Lower Airways Anatomy

The lower airways begin at the trachea and extend through successive branchings to the alveolar sacs where gas exchange occurs. Understanding this hierarchy is essential for interpreting imaging and understanding the distribution of pulmonary disease.

The trachea is a fibrocartilaginous tube approximately 10–12 cm long, supported by 16–20 C-shaped cartilage rings that are open posteriorly where the trachealis muscle provides a flexible posterior wall. The trachea begins at the lower border of the cricoid cartilage (C6) and bifurcates at the carina, which lies at the level of the sternal angle (T4/5 junction). The right main bronchus is shorter, wider, and more vertically oriented than the left, which is why aspirated foreign bodies more commonly enter the right lung.

At the carina the trachea divides into right and left main (primary) bronchi. Each main bronchus divides into lobar (secondary) bronchi: three on the right (supplying upper, middle, and lower lobes) and two on the left (supplying upper and lower lobes). Lobar bronchi divide into segmental (tertiary) bronchi, each supplying a bronchopulmonary segment — the smallest independently resectable unit of lung tissue, with its own bronchus, artery, and vein.

The right lung has three lobes (upper, middle, lower) and ten bronchopulmonary segments. The left lung has two lobes (upper and lower) and eight to ten segments (the lingula is the equivalent of the right middle lobe). The oblique fissure is present in both lungs; the horizontal fissure is present only in the right lung, separating the upper and middle lobes.

Beyond the segmental bronchi, airways progressively lose cartilage support and become bronchioles. Terminal bronchioles are the last purely conducting airways. Respiratory bronchioles begin to bear alveoli in their walls and therefore participate in gas exchange. Beyond the respiratory bronchioles lie alveolar ducts and finally alveolar sacs, the clusters of individual alveoli where diffusion occurs.

Pulmonary arteries run with airways (bronchovascular bundles) from the hilum. Pulmonary veins run in the interlobular septa independently of the airways. Bronchial arteries (systemic circulation) supply the airway walls. The lung has an extensive lymphatic network that drains towards the hilar lymph nodes, which are enlarged in many respiratory and systemic diseases.