Passage VI

In the human body, O₂ and CO² are exchanged between the lungs and the blood and between the blood and the tissues as indicated in Figure 1. These exchanges occur by the diffusion of each gas from a region where its partial pressure (the pressure contributed by a single gas) is higher to a region where its partial pressure is lower. The figure gives the partial pressures P_O2 and P_CO2 in the lung alveoli (sacs), in the tissue cells, and in the blood of a healthy resting person at sea level.

Figure 2 shows how alveolar P_O2 and P_CO2 vary with alveolar ventilation (volume of air per unit time available to the alveoli for gas exchange).

Figures adapted from Arthur J. Vander, James H. Sherman, and Dorothy S. Luciano, Human Physiology: The Mechanisms of Body Function. ©1985 by McGraw-Hill, Inc.