The alveoli, located in the lungs, have thin, moist
walls and are surrounded by thin-walled capillaries.
Oxygen passes from the alveolar air into the
blood by diffusion through the alveolar and capillary walls. CO2 and H2O
pass out in the same manner. All exchanges at the alveoli involve passive diffusion.Since passive diffusion drives gas exchange, both in the lungs as well as the tissues, gases always diffuse from higher to lower concentration. In the tissues, O2 diffuses into tissues and CO2 leaves, while in the lungs this is reversed due to high oxygen pressure and low CO2 . CO2 is carried in blood mainly as dissolved carbonate ions.
This animation (Audio - Important) describes carbon dioxide transport.
Take a look at the Airflow in Mammals Tutorial .
REVIEW: As a person inhales, the alveolar pressure _____, the pressure in the pleural
cavities _____, and the atmospheric pressure _____.
REVIEW: What is the path by which air moves into the respiratory system.
REVIEW: The pressure in the pleural cavities is always lower than atmospheric pressure.
a. True
b. False
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