12.8: Partial Pressure and Dalton's Law
- Explain Dalton’s law of partial pressure and apply the partial pressure concept in respiration.
Dalton's law of Partial Pressures
Dalton's law applies to a mixture of two or more gases. This law states that in a mixture of gases, the total pressure is the sum of the partial pressures of all the components. If there are three different gases in a container then the law is given by the following mathematical equation.
P total = P 1 + P 2 + P 3
The partial pressure of a gas is the pressure that gas would exert if it occupied the container by itself. Partial pressure is represented by the letter P .
Figure 12.8.1 below demonstrates the concept of partial pressure in more concrete terms, showing the pressure of each gas alone in a container and then showing the gases combined pressure once mixed.
Figure \(\PageIndex{1}\): Alone, Hydrogen has a pressure of 0.4 atm, while Nitrogen has a pressure of 0.6 atm. When placed in the same container, the total pressure is 1 atm, with each gas contributing its partial pressure. Image credits: OpenStax College
Air is a mixture of 78.0 % nitrogen, 21.1 % oxygen, and 0.9 % of other gases such as argon, helium, carbon dioxide, water vapors, methane, and hydrogen. The atmospheric pressure is the sum of the partial pressures of components of air. At sea level the atmospheric pressure is 760 torr.
At sea level: P total = P atm = 760 torr = P N2 + P O2 + P others = 593 torr + 160. torr + 7 torr
At the top of Mount Everest the atmospheric pressure is 200 torr.
Mount Everest: P total = P atm = 200 torr = P N2 + P O2 + P others = 156 torr + 42.2. torr + 2 torr
Therefore as altitude increases, and atmospheric pressure decreases, the partial pressure of oxygen decreases from 160 torr at sea level to 42.2 torr at Mount Everest. So climbers have to carry oxygen supplies. Otherwise climbers face the risk of Acute Mountain Sickness . Some of the symptoms are headaches, nausea, and shortness of breath.
Some runners take advantage of high altitude training. Why? P O2 is low at high altitude and so training at high altitudes increases aerobic capacity. The person tends to make more red blood cells to increase the intake of oxygen.
The concept of partial pressures also explains why some people with respiratory problems are uncomfortable in low pressure air that is very moist. When the atmospheric pressure is low and the air is moist the P water vapor is high and so the P O2 is low.
Basic Principles of Gas Exchange during Respiration
Gas exchange during respiration occurs primarily through diffusion. Diffusion is a process in which transport is driven by a concentration gradient. Gas molecules move from a region of high concentration to a region of low concentration. Blood that is low in oxygen concentration and high in carbon dioxide concentration undergoes gas exchange with air in the lungs. The air in the lungs has a higher concentration of oxygen and lower in concentration of carbon dioxide than that of oxygen-depleted blood. This concentration gradient allows for gas exchange during respiration.
Partial pressure is a measure of the concentration of the individual components in a mixture of gases. The partial pressure of a gas above a liquid is directly related to the amount of gas dissolved in the liquid. The rate of diffusion of a gas is proportional to its partial pressure within the total gas mixture.
Partial pressure of oxygen in ambient air is 160 mm Hg. In the arterial blood, the P O2 = 100 mm Hg. The venous blood has a P O2 = 40 mm Hg. The P O2 in the remote tissues is less than 40 torr. This pressure gradient drives the oxygen into the tissue cells via diffusion. See figure 12.8.2 for visual connection.
At the same time, the venous blood has a P CO2 = 46 mm Hg. The arterial blood has a P CO2 = 40 mm Hg and the ambient air has a P CO2 = 0.2 mm Hg A net movement of CO 2 takes place. The pressure gradient drives CO 2 out of tissue cells. CO 2 (g) is a bi-product of metabolism.
Figure \(\PageIndex{2}\): The partial pressures of oxygen and carbon dioxide change as blood moves through the body.
In short, the changes in partial pressure of gases drives the oxygen into the tissues and the carbon dioxide into the blood from the tissues. The blood is then transported to the lungs where differences in partial pressures result in the movement of carbon dioxide out of the blood into the lungs, and oxygen into the blood.
Summary
The partial pressure of a gas is the pressure that gas would exert if it occupied the container by itself. Dalton's law applies to a mixture of two or more gases. This law states that in a mixture of gases, the total pressure is the sum of the partial pressures of all the components. Gas exchange during respiration occurs primarily through diffusion. Gas molecules move from a region of high concentration to a region of low concentration.