.

Alveolar period

Week 36–8 years

References:[9][10][11][12]

Clinical significance

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  • 3. Celis EA. Lung Anatomy. In: Mosenifar Z. Lung Anatomy. New York, NY: WebMD. https://emedicine.medscape.com/article/1884995. Updated December 7, 2017. Accessed December 15, 2018.
  • 4. Reid AT, Sutanto EN, Chander-Veerati P, et al. Ground zero—the airway epithelium. Elsevier; 2019: pp. 61–98.
  • 5. Faculty of Biological Sciences, University of Leeds. Functions of the Respiratory Portion. https://www.histology.leeds.ac.uk/respiratory/respiratory.php. Accessed December 15, 2018.
  • 6. Kalsheker N, Stockley RA. Alpha-1-Antitrypsin Deficiency: Biology, Diagnosis, Clinical Significance, and Emerging Therapies. Academic Press; 2017.
  • 7. Kuhn C, Senior RM. The role of elastases in the development of emphysema. Lung. 1978; 155(1): pp. 185–197. doi: 10.1007/bf02730693.
  • 8. Weiss ST. Chronic Obstructive Pulmonary Disease: Risk Factors and Risk Reduction. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/chronic-obstructive-pulmonary-disease-risk-factors-and-risk-reduction. Last updated August 22, 2018. Accessed December 29, 2018.
  • 9. Reuter S, Moser C, Baack M. Respiratory distress in the newborn. Pediatr Rev. 2014; 35(10): pp. 417–429. doi: 10.1542/pir.35-10-417.
  • 10. Le T, Bhushan V,‎ Sochat M, Chavda Y, Abrams J, Kalani M, Kallianos K, Vaidyanathan V. First Aid for the USMLE Step 1 2019. New York, NY: McGraw-Hill Medical.
  • 11. Human Embryology. 18.1 Phases of Lung Development. http://www.embryology.ch/anglais/rrespiratory/phasen06.html. Updated January 1, 2018. Accessed December 15, 2018.
  • 12. Hislop AA, Wigglesworth JS, Desai R. Alveolar development in the human fetus and infant. Early Hum Dev. 1986; 13(1): pp. 1–11. doi: 10.1016/0378-3782(86)90092-7.
  • Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier Health Sciences; 2016.
  • Kasper DL, Fauci AS, Hauser SL, Longo DL, Lameson JL, Loscalzo J. Harrison's Principles of Internal Medicine. New York, NY: McGraw-Hill Education; 2015.
  • Theodore AC. Oxygenation and Mechanisms of Hypoxemia. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/oxygenation-and-mechanisms-of-hypoxemia. Last updated April 12, 2016. Accessed January 7, 2016.
  • 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.
last updated 06/22/2020
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