The bronchial tree conveys air from the trachea to the smallest passageways leading to the air sacs, or alveoli, where gas exchange takes place. It is composed of the:

The respiratory system is frequently divided into the:

  • Conducting zone - including the airways from the nose to the terminal bronchioles whose primary function is to conduct air
  • Respiratory zone - including the final portion of the bronchial tree including the respiratory bronchioles and alveoli whose function is to perform gas exchange

The structure of the bronchial tree accordingly varies from the proximal end nearest the trachea and the distal end towards the smallest bronchioles and alveoli. As the distance from the trachea increases, the:

  • Diameter of the airways decreases
  • Amount of hyaline cartilage holding the airways open decreases
  • Amount of smooth muscle surrounding the airways increases
  • Number of goblet cells and ciliated cells in the mucosa lining the airway decreases
  • Thickness of the mucosa decreases

The bronchial tree refers to the branching airways that penetrate each lung. It is known as the bronchial tree because of its resemblance to an actual tree, viewed upside down.

Figure 1: The conducting airways and respiratory zone of the bronchial tree. The left image shows the anatomical structure, while the table on the right is a graphical representation of the branching. The z column indicates the number of generations of branching.