Respiratory tree

The proximal respiratory tree from human lung, showing the trachea down to the conducting bronchioles.

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The respiratory tree describes the branching structure of the vertebrate lung. Beginning with the top of the respiratory system, the trachea, the order of branchings is as follows:

• trachea
  • main bronchus
    • lobar bronchus
      • segmental bronchus
        • lobular bronchus
          • conducting bronchiole
            • terminal bronchiole
              • respiratory bronchiole
                • alveolar duct
                  • alveolar sac
                    • alveolus

At each division point or generation, one airway branches into two or more smaller airways. The human respiratory tree may consist of up to 28 generations, while the respiratory tree of the mouse has up to 13 generations. Proximal divisions (those closest to the top of the tree, such as the bronchi) mainly function to transmit air to the lower airways. Latter divisions including the respiratory bronchiole, alveolar ducts and alveoli, are specialized for gas exchange.