Physiology of gas exchange
Alveolar gas exchange
Usually, gas pressure is considered in terms of total pressure, regardless of whether it is a single gas or a mixture of gases. However each gas in the mixture separately contributes to the total pressure.
It is defined as the pressure exerted by a particular gas in a mixture of gases. The sum of the partial pressures of the gases within a mixture equals the total pressure.
Because oxygen is consumed and carbondioxide is produced by cells, the venous blood will have a high PCO2 and a lower PO2 and vice-versa for arterial blood.
A more active location would consume more O2 and produce more CO2 than less active locations. Because of these differences, the jugular vein blood may not be representative of whole body venous blood (i.e., blood from the right atrium).
Gases diffuse from an area of their higher concentration to an area of their lower concentration.
Two gas laws are applicable to gas exchange .
Dalton’s law explains how gases move by diffusion based on pressure differences while Henry’s law describes the diffusion of gas based on its solubility.
Ventilation brings O2 to the alveoli and removes CO2. Because O2 is being consumed in the tissues, a pressure difference exists for its diffusion from alveoli to venous blood (which then becomes arterial) and from arterial blood to the tissues. Because CO2 is being produced in the tissues, a pressure difference exists for its diffusion from tissue to arterial blood (which then becomes venous) and from venous blood to the alveoli.
| Gases | Venous Blood | Alveolar Air | Arterial Blood | Tissues |
| O2 | 40 | 104 | 100 | 30 or less |
| CO2 | 45 | 40 | 40 | 50 or more |
| N2 | 569 | 569 | 569 | 569 |
| Water vapor | 47 | 47 | 47 | 47 |
| Total | 701 | 760 | 756 | 696 |
The total pressure in venous blood is somewhat less than atmospheric pressure because the volume of CO2 produced is lower than the volume of O2 consumed.
As because not all of the blood going to the lungs is arterialized (nonperfused alveoli) the difference in PO2 exists.
External and internal respiration
External respiration, also called pulmonary gas exchange, is the diffusion of O2 and CO2 from the alveoli to pulmonary blood. Blood circulating through the body picks up CO2 and delivers O2. As this blood travels through the pulmonary capillaries, CO2 diffuses into the alveoli while O2 diffuses from the alveoli to pulmonary blood. The exchange of these gases occurs independently and passively.
Pulmonary gas exchange is facilitated by a very thin respiratory membrane.
Ventilation is generally regarded as the process by which gas in closed places is renewed or exchanged. As it applies to the lungs, it is a process of exchanging the gas in the airways and alveoli with gas from the environment. The main function of breathing is to provide gas for ventilation.
In addition, there is a close association between the amount of gas reaching the alveoli (ventilation), and the blood flow through the pulmonary capillaries (perfusion).
Internal respiration, or systemic gas exchange, occurs at the tissue level, where there is an exchange of O2 and CO2 between systemic capillaries and tissue. O2 diffuses from the capillaries into the cells; CO2 diffuses from the cells into the systemic capillaries.