Exchange of Oxygen (O2) and Carbon Dioxide (CO2)
The exchange of O2 and CO2 between alveolar air and pulmonary blood occurs via passive diffusion governed by the behavior of gases. There are 2 Gas Laws: Dalton’s Law : explains how gases move down their pressure differences by diffusion Henry’s Law : explains how solubility of gas relates to its diffusion Dalton’s Law of Partial Pressures : each gas in a mixture of gases exerts its own pressure as if no other gases were present. The pressure of a specific gas in a mixture is called Partial Pressure (P x ). The total pressure of the mixture is simply calculated by summing all of the partial pressures.
P1 + P2 + P3 = P total
For example: atmospheric air is a mixture of gases – nitrogen (N2), oxygen (O2), water vapor (H20), carbon dioxide (CO2), plus other gases in small amounts. Therefore, atmospheric pressure is calculated as follows:
P N2 + P O2 + P H2O + P CO2 + P other gases = 760 mmHg
We can determine the partial pressure of any gas in the mixture by multiplying the % of that gas by the total pressure of the mixture. The % of each gas in atmosphere are:
Nitrogen (N2) – 78.6% Oxygen (O2) – 20.9% Carbon Dioxide (CO2) – 0.04% Other Gases – 0.06% Water Vapor (which is variable) - ~ 0.04% on a cool dry day. Therefore, the partial pressures of gases in inhaled air are as follows: P N2 = 0.786 x 760 mmHg = 597.4 mmHg P O2 = 0.209 x 760 mmHg = 158.8 mmHg P H2O = ------------“----------- = 3.0 mmHg P CO2 = ------------“----------- = 0.3 mmHg P other gases = ------------------- = 0.5 mmHg 760 mmHg
As you know, pressures determine movement of O2 and CO2 between the atmosphere and the lungs, between lungs and blood, and between blood and body cells. Each gas diffuses across a permeable membrane from an area where its partial pressure is higher (greater) to the area where its partial pressure is lower (less). The greater the difference in partial pressure, the faster the rate of diffusion. Henry’s Law of solubility : the quantity of a gas that will dissolve in a liquid is proportional to the partial pressure of the gas and its solubility.
281
Made with FlippingBook Publishing Software