Alveoli
Respiratory zone · About 300 million 50-100 square metres 0.3-0.5 micrometres Type I and II cells
About 300 to 500 million thin-walled sacs, each around 200 to 300 micrometres across, opening off the respiratory bronchioles, alveolar ducts and sacs, with a gas-exchange surface of 50 to 100 square metres. The blood-gas barrier of type I pneumocyte, fused basement membrane and capillary endothelium is only about 0.3 to 0.5 micrometres thick.
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
- Alveoli
Detail
- Number and area
- About 300 million (recent counts up to 480 million); 50 to 100 square metres, about 70 on average
- Type I pneumocyte
- Squamous, covers about 95 per cent of the surface, cannot divide
- Type II pneumocyte
- Cuboidal with lamellar bodies; secretes surfactant (mainly dipalmitoylphosphatidylcholine); the progenitor of type I cells and hyperplastic after injury
- Alveolar macrophage
- Clears dust and bacteria; becomes the haemosiderin-laden heart-failure cell after alveolar haemorrhage or oedema
- Pores of Kohn
- Openings between adjacent alveoli that allow collateral ventilation and the alveolus-to-alveolus spread of pneumococcal pneumonia
- Blood-gas barrier
- About 0.3 to 0.5 micrometres; diffusion is proportional to area and partial-pressure gradient and inversely to thickness (Fick); carbon dioxide diffuses about 20 times faster than oxygen
- Surfactant
- Lowers surface tension, more so in small alveoli (Laplace: pressure = 2T/r), so small alveoli do not empty into large ones; raises compliance and keeps alveoli dry; produced from about 24 weeks, adequate by about 34 to 35 weeks (lecithin to sphingomyelin ratio of 2 or more)
When it goes wrong
Prematurity under about 34 weeks
Neonatal respiratory distress syndrome: surfactant deficiency, high surface tension, widespread atelectasis and stiff lungs; ground-glass chest film with air bronchograms; antenatal steroids and exogenous surfactant
Acute respiratory distress syndrome (sepsis, aspiration, pancreatitis, trauma)
Diffuse alveolar damage with hyaline membranes and loss of surfactant: stiff lungs, refractory hypoxaemia and bilateral infiltrates without a raised left atrial pressure
Thickened barrier from interstitial fibrosis or pulmonary oedema
Reduced DLCO and desaturation on exertion; oxygen transfer becomes diffusion-limited long before carbon dioxide is affected
Lobar pneumonia (Streptococcus pneumoniae)
Spreads alveolus to alveolus through the pores of Kohn to fill an entire lobe: dense consolidation with air bronchograms
Left heart failure
Fluid crosses into the alveoli once interstitial lymphatic capacity is exceeded; frothy pink sputum and haemosiderin-laden macrophages
Practise this structure
1 question in the bank tagged Diffusion.