Blood-Alcohol Physiology

Blood-alcohol analysis is simply the attempt to measure the percentage by weight of alcohol within the DUI suspect's blood at the time of testing. This is done directly when dealing with a blood sample, or indirectly by analyzing the percentage in a breath or urine sample and applying conversion ratios to estimate the percentage in the blood; these conversion ratios used in DUI cases are inaccurate in that they simply represent statistical averages.

Blood Alcohol Level jpgIn dealing with a charge of violating Vehicle Code section 23152(b), driving with 0.08% blood-alcohol concentration (BAC) or more, the amount of alcohol in the blood at the time of driving is the only issue. Where, however, the DUI charge is violation of Vehicle Code Section 23152(a) (and it is common to charge both offenses), the amount of alcohol in the blood is of only secondary interest. It is the amount of alcohol actually absorbed into the brain that will affect the individual's ability to perceive, make judgments, and coordinate movements; the individual's ability to safely operate a motor vehicle. But in DUI cases there is no practical means of measuring the alcohol absorbed by the body beyond that found in the bloodstream, or, even further removed, in the urine or breath. Since the bones, brain, and fatty tissue contain a much lower percentage of water than does blood, and since the alcohol level in blood is about 17 percent higher than that in the soft tissues, the concentration of alcohol in the entire body, including the brain, is always less than that in the blood. However, science has offered the "Widmark Factor R," a designation of the ratio between the concentration of alcohol in the whole body divided by the concentration of alcohol in the blood. For men, this ratio averages about .67, with a range of .46 to .86; women usually have a somewhat lower ratio because of having a larger proportion of fatty tissue. Obviously, the fact that this ratio varies so widely according to the individual makes generalizations in a given DWI case very suspect.

In organs having a rich blood supply, such as the kidneys, brain, and liver, the tissues very quickly attain alcohol equilibrium with the arterial blood. Voluntary muscle tissue, however, has a much smaller blood flow per unit of weight, and as a result requires longer to reach alcohol equilibrium after ingestion. Since the muscles make up about 40 percent of body weight, this delay in alcohol absorption by the muscles results in high concentrations of alcohol in arterial blood and in the brain during active absorption of alcohol. The result is the common phenomenon that an individual may appear greatly affected only a few minutes after taking two or three drinks, and then rapidly sober up within 15 to 30 minutes, in apparent contradiction to normal expectations. This, of course, can raise serious doubts about the relevance of blood-alcohol tests in DUI cases.

There are many factors in blood alcohol physiology that can affect the rate of absorption and distribution of alcohol into the system and, ultimately, into the brain. The most common is that different individuals have different rates, and these rates can vary within a given individual. But external factors can also cause variation. The effects of cold weather or extreme stress, for example, can cause less blood to be delivered to the muscles and more to the brain. With more blood being delivered to the brain, more alcohol is also delivered, and an elevated BAC is the result. [Ritchie, The Aliphatic Alcohols, in Goodman and Gilman's The Pharmacological Basis of Therapeutics (7th ed., 1985)]. Thus, it would appear that the stress in DUI cases caused by such things as interrogation, field sobriety tests and handcuffing could themselves cause higher blood-alcohol levels when the individual is later tested at the police station.

The concentration of alcohol in a DUI suspect's body depends upon the amount of water contained in that body. The more water is present in the body, the more diluted the alcohol will become as it is absorbed into the system. And the simple fact is that individuals vary according to the percentage of water in their bodies.

In a study entitled Pharmacokinetics of Ethanol in Plasma and Whole Blood: Estimation of Total Body Water by the Dilution Principle, (Jones, Hahn, and Stalberg, 42 European Journal of Clinical Pharmacology 445 (1992)), researchers confirmed that the body-water content varies from person to person. The content in men, interestingly, decreases with age, with a result that the blood-alcohol concentration will increase. Further, where an individual has experienced trauma, as in an automobile accident, the body's percentage of water will decrease. The same can happen as a result of pathological conditions, such as in persons with diarrhea, heart failure, or impaired renal function.

Yet another factor affecting absorption rates among individuals is the sex of that individual. It has been known for some time that women are generally more susceptible to the effects of alcohol than men. This has generally been explained by pointing out that women are smaller and have relatively more fat and less water than men. But recent research seems to indicate that a more important reason may be that women have significantly lower amounts of an enzyme which provides a protective barrier in the stomach by breaking alcohol down before it circulates into the body.

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