The trachea conducts air to the two main bronchi, which are similar to the trachea histologically. Examine the diagram of trachea and major bronchi, keeping in mind that as the trachea branches into the bronchi and the bronchi branch into bronchioles, the components within the walls (lining epithelium, hyaline cartilage, and smooth muscle) gradually change. For example, when the trachea branches into the two main bronchi, the C-shaped cartilage rings of the trachea give way to irregular plates of hyaline cartilage in the bronchi. Progressive changes to the walls of the airways will be a recurring theme as we examine the lungs.

• Examine this longitudinal section of larynx that leads into the trachea and this H&E stained slides of trachea (sample 1 and sample2).
• Identify the layers in the wall of the trachea, as seen in the slides and images above and at the right, including the respiratory epithelium, noting especially the cilia and thick basement membrane, the lamina propria with serous-mucous glands, the hyaline cartilage, and trachealis muscle.
• Study the epithelium in the trichrome stained slide, noting the cilia, goblet cells, as well as the thick basement membrane that is nicely revealed by this stain.

On to the lung.