Alcohol in the Breath
The blood circulatory system is closed. Oxygen gets into the blood and carbon dioxide and other volatile waste products are eliminated from the blood by diffusion from and to air that is inhaled into the lungs via the brachial tube which is, in turn, connected to the mouth and nose. The pulmonary artery branches out into millions of capillaries that end in small sacs called alveoli. Diffusion of oxy gen and carbon dioxide are exchanged between the blood and air through these alveoli. If there is alcohol in the blood, then that too is eliminated through the alveoli and is exhaled from the mouth and nose. Figure 1 shows the human respiratory system.
The amount of alcohol that gets into the breath from the alveoli is proportional to the amount of alcohol in the blood and is governed by Henry’s law. This law states that when a volatile substance, such as alcohol, is dissolved in a liquid, such as blood, and then that liquid is brought in contact with a closed air space, such as alveolar breath, the ratio of the concentration of alcohol in the blood and breath is a constant at a given temperature. If the temperature is raised, the equilibrium shifts toward the concentration in the air. As the alcohol is removed from the lungs by respiration, more of it will move from the blood to the replaced air in the lungs in an effort to reestablish the equilibrium. Ultimately all of the alcohol will be removed from the blood as it passes through the lungs. In the United States, most states have adopted 2100:1 (1 mL of blood contains as much alcohol as 2.1 L of air) at 34 °C for the ratio between blood alcohol and breath alcohol, although research indicates that the actual ratio at this temperature is closer to 2300:1. This ratio is somewhat person-dependent and the lower 2100:1 ratio favors the subject. There is a lot of variation among people.
FIGURE 1 Diagram of human respiratory system. The lungs contain many branches called bronchi. These are further subdivided into alveoli. As the circulating blood reaches the alveoli, it releases carbon dioxide and picks up oxygen. If there are other dissolved volatile substances in the blood such as alcohol, they will also be released into the alveoli. These gases will be exhaled through the mouth. A breath alcohol testing instrument can measure the amount of alcohol in the exhaled breath.
