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Airways and lungs

Last updated: May 12, 2020


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


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).


  • 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


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


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


  • 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



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)


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



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


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


Clinical significance


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