The science of alcohol
The science of alcohol

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The science of alcohol

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).

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The gut is a long tube which runs all the way from the mouth to the anus. Its purpose is to extract the chemicals from the diet that the body requires for survival. When food has been broken down into molecules small enough to be absorbed through the wall of the gut, they can enter the bloodstream. Dissolved molecules travel to all the tissues of the body, carried along in the blood, which is pumped around by the heart. This system of absorption and circulation is responsible for taking the ethanol from an alcoholic drink to the organs in the body, including those such as the brain, skin, and kidneys, where it has its characteristic effects. The start of the ethanol journey is in the mouth, the entrance to the gut. Ethanol, like all drinks, passes quickly through the mouth and is swallowed, passing down the oesophagus and entering the stomach. Food is kept in the stomach by a constriction of the pyloric sphincter. This means that ethanol is also retained in the stomach if food is present. When sufficient digestion has taken place, the pyloric sphincter opens, and the food and ethanol are released into the small intestine, where small molecules can be absorbed. But an alcoholic drink arriving in an empty stomach is not sufficient to initiate closure of the pyloric sphincter. The ethanol doesn't linger in the stomach and instead passes quickly into the small intestine. This is why people become intoxicated more quickly on an empty stomach. The small intestine is the part of the gut that is specially constructed to facilitate the absorption of digested food and other small molecules into the surrounding blood vessels. It is highly adapted for absorption. This is partly because it is five metres long, and the contents pass down it slowly. But if you look at a cross section of the small intestine, it also has millions of projections on its inner surface, which maximise the area through which small molecules can pass out of the gut. These finger-like projections are called villi. They project out from the gut wall into the lumen, the region inside the tube. The surface of the villi is formed from specialised cells called epithelial cells. The epithelial cells have projections that increase their surface area still further. These microscopic structures are called microvilli. The epithelial cells form a thin barrier between the lumen of the gut and the numerous blood vessels that are intimately associated with the gut wall in this region. This barrier is so thin that small dissolved molecules such as ethanol can easily pass through to be transported away in the bloodstream. When ethanol molecules move from the gut to the blood in this way, their absorption is driven by a process called diffusion. Diffusion is an important process in the body whereby many different molecules move from an area of higher concentration to one of lower concentration. The diagram shows two compartments separated by a permeable barrier through which the molecules can move. They are free to move in either direction through the barrier. But because the concentration on the right is much higher than on the left, the net movement is from right to left. More molecules are moving from the area of high concentration to the area of lower concentration than are coming back the opposite way. This is referred to as molecules diffusing down their concentration gradient. When the concentration in both compartments becomes equal, the concentration gradient has been abolished, and the number of molecules moving back and forth is equal. There is no net movement in either direction. Diffusion is the process by which ethanol moves from the gut lumen through the gut wall and into the bloodstream. When ethanol molecules arrive in the small intestine, the concentration is higher inside the gut lumen than it is in the blood. This means that a concentration gradient exists between these two compartments, which drives the diffusion of ethanol through the gut wall and into the blood. The higher ethanol concentration in the small intestine is maintained as long as alcohol drinking continues. The lower concentration in the blood is maintained by the blood flow, which rapidly transports the ethanol molecules away. This is how the ethanol concentration gradient across the gut wall drives ethanol to diffuse into the bloodstream. The blood vessels around the gut into which the ethanol has diffused all drain into one big vein called the portal vein. This leads directly from the gut to the liver, which ensures that all substances that are absorbed into the blood from the gut must pass through the liver before they can reach the rest of the body. So ethanol absorbed from the gut is carried straight to the liver via the portal vein. The cells in the liver have the ability to remove some ethanol from the blood and convert it to other chemicals. This means that the blood flowing out of the liver contains a lower concentration of ethanol than the blood flowing in through the portal vein. The blood leaving the liver flows to the heart, which pumps blood first to the lungs and then all around the rest of the body to other organs, such as the brain and kidneys. This route around the body includes the liver again, because the liver gets blood via arteries from the heart as well as from the portal vein. The cells in the liver continue to remove ethanol from the blood each time it passes through. Eventually, after ethanol intake has ceased and the blood has circulated through the liver enough times, all of the ethanol is removed from the blood, and the organs of the body are no longer exposed to it. However, the breakdown of ethanol may leave enough toxic chemicals in the body to cause a hangover.
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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.


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