Gas Exchange

In a mixture of different gases, each gas contributes to the total pressure of the mixture. The contribution of each gas, called the partial pressure, is equal to the pressure that the gas would have if it were alone in the enclosure. Dalton's Law states that the sum of the partial pressures of each gas in a mixture is equal to the total pressure of the mixture.

The following factors determine the degree to which a gas will dissolve in a liquid:

• The partial pressure of the gas. According to Henry's Law, the greater the partial pressure of a gas, the greater the diffusion of the gas into the liquid.
• The solubility of the gas. The ability of a gas to dissolve in a liquid varies with the kind of gas and the liquid.
• The temperature of the liquid. Solubility decreases with increasing temperature.

Gas exchange occurs in the lungs between alveoli and blood plasma and throughout the body between plasma and interstitial fluids. The following factors facilitate diffusion of O 2 and CO 2 at these sites:

• Partial pressures and solubilities. Poor solubility can be offset by a high partial pressure (or vice versa). Compare the following characteristics of O 2 and CO 2:

• Oxygen. The partial pressure of O 2 in the lungs is high (air is 21percent O 2), but it has poor solubility properties.

• Carbon dioxide. The partial pressure of CO 2 in air is extremely low (air is only 0.04 percent CO 2), but its solubility in plasma is about 24 times that of O 2.

• Partial pressure gradients. A gradient is a change in some quantity from one region to another. Diffusion of a gas into a liquid (or the reverse) occurs down a partial pressure gradient—that is, from a region of higher partial pressure to a region of lower partial pressure. For example, the strong partial pressure gradient for O 2(pO 2) from alveoli to deoxygenated blood (105 mm Hg in alveoli versus 40 mm Hg in blood) facilitates rapid diffusion.

• Surface area for gas exchange. The expansive surface area of the lungs promotes extensive diffusion.
• Diffusion distance. Thin alveolar and capillary walls increase the rate of diffusion.
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