A. Consider a capillary in an alveolus in the lung. The density of oxygen inside

Question

A. Consider a capillary in an alveolus in the lung. The density of oxygen inside the capillary is 0.3 million molecules/(m)3 and in the air outside is about 5 million molecules/(m)3. Which way will the oxygen flow across the membrane? Explain why.

B. Consider an individual molecule in the air outside the alveolus. How is it moving? How does it know how to get into the capillary?

C. Now consider a capillary inside a muscle. The muscle has done work and used up its oxygen. The density of oxygen inside the capillary is 1 million molecules/(m)3 and in the fluid surrounding the muscle is 0.3 million molecules/(m)3. Which way will the oxygen flow across the membrane? Explain why.

D. Consider an individual molecule in the capillary in part C (after is has been released by the hemoglobin). How is it moving? How does it know how to get into the capillary?

E. Fick's Law tells us how a concentration difference drives flow: J = -D n/x. If the membrane is 7 nm thick, and the diffusion constant, D, for oxygen through the membrane is 6 x 10-3 (m)2/s, calculate the rate of flow through the membrane.

Explanation / Answer

A. The density of oxygen in the air outside is higher than the density of oxygen inside the capillary. Hence, oxygen will flow from outside to inside across the membrane. Higher density corresponds to higher pressure (as density is directly proportional to pressure). A gas always flows from higher pressure to lower pressure.
B. An individual molecule in the air outside the alveolus is moving from higher pressure region to lower pressure region. It can move into the capillary as the pressure inside the capillary is lower than the pressure outside the capillary.
C.The density (and hence the pressure) of oxygen inside the capillary in muscle is higher than the density (and hence the pressure) of oxygen in the fluid surrounding the muscle.Hence, the oxygen will flow from inside to ouside the capillary.
D. An individual molecule in the capillary is moving from higher pressure region to lower pressure region. It can move out of the capillary as the pressure inside the capillary is higher than the pressure outside the capillary.

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