How Stuff Works: How the liver works

HowStuffWorks.com (MCT) - There is lots of talk these days about the gas tank in your car, especially when it comes to the cost of filling it up. But here's a question: Have you ever thought about the gas tank inside your body? Yes, each human being has a "gas tank". It's your liver. Let's take a look at how it works.

To understand your body's gas tank, we first need to talk about the fuel your body uses. The primary fuel is glucose. When you eat any carbohydrate, from bread to rice to potatoes to a sugary soft drink, your body chops the carbohydrate molecules up into individual sugar molecules and absorbs them into the bloodstream via the small intestine. When there is glucose pouring into the bloodstream after a meal, times are good.

But imagine that you are waking up in the morning. It's a new day, and you haven't eaten since dinner last night at 6. The food that you put into your stomach at supper got digested hours ago. But your body still needs glucose to operate. Your brain, for example, depends on glucose in the bloodstream to keep its neurons firing. This is one place where your liver steps in.

Your liver is largest internal organ that you've got, weighing about 3 pounds. One of its jobs is to keep your energy supply going when food isn't available. When your liver sees a surplus of glucose in the blood (for example, after a meal), it absorbs the excess and converts it to a starch called glycogen to store it. Then, when your body needs glucose (for example, as you are waking up in the morning after your nighttime fast), the liver converts the glycogen back to glucose and releases it into the bloodstream. Your liver "gas tank" holds approximately 12 hours worth of glycogen.

Once your liver runs out of glycogen, it has several other tricks up its sleeve to handle your body's energy needs. For example, your liver can convert amino acids to glucose if it needs to. It can also convert fatty acids into something called ketone bodies. Many cells can use ketone bodies as an alternative to glucose for energy. When you think about your body "burning fat", this is how it happens. The fat comes out of your fat cells and into the bloodstream as fatty acids. Your liver converts the fatty acids to ketone bodies and sends them out in the bloodstream. Some cells absorb the ketone bodies and use them for energy. Other cells can use the fatty acids directly.

Speaking of fat, your liver plays a big role in digesting fat. Imagine that you eat a plateful of greasy french fries. All of the fat from the fries makes its way into your small intestine, and now it needs to get digested. But oil and water don't mix. Your liver helps out by producing a greenish fluid called bile. The bile flows into the small intestine, and it helps to convert fat into a water soluble form. Think about soap _ soap molecules are able to connect to fat molecules so they can dissolve in water. Bile works in a similar way to emulsify fats. Then other chemicals in the small intestine called lipases break the fat molecules down.

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Another big job for the liver is to remove foreign substances from the bloodstream. For example, you may have noticed that you have to take most medicines every four to six hours. If you take aspirin, its effects wear off four hours later. It's the same for Tylenol, nicotine and most other drugs. The liver metabolizes all of these different drugs so that they get cleared out of the bloodstream.

Alcohol is another drug that the liver processes. However, if you drink too much alcohol, the liver gets overloaded. In the worst cases, heavy drinkers get cirrhosis of the liver, where sections of the liver die and turn into scar tissue. You can see how important the liver is. Once too much of the liver dies, it stops doing its many different jobs and the liver's owner dies as well.

The good news about liver disease is that, in some cases, your liver can recover. A liver transplant is also an option. A donor liver can come from someone who has recently had an accident, or by taking half of the liver out of a living donor. The other half of the liver regenerates in both the donor and the recipient. When successful, both donor and recipient end up with full-sized, completely normal livers.

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(Looking for more? For extra info on this or the scoop on other fascinating topics, go to HowStuffWorks.com. Contact Marshall Brain, founder of HowStuffWorks, at marshall.brain@howstuffworks.com.)

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