1.2 The journey of ethanol through the body
In this video you can see the journey of ethanol through the body, beginning in the mouth, passing down the oesophagus to enter the stomach and then into the small intestine where most of its absorption takes place. The video also illustrates the diffusion of ethanol molecules from the small intestine into adjacent blood capillaries which drain into the portal vein. This vein leads directly from the gut to the liver where some of the alcohol is converted into other molecules (you will learn more about this process shortly).
As blood passes through the capillaries in the lungs, some of the ethanol molecules diffuse from the blood into the lungs. From the bloodstream, ethanol is able to diffuse into cells in all of the organs in the body. The rate at which ethanol leaves the blood and enters the organs is dependent upon how rich the blood supply is to the particular organ. Organs with a particularly rich blood supply include the brain and the lungs, so ethanol will tend to affect these organs sooner than others.
As it can diffuse freely, ethanol quickly becomes distributed throughout the water-based components of the body, that is, the blood and the cells of most tissues such as muscles and the brain. However, very little ethanol diffuses into the fatty tissue because ethanol is much more soluble in water than it is in fat.
Box 1 The breathalyser
A person suspected of drinking and driving is asked to blow into a detector device and the concentration of ethanol in this sample of exhaled air is measured and gives an estimate of the corresponding blood–alcohol concentration (BAC), that is how much alcohol is in the bloodstream. One way in which these devices operate is by using a chemical that undergoes a colour change in the presence of ethanol. Potassium dichromate undergoes a reaction with alcohol and, in so doing, it changes from orange to green, a colour change that can be measured quantitatively. You will see this reaction in Week 8 when you study some of the techniques scientists use to measure ethanol.