Lower Airway Anatomy
The blood/gas diffusion barrier is ~50-80m². There are 200-600 million alveoli in the lungs. Each alveolus is polyhedral and ~0.2mm in diameter at FRC.
Cells in the respiratory tract:
- Capillary endothelium - ~0.1µm thick
- Alveolar type I cells - ~0.1µm thick, have 1nm gap junctions, form the structure of the alveoli. Are impermeable to albumin, allow extravasation of macrophages. These are highly sensitive to hyperoxia
- Alveolar type II cells - rounded cells at septal junctions which produce surfactant. These are resistant to hyperoxia.
- Alveolar macrophages - present in alveoli and airways. Involved in defence and scavenging.
- PMN's (in smokers/CF), mast cells, clara cells
There are 23 generations of airway branches/zones
- 1 - 16 are the conducting zone, which don't contribute to gas exchange therefore are anatomical dead space:
- R/L main bronchi
- Segmental bronchi
- Terminal Bronchioles - the smallest airways without alveoli
- 17 - 23 are the respiratory zone - airways with alveoli coming off them - ~3 litres of air volume
- Respiratory bronchioles - have a few alveoli in their walls
- Alveolar ducts - completely lined with alveoli
- Alveolar sacs
- Inspired air travels down the conducting zone by bulk flow, but below this the cross sectional area is so large and distances to travel so small (<5mm) that it moves by diffusion (<1 second)
- Very easy to distend, only 3cm of water required to create a 500ml tidal volume - due to high compliance of tissue and low resistance to flow - C ~ 180ml/cmH2O