Fick’s Law tells us how a concentration difference drives flow:

Fick’s Law tells us how a concentration difference drives flow:

J = −DΔn/Δx

If the membrane in the alveolus (parts A and B) is 7 nm thick, and the diffusion constant, D, for oxygen through the membrane is 6 ✕ 10−3 (µm)2/s, calculate the rate of flow through the alveolar membrane. 

__________________= molecules/µm2/s.


(PART A and B)

Capillaries are the narrowest elements in the blood transport system of animals. They connect the arteries (the vessels that carry blood away from the heart) to the veins (the vessels that carry blood back to the heart). They are the place where the chemicals that the blood is carrying are taken from source regions (lungs for air, intestines for food, glands for hormones) and delivered to the cells that need them. They are pretty small—about 5 µm in diameter. One of the interesting questions about this process is: how do oxygen molecules know where to go? For example, Wikipedia says that red blood cells “take up oxygen in the lungs or gills and release it while squeezing through the body’s capillaries.” In order to consider this let’s make an oversimplified model in which oxygen diffuses into the blood stream in the lungs, is carried by fluid flow out to the cells where it diffuses out. (In reality, the oxygen is bound to hemoglobin, carried, and released, but the beginning and end processes are as described here.) 

A. Consider a capillary in an alveolus in the lung. The density of oxygen inside the capillary is 0.3 million molecules/(µm)3and in the air outside is about 5 million molecules/(µm)3

B. Consider an individual molecule in the air outside the alveolus

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…