Alveolar ducts and sacs
Respiratory zone · Gen 20-23 Diffusion, not bulk flow
The respiratory bronchioles open into alveolar ducts (generations 20 to 22), passages whose walls consist entirely of alveolar openings ringed by knobs of smooth muscle and elastin, which end in blind alveolar sacs (generation 23), the last generation of the tree.
Traced from the start
- Trachea
- Right main bronchus
- Right upper lobe bronchus
- Right middle lobe bronchus
- Right lower lobe bronchus
- Left main bronchus
- Left upper lobe bronchus
- Left lower lobe bronchus
- Right upper lobe segmental bronchiRight middle lobe segmental bronchiRight lower lobe segmental bronchiLeft upper lobe upper division bronchiLingular bronchus and segmentsLeft lower lobe segmental bronchi
- Bronchioles and terminal bronchiole
- Alveolar ducts and sacs
Detail
- Generations
- 20 to 22 for the ducts, 23 for the sacs; arise from the respiratory bronchioles
- Wall
- No continuous epithelium of its own: a ring of smooth-muscle and elastic-fibre knobs between the mouths of the alveoli
- Gas movement
- The respiratory zone holds about 2.5 to 3 L of gas; from here on gas moves by diffusion rather than bulk flow, and the total cross-sectional area is so large that velocity falls almost to zero
When it goes wrong
Alpha-1 antitrypsin deficiency
Panacinar emphysema: the whole acinus from respiratory bronchiole to alveolar sac is destroyed uniformly, worst in the lower lobes; early-onset emphysema in a non-smoker with cirrhosis
Loss of ductal and septal elastin
Reduced elastic recoil raises compliance and shifts the pressure-volume curve up and to the left; FRC and TLC rise
Practise this structure
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