The structure indicated is the right main bronchus.

The trachea divides into two main bronchi – the right main bronchus and the left main bronchus (also referred to as primary bronchi). The trachea divides at the level of the sternal angle and the point at which it bifurcates is known as the carina – a cartilaginous ridge which runs between the left and right main bronchi.

The main bronchi then divide into smaller and smaller airways in the following sequence:

• Lobar bronchi (secondary bronchi)
• Segmental bronchi (tertiary bronchi)
• Bronchioles
• Terminal bronchioles
• Respiratory bronchioles
• Alveolar ducts
• Alveolar sacs

The right main bronchus is wider than the left main bronchus and is oriented more vertically than the left main bronchus. Due to this difference in structure, inhaled foreign bodies are more likely to become lodged in the right main bronchus than the left main bronchus.

The histology of the main bronchi is similar to that of the trachea, consisting of cartilage and mucous membrane. As the bronchi divide into smaller and smaller airways, the amount of hyaline cartilage decreases, and is absent in the bronchioles. The bronchi are lined with ciliated pseudostratified columnar epithelium – this respiratory epithelium is specialised to protect the airways from pathogens and infection.

Learn more about the anatomy of the respiratory system and bronchi in this tutorial!