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Name: Mary
Status: Educator
Age: 50s
Location: N/A
Country: N/A
Date: April 2003

I assume the walls of capillaries surrounding alveoli prevent the entrance of carbon dioxide into the bloodstream?

Quite the opposite. Alveoli are VERY permeable to carbon dioxide. The important thing is that carbon dioxide is at a quite low concentration in air, and at a fairly high activity in deoxygenated blood. So carbon dioxide partitions from the blood into the air in the lungs, to be exhaled.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois

No, respiration is by simple diffusion. Usually the concentration of CO2 is higher inside the cells than outside so the CO2 diffuses out of the blood cells/plasma and into lung cells. If the concentration should change to higher outside the blood cells at the lungs, the CO2 would diffuse that way.


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