Partial Pressure of Oxygen and Carbon Dioxide

With each breath, carbon dioxide leaves the alveoli and is replaced with oxygen. This exchange happens through the process of diffusion. When blood has left the heart and is sent to the lungs, the concentration of carbon dioxide in the deoxygenated blood is greater than the concentration of carbon dioxide in the atmosphere. As a result, the gases in the blood and the lungs move toward equilibrium—carbon dioxide diffuses from an area of higher concentration (the blood) through the capillaries to an area of lower concentration (the lungs). Conversely, the oxygen concentration in the external environment is far greater than that in the deoxygenated blood in the lungs, so it diffuses into the body.

Dalton's law of partial pressure states that the total gas pressure in a system is the sum of the pressures of each individual gas in that system. Partial pressure is the pressure put forth by each of the gases in a system, where the pressure is directly proportional to the amount of the particular gas. For example, if the total air pressure is 760 mmHg, the partial pressure of dry air can be calculated because each gas's partial pressure is proportional to its percentage composition in the air. In dry air, the partial pressure of oxygen is 158 mmHg or 20.8 percent ($\text{[158 mmHg /760 mmHg]} \times \text 100$). In the alveoli, oxygen makes up approximately 13.7 percent of the total gases, therefore its partial pressure in alveolar air is 104 mmHg ($\text{[13.7 percent/100]} \times \text 760 \; \text {mmHg}$). Carbon dioxide is only 5.2 percent of the total gases in the alveoli, so its partial pressure is 40 mmHg ($\text{[5.2 percent/100]} \times \text 760 \; \text {mmHg}$). The movement of gases between the capillaries of the alveoli and the external environment is the result of pressure gradients between the concentrations of gases. Negative respiratory pressure occurs when a respiratory area has a pressure that is lower than the pressure of the atmosphere. A positive respiratory pressure occurs when a respiratory area has a pressure that is higher than the atmospheric pressure. A zero respiratory pressure is equal to that of the atmosphere. It is these changes in pressure that allow for the movement of gases down their concentration gradients.

The partial pressures of gases also determines the amount of a gas that can dissolve into a liquid. Henry's law states that gases dissolving in liquids will do so in proportion to their partial pressures. Additionally, the amount of a particular gas in a system determines the rate at which it will dissolve into solution. This is important because carbon dioxide and oxygen need to diffuse into and out of the blood, which is made mostly of water. The same law applies to gases diffusing out of blood and leaving the body for the external environment.

Gas transport occurs because of the large partial pressure gradients that exist across the alveolar capillaries. The gradient of oxygen between the alveoli and the pulmonary capillaries is so large that oxygen can diffuse across the one micron capillary membrane rapidly. Approximately five to eight liters of air are brought into and out of the lungs every minute, and 0.3 liters of oxygen moves from the alveoli to the blood every minute. Similarly, carbon dioxide is exchanged from the blood to the alveoli at a similar rate. An equilibrium in partial pressures of carbon dioxide and oxygen on both sides of the capillary membrane is reached in 0.25 seconds. Each red blood cell only spends about 0.75 seconds in the pulmonary capillary. Therefore, blood can move much faster through the capillaries and still get a sufficient supply of oxygen. Carbon dioxide, on the other hand, is removed at a much slower rate due to the smaller difference in its partial pressure between blood (46 mmHg) and air (40 mmHg). However, carbon dioxide dissolves in the blood much more easily than oxygen due to the fact that the solubility of carbon dioxide in blood is greater than that of oxygen in blood.