Respiratory Anatomy

Introduction

• Development of the respiratory system begins early in gestation and continues into childhood.¹
• The airway begins with the largest segment, the trachea, and progressively subdivides into the smallest segment, the bronchioles.
• The trachea, main stem bronchi, and terminal bronchioles do not have gas exchange abilities, thus are considered “dead space” or the “conducting zone.”³
• The respiratory bronchioles and the beginning of the alveoli represent the “transitional zone”. There is some gas exchange capability in this region, despite this part of the airway still having conducting capabilities.³
• At the end of the bronchioles are the alveoli, which constitute the gas-exchange unit of the respiratory system. This is called the “respiratory zone.”³

Airway Development

• Airway development begins in utero and continues for about 8-10 years after birth.¹
• There are five stages of embryologic development: embryonic, pseudoglandular, canalicular, saccular, and alveolar.¹
• In infants born prematurely, lung development and function are related to the embryologic stage at which they were born.¹
• Please see the OA summary on embryology of the airway and lungs for more details. Link

Airway Cell Types

Different regions of the airway are composed of distinct cell types. Understanding the differences between these cell types is important for anesthetic management.

Nasal epithelium: Pseudostratified columnar epithelium.

Oral and Tracheal Epithelium: Stratified squamous epithelium.

• Clinical Pearl: In patients with epidermolysis bullosa, the squamous epithelium of the oropharynx is at higher risk for blistering and damage than the columnar epithelium of the nasal tract; therefore, oral endotracheal intubation is avoided, and nasal intubation is preferred.

Bronchial Mucosa: Pseudostratified, ciliated, columnar epithelial cells, goblet cells, and basal cells.³

• Ciliated cells: help with particulate clearance.³
• Goblet cells: produce mucin to trap particulates.³

Terminal Bronchioles: Ciliated, cuboidal cells, Clara cells.³

• Clara cells are progenitor cells. They secrete surfactant apoproteins and help with acid-base balance in the airway.³

Alveolar Epithelium: Squamous epithelial respiratory cells.³

• A pneumocyte is a specialized cell that lines the alveoli. There are two types of pneumocytes—type I and type II—that differ in their properties and functions.
• Other cell types in the alveoli that are not pneumocytes include pulmonary alveolar macrophages, neuroendocrine cells, mast cells, and lymphocytes.³
• To facilitate gas exchange, the air-blood barrier within each alveolus is less than 1 μm thick.⁵
• Table 2 below summarizes the differences and functions of type 1 and type 2 pneumocytes.

Airway Anatomy

• Broadly speaking, the airway can be subdivided into two areas: the upper and lower airway.
  • The upper airway is defined as: the nasal cavity, pharynx, and larynx (to the vocal cords).⁵
  • The lower airway is defined as: the trachea (below the vocal cords) and distal to the alveoli.⁵
• The lungs are divided into lobes.^3,5
  • The right lung has three lobes: the upper, middle, and lower lobes.^3,5
  • The left lung has two lobes: the upper and lower lobes.^3,5
  • The lungs are covered by two layers of pleura: the parietal and visceral pleura.⁵
• The trachea bifurcates into the right and left mainstem bronchi at the carina, which is approximately at the T5 vertebrae.³
  • The trachea is supported by 15-20 C-shaped cartilage rings known as “tracheal rings” to assist in maintaining its structure.⁵
  • Right mainstem bronchi: typically shorter, ~2.5cm in adults, and more vertical.³
    • Clinical Pearl: Because the right mainstem is more vertical than the left, aspirated contents will preferentially enter the right mainstem bronchus.³
  • Left mainstem Bronchi: typically longer, ~5cm in adults, more oblique compared to the right mainstem bronchus.³
• The mainstem bronchi continue to separate into secondary (lobar) and tertiary (segmental) branches.³
  • Bronchi contain cartilage, while bronchioles do not.³
    • Clinical Pearl: Because the bronchioles and distal portions of the airway do not have cartilage, they are more susceptible to compression and atelectasis.^3,5
• There are three different “zones” of the airway that reflect their gas exchange capabilities:
  • “Conducting zone”: The trachea, main stem bronchi, and terminal bronchioles; this area does not have alveoli, therefore, no gas exchange capability, thus it is considered “dead space.”^3,5
  • “Transitional zone”: Includes the respiratory bronchioles and the beginning of the alveoli. There is gas exchange capability in this region.³
  • “Respiratory zone”: At the end of the bronchioles are the alveoli, which are the gas exchange units of the respiratory system.3
    • An average adult has 300-480 million alveoli, which is approximately 50m2 – 100m2 of surface area for gas exchange.³

Common Diseases and Pathophysiology of the Pulmonary Anatomy

• Airway malformations can lead to a range of diseases across various aspects of the respiratory system. Understanding the location of the airway malformation can assist with disease identification and management.
• Below is a non-comprehensive table summarizing common diseases and the pathophysiology of the pulmonary anatomy.

See OA Summary on esophageal atresia and tracheoesophageal fistula for more details. Link