Table of Contents

• Prequiz...
• Fick's Laws, Part II
• 1: Fick's Second Law ...
• 2: ... is about "time to diffuse"
• 3: A Very Useful Formula
• 4: Time-to-diffuse increases dramatically
• 5: General Functions and Diffusion
• Biological Applications
• 6: Biological Applications
• 7: Why do rhinos have lungs and amoebas don't?
• 8: Diffusion is efficient in small organisms, but not big ones
• 9: Why we have lungs
• 10: How many macrophages does it take to kill a virus?
• 11: You need a lot of macrophages - or one smart one!
• Review
• 12: Summary

Why we have lungs

Does that mean then, that diffusion has no place in gas exchange of larger organisms? Not at all, once air is in the lungs, gases get exchanged though...none other than diffusion. The lung is subdivided, ultimately, into millions of tiny sacs called alveoli, each of which has a thickness of only 0.5 to 1 micrometre. Each alveolus is surrounded by capillaries,and it takes blood about 1 second to pass through these capillaries.

Let’s do the maths: Plenty of time!

However, when you get a nasty cold, or a case of pneumonia, the lung tissue can swell and thicken enough to endanger your ability to breathe!

There’s a second place where diffusion is vital in gas exchange: capillaries are the very thin, small blood vessels through which gases are exchanged with cells. The cells making up the walls of the capillaries are around 1 micrometre thick, so gases can diffuse directly through them in the same amount of time as above (0.00028 seconds).