The alveoli of the lungs are small air sacs about 0.1 mm (10−4 m) in radius. The membrane walls of the sacs that separate the air space from the blood capillaries are about 0.25×10−4 m thick. The capillaries themselves have a radius of about 5 × 10−6 m.
(a) Assuming that O2 diffuses through the wall and the blood as it does through water, what average time is required for O2 to diffuse from the center of an alveolus to the cen- ter of a capillary? Note that the diffusion constant of oxygen through water, at 300 K, is 1.0 × 10−9 m2/s whereas the diffusion constant of oxygen through air is 2.0 × 10−5 m2/s. Take care calculating this; diffusion is an “all-or-nothing” random-walk process, so you have to treat the total distance through all the different media oxygen diffuses in one go. How then do you handle the situation when there’s more than one diffusion constant? (As in chemistry, think about “rate-limiting steps”.)

(b) Now the time that moving blood cells are in transit around an alveolus is about 0.1 s. Considering how this compares for the time for oxygen to diffuse into the blood from the alveolus, what does this tell us about the anatomy of blood traveling