Explain how alveoli are adapted for gas exchange by diffusion between air…

The idea

Alveoli are tiny air sacs at the ends of bronchioles, and each is wrapped in a dense network of blood capillaries. Gas exchange happens by diffusion: oxygen moves from the air in the alveolus, where its concentration is relatively high, down a concentration gradient across the alveolar wall and capillary wall into the blood, while carbon dioxide moves the opposite way, from blood to air, to be breathed out. Their adaptation comes from several features working together. The millions of alveoli give an enormous total surface area. Both the alveolar wall and the capillary wall are one cell thick, so the diffusion distance is very short. A steep concentration gradient is maintained because breathing constantly refreshes the air and blood flow constantly removes oxygen and delivers carbon dioxide. The moist lining helps gases dissolve. A rich blood supply keeps the gradient steep.

What earns the marks

• Large total surface area from millions of alveoli increases the rate of diffusion of oxygen and carbon dioxide.
• Alveolar wall and capillary wall are each one cell thick, giving a short diffusion distance.
• Breathing refreshes air and blood flow carries gases away, maintaining a steep concentration gradient.
• Oxygen diffuses from alveolar air into capillary blood; carbon dioxide diffuses from blood into alveolar air.
• A moist lining allows gases to dissolve before diffusing; a rich capillary network supports the gradient.

Common mistakes and how to put them right

• Saying alveoli have a large surface area 'to store air'.

  Put it right: The correct reason is to increase the rate of diffusion.

• Claiming oxygen diffuses from blood into the alveolus.

  Put it right: The direction is air to blood for oxygen and blood to air for carbon dioxide.

• Describing the walls as 'thin' without linking this to a short diffusion distance that speeds diffusion.

Exam tips

• Read the question stem to see whether it asks for a structural feature or the reason it speeds diffusion, then choose the option that matches.
• Eliminate options that reverse the direction of gas movement before comparing the remaining choices.
• Check that any option mentioning a gradient also explains how the gradient is maintained, such as by breathing or blood flow.