Question 4

Explain the structural features of the alveolus that facilitate its function?

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College Answer

The question asked for an explanation of the structural features of the alveolus that facilitate its function. Candidates who scored well integrated the specific anatomical and structural elements of the alveolus with the multiple functions of the alveolus, including the relevant explanation regarding mechanisms. No marks were given for simply listing the structural features of an alveolus without an explanation on how this facilitates function, nor for listing functional requirements of an alveolus without explaining whether or how they are met by its structure. For the same reason, this was one question where simply listing equations with no discussion as to how these relate to the structure and function of the alveoli garnered no marks. Equations were not required for full marks but may be an efficient way to represent physical relationships that are hard to write in a few words. Common omissions included a description and the role of collagen and elastin fibres, capillary structure, the filtration function of the membrane, lymphatic drainage, recruitment and distensibility and metabolic functions. Candidates are encouraged to practice model answer templates for these integrative questions in the months leading up to the exam.


Structure and Function Relationships of the Alveoli



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: thin cells 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


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