After consumption of alcoholic bevarages the alcohol absorption takes place first into the gastrointestinal tract, then into the bloodstream, thus resulting in the physiological effects of alcohol consumption. The concentration is measured in the venous blood as blood alcohol concentration (BAC) in mg/g (per mille) resp. in Britain in mg/dL.
Since alcohol is also a natural metabolic product, it also occurs in blood. This as endogenous blood alcohol designated quantity of alcohol is in a range of 0.001 to 0.003 per mille (0.1 to 0.3 mg/dL) and is therefor too small to be measured in normal blood alcohol determination.
A direct correlation between measured BAC and the resulting physiological effects is not given in a particular case, since the level of alcohol habituation hereby is to consider. Statisticly, however, connections between BAC and alcohol effects derived. From 0.3 per mille (30 mg/dL) first physiological damages exists. Concentrations above 3 per mille (300 mg/dL) can lead to death. For trafficers legal limits exists. The BAC is not only a fact of importance in criminal law, but also for the assessment of debt capacity.
As an alternative - but not compatible - reference system the breath alcohol is increasingly used. The individual EU states set different legislation rules especially for the replacemant of the blood alcohol determination.