18.1 Phases of lung development

• Introduction
  
• Embryonic phase
  
• Pseudoglandular phase
  
• Canalicular phase
  
• Saccular phase
  
• Alveolar phase
  
• Classification in the adult lung

Classification in the adult lung

In the adult lung one distinguishes between conducting and respiratory zones.
In the conducting zone, all branches of the bronchial tree, the walls of which contain cartilage tissue and seromucous glands, are bronchi. As soon as cartilage and glands are no longer present, bronchioli are involved.

Fig. 11
Diagrams for comparing the constructions of the walls in the respiratory tract.

According to their function the respiratory tract passages are divided into conducting and respiratory zones:

Conducting zone = 16 generations

• Segmental bronchi are continued by several generations of
• Intersegmental bronchi (up to ca. 1 mm diameter). After these follow the
• Bronchioli (< 1mm diameter) that after several divisions go over into
• Terminal bronchioli (ca. 0.4 mm diameter). They subdivide numerous times and represent the end stretch of the purely conductive respiratory tract. The measurements come from histological findings.

Respiratory zone = 7 generations

• Out of the terminal bronchioli several generation of
• Respiratory bronchioli (= 3 generations) proceed. From them follow several generations of
• Alveolar ducts (= 3 generations) that in
• Alveolar sacculi (last generation = 23rd generation) end