Key structures:

Trachea:

• Cartilaginous “trunk” of the tree.
• Comprises 15 – 20 C-shaped cartilaginous rings, are stacked vertically and connected via anular rings.
• Trachealis posterior forms posterior wall of trachea; moves to accommodate foods passing posteriorly through the esophagus.

Bronchi:

• Primary bronchi enter the lungs.
• Secondary bronchi serve lobes of lung (“lobar” bronchi).
• Tertiary bronchi serve bronchopulmonary segments (“segmental” bronchi); 10 on the right, 8-10 on the left.

Bronchioles:

• Numerous, and narrow as they branch.
• Have more smooth muscles in their walls, but still have cartilage in their walls.
• Terminal bronchiole is the final passageway of the conduction portion of the respiratory system.

Respiratory bronchioles:

• Demarcate the respiratory portion of the respiratory tract.
• Thin walls allow some gas exchange.

Alveolar ducts:

• Arise from respiratory bronchioles.

Alveolar sacs:

• Terminal ends of the alveolar ducts.

Alveoli:

• Thin-walled out-pockets of the alveolar sacs.
• Surrounded by pulmonary capillaries.
• Facilitate gas exchange between the respiratory and cardiovascular systems.

Lungs:

• Hundreds of millions of alveoli.
• Left lung = superior and inferior lobes; heart nestles into medial left lobe.
• Right lung = superior, middle, and inferior lobes.

Clinical Correlations:

Lung cancer often originates in the bronchi.
Pulmonary embolism (aka PE) obstructs arterial supply. In a PE, gas exchange is reduced, and blood oxygen levels drop.