3.2 What does the liver do?

The liver weighs about 1.5 kg in an adult and sits just below your ribs. The liver is reported to have 500 different functions. One of them is to make bile and store it in the gall bladder, so that it can be released to help with digestion, as mentioned earlier.

You know already that the liver contains powerful enzymes. You saw in Do the liver experiment just how quickly catalase can break down hydrogen peroxide, a dangerous by-product of some chemical reactions in the body.

Enzymes in the liver also break down any unwanted substances that arrive in the hepatic portal vein from the intestines. Alcohol is one of the main ones. If a person consumes alcoholic drinks, the alcohol is absorbed into the blood through the walls of the stomach or through the villi of the small intestine into the blood. Up to 5% of this alcohol is removed in urine by the kidneys and up to 5% is breathed out (that is the alcohol that breathalysers detect). At least 90% of the alcohol a person consumes is dealt with by the liver. It is broken down by enzymes to acetate molecules, which can be used to produce energy. Excessive consumption of alcohol can damage the liver, causing cirrhosis.

Any spare sugars are stored by the liver as glycogen. When needed, the glycogen can be released back into the blood to keep the blood-sugar level constant. The level of sugar (glucose) in the blood needs to be quite closely regulated. This is done using insulin produced by the pancreas. This ensures that the brain, the muscles and the rest of the body are supplied with the correct level of glucose they need for energy.

The liver also deals with the fatty acids and monoacylglycerols. Some are joined back together to make the fats that the body needs, possibly after some modification. Others are used for energy, and any spare ones are probably sent to the fat cells, mostly under the skin, where they are stored as fat. Excess amino acids in the blood arriving at the liver are broken down to ammonia and then to urea. This is filtered out of the blood when it reaches the kidneys, which is where urine is produced.