8.4 Gas Exchange
Learning Objectives
By the end of this section, you will be able to:
- Compare the composition of atmospheric air and alveolar air
- Describe the mechanisms that drive gas exchange
- Discuss the importance of sufficient ventilation and perfusion and how the body adapts when they are insufficient
- Discuss the process of external respiration
- Describe the process of internal respiration
The purpose of the respiratory system is to perform gas exchange. Pulmonary ventilation provides air to the alveoli for this gas exchange process. At the respiratory membrane, where the alveolar and capillary walls meet, gases move across the membranes, with oxygen entering the bloodstream and carbon dioxide exiting. It is through this mechanism that blood is oxygenated and carbon dioxide, the waste product of cellular respiration, is removed from the body.
Gas Exchange
In order to understand the mechanisms of gas exchange in the lung, it is important to understand the underlying principles of gases and their behaviour. In addition to Boyle’s law, several other gas laws help to describe the behaviour of gases.
Gas Laws and Air Composition
Gas molecules exert force on the surfaces with which they are in contact; this force is called pressure. In natural systems, gases are normally present as a mixture of different types of molecules. For example, the atmosphere consists of oxygen, nitrogen, carbon dioxide, and other gaseous molecules, and this gaseous mixture exerts a certain pressure referred to as atmospheric pressure (Table 8.4.1). Partial pressure (Px) is the pressure of a single type of gas in a mixture of gases. For example, in the atmosphere, oxygen exerts a partial pressure, and nitrogen exerts another partial pressure, independent of the partial pressure of oxygen (Figure 8.4.1). Total pressure is the sum of all the partial pressures of a gaseous mixture. Dalton’s law describes the behaviour of nonreactive gases in a gaseous mixture and states that a specific gas type in a mixture exerts its own pressure; thus, the total pressure exerted by a mixture of gases is the sum of the partial pressures of the gases in the mixture.
Table 8.4.1. Partial pressures of atmospheric gases
| Gas | Percent of total composition | Partial pressure (mm Hg) |
| Nitrogen (N2) | 78.6 | 597.4 |
| Oxygen (O2) | 20.9 | 158.8 |
| Water (H2O) | 0.04 | 3.0 |
| Carbon dioxide (CO2) | 0.004 | 0.3 |
| Others | 0.0006 | 0.5 |
| Total composition/total atmospheric pressure | 100% | 760.0 |
Partial pressure is extremely important in predicting the movement of gases. Recall that gases tend to equalise their pressure in two regions that are connected. A gas will move from an area where its partial pressure is higher to an area where its partial pressure is lower. In addition, the greater the partial pressure difference between the two areas, the more rapid is the movement of gases.
Solubility of Gases in Liquids
Henry’s law describes the behaviour of gases when they come into