Describe the structure and function of the alveolus.

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College Answer

Better answers related structure to function. Many answers lacked key anatomical features (for 
example pores of Kohn, basement membrane, interconnecting walls / alveolar 
interdependence etc.). There was little understanding of the role and origin of the basement 
membrane of the alveolus. Some candidates went into detailed discussions of Work of 
Breathing, respiratory mechanics and the renin-angiotensin system which were not asked for.
Answers not reaching a pass mark generally suffered from lacking detail and suggested only a 
superficial understanding of the area. 

Discussion

Though this resource is often at fault of being excessively critical of the college examiners, at one stage or another one must share in their exasperation, as to digress on the renin-angiotensin-aldosterone system would be extreme for even this digression-prone author.

Anyway. The college asked for the structure and function of the alveolus. It is possible to do this thing in a number of ways. One might first discuss the structure, and then the function:

Structure of the alveolus

Macroscopic characteristics:

  • Large surface area, as a group: for the lungs, this is approximately 140m2
  • Short diffusion distance: 0.2-0.5 µm for the blood-gas interface
  • Largely polyhedral shape 
  • Open at one end, like a cup
  • Walls of the alveoli are composed of the pulmonary capillary sheet 
  • Alveolar surfaces are covered in a thin (200nm) layer of surfactant

Gas exchange surface

  • The blood-gas barrier is a thin trilamellar membrane
  • It is composed of three layers:
    • Capillary endothelial cell
    • Basal lamina
    • Alveolar Type 1 cell
  • The total thickness is usually around 300-500 nm

Cell types

  • Cellular population of the alveolar walls consists of:
    • Type I alveolar cells, which cover 95% of the surface area and have mainly barrier functions
    • Type II alveolar cells, which secrete surfactant and replenish the Type I cell population
    • Capillary endothelial cells
  • Additional cell types 
    • Alveolar macrophages
    • Mast cells
    • Fibroblasts

Interstitial fibres and basal lamina

  • Pores of Kohn interrupt these septa and allow communication between alveoli
  • Mechanical shape stability (resistance to collapse, facilitated by surfactant and alveolar intedependence)
  • Flexibile (facilitated by collagen and elastin fibres)

Function of the alveolus

  • Gas exchange surface: High permeability to gases, but low permeability to water, achieved by the lipid bilayer of the alveolar cell membrane
  • Secretion of surfactant (by Type 2 ccells)
  • Self-repair following damage (Type 2 cells)
  • Collateral ventilation assists in V/Q matching (Pores of Kohn)

Alternatively, it may be able to marry the two categories in a table which pairs the structures with whatever their function is supposed to be:

Structure and Function Relationships of the Alveoli

Structure

Function

Alveolar macrostructure: large number of (mostly spherical) air spaces connected by septa

  • Large surface area, to facilitate diffusion.
  • The interconnected network of walls allows mechanical stress to be shared across a larger area of lung parenchyma (this is alveolar interdependence)
  • Spherical in full distension, but folding along pleats with deflation to maintain surface area

Alveolar blood-gas barrier: a thin trilamellar membrane composed of three layers:

  • Capillary endothelial cell
  • Basal lamina
  • Alveolar Type 1 cell

Short diffusion distance: 0.2-2 µm for the blood-gas interface

  • Flexibility (facilitated by collagen and elastin fibres in the basement membrane)
  • High permeability to gases, but low permeability to water, achieved by the lipid bilayer of the alveolar cell membrane

Elastic basement membrane, containing the septal interstitial fibre network:

  • Axial collagen fibres
    (from the hila along the bronchi  and alveolar ducts)
  • Peripheral fibres (from the visceral pleura via interlobular septa)
  • Septal fibres (along the alveolar septa).
  • Increases the elastic recoil of the distended lung
  • Increases the resistance to atelectasis
Type I alveolar cells: think cels with extended cytoplasmic plates which cover a large surface arrea
  • Barrier function (very poor permeability to water-soluble substances)
  • Gas exchange function (very high permeability to gases)
Type II alveolar cells: 
  • Secrete surfactant (which decreases the surface tension of the fluid on the alveolar walls, preveing their collapse)
  • Act as stem cells to replenish Type I alveolar cells, which cannot replicate
Pores of Kohn: defects in alveolar septal walls
  • Their main function is to allow collateral ventilation between alveoli
  • This is another mechanism of matching ventilation and perfusion

References

References

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Yamaguchi, Kazuhiro, et al. "Anatomical backgrounds on gas exchange parameters in the lung." World Journal of Respirology 9.2 (2019): 8-28.

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Gil, J., et al. "Alveolar volume-surface area relation in air-and saline-filled lungs fixed by vascular perfusion." Journal of Applied Physiology 47.5 (1979): 990-1001.

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Evans, Michael J., et al. "Renewal of alveolar epithelium in the rat following exposure to NO2." The American journal of pathology 70.2 (1973): 175.

Pastor, Luis Miguel, et al. "Morphogenesis of rat experimental pulmonary emphysema induced by intratracheally administered papain: changes in elastic fibres." Histology and histopathology (2006).

Bastacky, J., and J. Goerke. "Pores of Kohn are filled in normal lungs: low-temperature scanning electron microscopy." Journal of Applied Physiology 73.1 (1992): 88-95.

Boatman, E. S., and H. B. Martin. "Electron microscopy of the alveolar pores of Kohn." American Review of Respiratory Disease 88.6 (1963): 779-784.

Desplechain, C., et al. "The pores of Kohn in pulmonary alveoli." Bulletin europeen de physiopathologie respiratoire19.1 (1983): 59-68.

Weibel, Ewald R. "Lung morphometry: the link between structure and function.Cell and tissue research 367.3 (2017): 413-426.

Cordingley, J. L. "Pores of Kohn." Thorax 27.4 (1972): 433-441.

Terry, Peter B., and Richard J. Traystman. "The clinical significance of collateral ventilation." Annals of the American Thoracic Society 13.12 (2016): 2251-2257.

Reich, Stanley B., and Jacob Abouav. "Interalveolar air drift." Radiology 85.1 (1965): 80-86.

Kuriyama, T., and W. W. Wagner Jr. "Collateral ventilation may protect against high-altitude pulmonary hypertension." Journal of Applied Physiology 51.5 (1981): 1251-1256.