The essential features of the alveolar epithelium as a surface over which…

The idea

The alveolar epithelium is a single layer of flattened squamous cells, and its features follow directly from what a gas exchange surface needs. It is one cell thick and lies against a capillary endothelium that is also one cell thick, so the diffusion pathway between air and blood is extremely short. The epithelium is permeable to oxygen and carbon dioxide and is kept moist so that gases dissolve before diffusing. The large number of alveoli gives a large total surface area for diffusion. A steep concentration gradient is maintained by ventilation bringing fresh air and by blood flow carrying oxygen away, although these are features of the surrounding system rather than of the epithelium itself. If the epithelium is thickened, for example by scar tissue, the diffusion pathway is lengthened and the rate of gas exchange falls.

What earns the marks

• Award credit for stating that the alveolar epithelium is one cell thick, or made of flattened squamous cells, giving a short diffusion pathway.
• Award credit for stating that the epithelium is permeable and moist, so gases dissolve and diffuse across it.
• Award credit for stating that the large number of alveoli gives a large total surface area for diffusion.
• Award credit for explaining that a thicker epithelium, or more cells between air and blood, gives a longer diffusion pathway and so a slower rate of gas exchange.
• Award credit for stating that a steep concentration gradient is maintained by ventilation and by blood flow, and that this increases the rate of diffusion.

Common mistakes and how to put them right

• Listing features with no consequence.

  Put it right: 'thin' earns nothing until it is linked to a short diffusion pathway and faster diffusion.

• Saying alveoli are small so they have a large surface area.

  Put it right: It is their number and the folding of the lung that raise the total area.

• Writing that the alveolus has a good blood supply with no reference to the concentration gradient.

• Confusing epithelium with endothelium.

  Put it right: The epithelium lines the alveolus, while the endothelium lines the capillary.

Exam tips

• Answer as a table in prose: feature, then consequence, for each of thin, large area, moist and permeable, and well ventilated and perfused.
• If the question describes a disease or damage, reverse the logic and state which feature is lost and what happens to the rate of diffusion.