Bronchi and Bronchioles – Tutorial

Please read Unit 9 – Introduction to Respiratory System Tissues prior to completing the activities in this chapter.

Introduction to Bronchi and Bronchioles

Bronchi and bronchioles are passageways of the respiratory tract that conduct air from the trachea into and out of alveoli during inhalation and exhalation.  Bronchi are the first series of respiratory passageways that branch off the inferior end of the trachea.  Bronchi exhibit many structural similarities to the trachea such as the presence of hyaline cartilage supports in their walls and an endothelium consisting of pseudostratified ciliated columnar epithelium with numerous goblet cells.

The first bronchi that branch off the inferior trachea are known as the right and left main bronchi (primary bronchi).  The main bronchi then enter the lungs at the hilum (hilar region) and immediately branch into lobar bronchi (secondary bronchi) which conduct air into each lung lobe.   Within each lung lobe, the lobar bronchi continue to branch into smaller segmental bronchi (tertiary bronchi).  As main, lobar, and segmental bronchi progressively branch into smaller diameter tubes, there is a gradual reduction in the size of the cartilage supports, height of epithelial cells, number of cilia on epithelial cells, and number of goblet cells in the endothelium.  As the amount of supportive cartilage in the walls of smaller bronchi decreases there is also an increase in the amount of smooth muscle in the walls.

Bronchi continue to branch into progressively smaller tubes as they extend deeper into the lungs eventually leading to smaller diameter tubes known as bronchioles.  Bronchioles are small respiratory passages that have smooth muscle in their walls but lack the cartilage supports found in bronchi.  The circular arrangement of smooth muscle and the absence of cartilage supports in the walls of bronchioles allow the diameter of the bronchiole lumen to change.  The tone of smooth muscle in the walls of bronchioles is controlled autonomically and relaxation of smooth muscle increases lumen diameter in a process known as bronchodilation.  Alternatively, an increase in smooth muscle tone decreases lumen diameter and causes bronchoconstriction.

Bronchioles continue to branch into smaller passageways known as terminal bronchioles, each of which continue to branch into several respiratory bronchioles that eventually lead to alveolar ducts and alveoli.  Bronchiole lumens are lined with endothelial cells that are shorter and wider than the columnar cells lining larger respiratory passages such as the trachea and bronchi.  This gives the bronchiole endothelium a cuboidal-like appearance.  Furthermore, cilia and goblet cells are completely absent in the endothelium of small bronchioles.

The tutorials in this chapter will focus on how to histologically differentiate between bronchi and bronchioles.  The tutorials will not focus on how to differentiate between the different categories of bronchi (main, lobar, and segmental) and the different categories of bronchioles (terminal and respiratory).

Tutorial:  Use the image slider below to learn more about the structure and characteristics of bronchi and bronchioles. 

Microscopy:  Use the image slider below to learn how to use a microscope to study bronchi and bronchioles on a microscope slide.

Tutorial:  Use the hotspot image below to learn more about the structure of bronchi.  

Tutorial:  Use the hotspot image below to learn more about the structure of large bronchioles.

Tutorial:  Use the hotspot image below to learn more about the structure of small bronchioles.

Tutorial:  Use the image slider below to study numerous examples of tissue from bronchi and bronchioles.