Alcohol that you have ingested, but that has not yet been absorbed into your bloodstream, has no neurological effects and cannot cause driving impairment. The time-frame in which alcohol is absorbed into the bloodstream and the volume of alcohol absorbed are significant in determining both impairment and blood alcohol concentration. Therefore, characteristics of the absorption, distribution and elimination of alcohol are the keystones to the determination of BAC at the time of driving.
Circulation through the blood stream
Alcohol is categorized as a depressant. For the brain to be influenced by the depressant effects, the alcohol must be absorbed into the body and circulate through the blood stream to the brain and other organs.
There are two main circulatory routes in the body:
- Pulmonary circulation to the lungs.
- Systemic circulation to the rest of the body.
Arteries carry blood away from the heart and veins carry blood back to the heart. In the systemic circulation, the arteries carry oxygenated blood from the heart to the organs and tissues. After the blood releases its oxygen to the organs and tissues, the veins carry deoxygenated blood back to the heart. In the pulmonary circulation, the flow of oxygenated and deoxygenated blood is reversed. The pulmonary artery carries deoxygenated blood from the heart to the lungs where it receives oxygen. The pulmonary vein carries oxygenated blood from the lungs to the heart. Alcohol follows this route, distributing and circulating in the body until it is finally metabolized (broken down or excreted).
Once ingested, alcohol is constantly absorbed into and eliminated from the body. The entire process of absorption, distribution, and elimination is often displayed as a graph, with time as the “x” axis and blood alcohol concentration (BAC percent) as the “y” axis. [See Graph 1.]
The graph is a simplistic representation, and is not meant to depict a particular individual’s alcohol curve in a particular situation. However, the curve demonstrates the concept of how alcohol levels change over time. That is, it shows the general trend towards increasing blood alcohol (BA) levels, followed by a peak (or maximum level), and concluding with a decreasing BA level.
The absorption or rising side of the curve
The left portion of the curve is the absorption (or “rising”) side, showing an increase in concentration over time. Absorption takes place at a variable rate, depending on many factors.
Some of these factors include:
- The presence or absence of food in the stomach.
- The concentration of the beverage consumed.
- The rate of consumption.
- Vascularization (how many blood vessels the person has through which the alcohol can pass).
- The individual’s emotional state.
- The individual’s ratio of muscle mass to fat.
As long as the amount of alcohol entering the blood is greater than the amount the liver can oxidize, BAC will rise.
The rising alcohol concept is important in DUI cases because a person who is stopped for DUI may be in the absorption or rising phase of the alcohol curve when he or she is tested. This means the person’s BAC while driving will have been lower than at the time of the chemical test. Similarly, if the breathalyzer or blood alcohol test is taken when the alcohol level has peaked, or reached its plateau, the BAC level will test as higher than it was earlier in the absorption phase. In these situations, a rising alcohol defense may be possible.
On the other hand, during the elimination phase, the BAC level starts to decline. Thus, a blood alcohol test taken during the elimination phase may report a lower alcohol value than the actual value at the time of driving.
The absorption phase and elimination phase are simply trends. Short term fluctuations up or down may occur during either of these phases.
The elimination or falling side of the curve
The right portion of the curve is the elimination (or “falling”) side, showing a decrease of the BA concentration over time. At some time after the cessation of drinking, when the amount of alcohol entering the blood stream equals the amount the liver can handle, the BA level will peak (the maximum concentration). After the peak, the amount entering the blood stream will be less than the liver can handle, and the liver can handle what is entering and start eliminating alcohol from the blood stream, causing BAC to fall.
Primary method of alcohol absorption: through the GI tract
Toxicants, such as alcohol, are absorbed through the skin, lungs, and the gastrointestinal (GI) tract. The main route of alcohol absorption is through the entire length of the GI tract, from mouth to rectum.
Because of alcohol’s small molecular size, it readily diffuses through membranes of the GI tract by passive diffusion. Although the entire GI tract is capable of alcohol absorption, the vast majority of absorption takes place in the small intestine, where microvilli (very small finger-like projections) greatly increase the surface area available for absorption. Thus, most nutrients, drugs, and alcohol are primarily absorbed from the proximal end (end nearest the stomach) of the small intestine.
After the consumption of a drink, alcohol travels down the esophagus and enters the stomach. As it arrives in the stomach, a small portion of the dose (25%) is absorbed via passive diffusion from the stomach into the blood. The remainder of the alcohol is absorbed from the small intestine, notably the duodenum and the jejunum. The absorbed alcohol travels from the stomach and into the intestine through the pyloric valve, which opens approximately three times a minute to allow the passage of food and liquid from the stomach. Alcohol from the stomach and intestine travels through the mesenteric veins, into the portal vein, to the liver, where the major site of oxidation takes place.
Other methods of alcohol absorption
Absorption can also occur by routes other than the GI tract. For example:
- Absorption through the intact skin is possible, yet negligible in the overall process.
- Diffusion from the bladder back into the blood is possible, especially if the alcohol concentration in the bladder is greater than the alcohol concentration in the blood. However, because the bladder wall receives a poor blood supply and the bladder wall is muscular in nature, this amount is rarely more than 10%. The net result is an insignificant increase in levels in the blood due to diffusion through the bladder wall.
- A small portion of the consumed alcohol may be absorbed directly from the mouth. Absorption from the mouth occurs through the mucosa into the blood circulating in the veins in the buccal (cheek) tissue. Any such absorption, if it is to occur, will occur within the first 15 minutes after a drink.