In the lungs, ethanol moves by passive diffusion from the capillaries into the surrounding air sacs called alveoli. Unlike the process of active transport, passive diffusion requires no additional energy to move molecules. The energy comes from the difference in the concentration of ethanol between the two compartments: blood and lung alveoli (substances move from areas of high concentration to areas of low concentration).
As ethanol diffuses out of the pulmonary capillaries, it contacts a thin water-based mucous layer on the inner surface of the alveolar sacs (Figure 4.3). It is inside the alveolar sacs of the lung—at the interface between the liquid mucous layer and the air—where some of the ethanol changes from a liquid into a gas. The process is known as vaporization.
Figure 4.3 Ethanol from the heart reaches the lung capillaries (purple), which surround the alveoli. As ethanol diffuses into the alveoli, it vaporizes to a gas.
Learn more about the structure and function of the lungs.
How does ethanol vaporize into a gas?
Let’s look more closely at the process of ethanol vaporization.
At room and body temperatures, ethanol can exist as a liquid or a gas.
- Liquid – individual ethanol molecules are tethered to one another by two types of chemical forces: hydrogen bonds and Van der Waals forces.
- Gas – when liquid ethanol molecules have enough energy to break the hydrogen bonds connecting them together, they escape into the gas state.
Compounds, like ethanol, that can easily change from a liquid to gas are called volatile compounds.
Volatility is related to the number of hydrogen bonds. Let’s compare ethanol and water. Individual ethanol molecules can only form three hydrogen bonds with neighboring ethanol molecules, while water molecules can form as many as four hydrogen bonds with neighboring water molecules. So, ethanol requires less energy to break three hydrogen bonds (ethanol) than four hydrogen bonds (water). Therefore, ethanol is more volatile than water.
Figure 4.4 Ethanol vaporizes when its hydrogen bonds are broken. This happens more easily compared to water because ethanol molecules are bound to each other with up to three hydrogen bonds, while water molecules are bound to each other with up to four hydrogen bonds.
Learn more about vaporization and the volatility of a compound.