Alveoli adaptations for gas exchange

Predict, then check

Think about what the oxygen concentration in the blood would do.

Imagine the blood stopped flowing through the capillaries around an alveolus. What would happen to the rate at which oxygen diffuses into the blood?

Tap any cell to reveal it. Tap a column header to read one property down every item.

What you need to know

• Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
• Gases are exchanged between the air in the alveoli and the blood in the capillaries by diffusion.
• Oxygen diffuses from the alveoli into the blood: there's a lot of it in the alveoli and much less in the blood.
• Carbon dioxide goes the other way, from the blood into the alveoli, because there is a greater concentration of it in the blood.
• Have a goSam says: 'Gases move from where there's less to where there's more, so carbon dioxide should head into the blood.' Using the last four bullets, say what Sam has backwards.

  Diffusion goes from higher to lower concentration, and carbon dioxide is more concentrated in the blood, so it moves out.

  Net movement is always from higher to lower concentration, so Sam's rule sends carbon dioxide the wrong way.

• The alveoli give the lungs a very large surface area, and a larger surface area increases the rate of diffusion.
• Have a goZara claims her lungs would work just as well as one smooth balloon. What does that idea throw away?

  The very large surface area the alveoli give the lungs.

  A larger surface area increases the rate of diffusion, so losing it would lower the rate.

• Steep concentration gradients between the alveolar gases and the blood flowing through the capillaries mean a fast rate of diffusion is maintained.
• Cells lining the alveoli and capillaries are very thin and flattened, so the diffusion distance is short, and a thinner membrane gives a faster rate.
• Capillary walls are one cell thick, and the very narrow lumen slows red blood cells into single file, giving more time for exchange.
• Have a goSuppose red blood cells sprinted through a capillary instead of lining up in single file. What would exchange lose?

  Time for exchange.

  The very narrow lumen slows red blood cells into single file precisely to give more time for exchange.

• Together, a large surface area, a steep concentration difference and very thin cells give a high rate of diffusion between lungs and bloodstream.
• Fick's law sums it up: rate of diffusion is proportional to surface area × concentration difference ÷ membrane thickness.

The big picture

Gases are exchanged between the air in the alveoli and the blood in the capillaries by diffusion. A very large surface area, steep concentration gradients and very thin, flattened cells together give a high rate of diffusion, which Fick's law sums up.

⚠ Watch out

Writing 'the alveoli have a large surface area' and stopping there. Say what it does (a larger surface area increases the rate of diffusion), then do the same for the gradient and the thin cells.

Memory hook

Big, Steep, Thin: a big surface, a steep gradient and a thin barrier. Two of them go on the top of Fick's fraction. Thin goes on the bottom.

Check yourself

Cover the page and finish three sentences: 'A large surface area helps because...', 'Steep concentration gradients help because...', 'Very thin, flattened cells help because...'. Which of the three goes on the bottom of Fick's fraction?