Explain the structural features of the alveolus that facilitate its function?
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 |
Function |
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Alveolar macrostructure: large number of (mostly spherical) air spaces connected by septa |
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Alveolar blood-gas barrier: a thin trilamellar membrane composed of three layers:
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Short diffusion distance: 0.2-2 µm for the blood-gas interface
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Elastic basement membrane, containing the septal interstitial fibre network:
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| Type I alveolar cells: thin cells with extended cytoplasmic plates which cover a large surface arrea |
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| Type II alveolar cells: |
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| Pores of Kohn: defects in alveolar septal walls |
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