Page 92 - The Cell in 40 Topics

he main purpose of respiration is the expulsion of the carbon
dioxide (CO ) from the body and its replacement by life-giving
2
oxygen. These processes take place in a site far distant from the body tis-
sues—in the lungs. That being so, the oxygen entering the body by way of
the lungs needs to be carried to the tissues in some way, and the carbon
dioxide forming in the tissues has to be removed from the lungs in the same
way. How is that transportation carried out?
Erythrocytes, or red blood cells, are the tireless, indefatigable carriers
of the oxygen and carbon dioxide in the blood. The erythrocytes that come
into contact with the air in the lungs release their carbon dioxide—a waste
product—they've brought from the cells into small sacs, and absorb the
oxygen in those sacs. This process takes place along a very special mem-
brane. One side of this membrane receives oxygenated air in the sac or
alveolus, while on the other side, there are capillary extensions of such nar-
row width as to permit only one erythrocyte to pass. In this way, oxygen
molecules has no difficulty in making contact with the erythrocytes.

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