The concentration of alcohol in venous blood reflects the concentration of alcohol in tissue of the forearm muscle, whereas the concentration of alcohol in breath reflects the concentration entering the pulmonary capillaries of the lungs, which is closer to the concentration of ethanol in arterial blood supplying the brain. The magnitude and time course of arterial–venous differences in BAC change as a function of time after drinking, which has a bearing on the closeness of agreement between venous BAC and estimated BAC from the analysis of breath [10,23].