The Human Digestive System

What's 30 feet long and devours more than 50 tons of food in its life? A python? The shark from Jaws? A genetically modified monster turkey? No, it's your digestive system.

OK, so maybe digestion isn't as sexy as reproduction or as thought-provoking as your brain. But your digestive system keeps you alive. Most of the food you eat, no matter how nutritious it is, can't be immediately absorbed by your body. It needs to be digested first, and that's hard work that deserves your respect--especially after a holiday feast.

So, to find out more about this important process, we're going to send someone down there: namely, you. It's Thanksgiving, and you're not carving turkey--you are a turkey, and you're going to get gobbled. Good luck. In less than two days, we'll see you on the other side. Well, sort of.

Usually when they stick a fork in you, you're done. But not this time. Your journey is only beginning, as the entrance to a wild ride is closing in: the mouth. Immediately, two sets of hard enameled teeth go to work on you. The front ones--incisors and canines--rip and tear you into small pieces, while the back ones--bicuspids, molars, and wisdom teeth--pound and mash you into a pulp.

As if that weren't enough, you're being spit on, too. You're getting soaked in saliva, the first of many secretions the digestive system will squirt at you. You quickly become softer and easier to swallow, and digestive enzymes start to break apart the starches inside you. After just a few juicy chews, your once-tough flesh has been broken down into a small, mushy mass that medical types call a bolus.

Now it's time for the squeeze, and you better get used to it because it's going to happen a lot. You're pushed to the back of the mouth, where the upper muscles of the throat contract and force you down into the esophagus--you're getting swallowed. After the swallow comes a pattern of muscular contraction called peristalsis, in which muscles contract in sequence along the length of the digestive tract, moving food along.

Your journey lasts only a few seconds before you splash down in the stomach. If you thought the mouth was bad, try getting comfortable in here. The stomach is a muscular compartment that holds about a liter of food, though it can expand to hold as much as four (a useful talent on days like Thanksgiving). Like the mouth, the stomach rhythmically contracts to break up and mix food. But it has far worse secretions to drench you in than saliva.

That stinging sensation comes from the hydrochloric acid in the gastric juice you're bathing in. One of the most powerful acids known, it quickly kills off any microbial life you may have brought in with you. Meanwhile, another gastric juice component, the enzyme pepsin, starts breaking down your proteins. After stewing in the stomach for a few hours, you're really not looking much like your old self. By this time, you're a soupy, nutrient-rich mass of small particles called chyme, and you're on your way to the small intestine.

As you arrive, you're joined by new secretions that will digest you even more. Coming in from the liver, by way of the gallbladder, is bile. A complex mix of acids, lipids, and electrolyte chemicals, bile breaks down fats, so if you weren't lean meat before, you sure are now. In addition to bile, you're also being mixed with a variety of chemicals coming in from the pancreas. One is bicarbonate, which helps neutralize the acid from the stomach. The others are enzymes that break down starches, fats, and proteins.

The small intestine itself doesn't secrete much except water, but it doesn't have to. All the good stuff in you has been broken down and is ready to go. Over the course of about five hours, you're pushed along 20 feet (6 meters) of narrow, winding curves. All the while, millions of tiny fingerlike projections called villi suck up your nutrients and disperse them into the body. Proteins, fats, carbohydrates, vitamins, electrolytes, water--all are absorbed. Finally, what's left of you is ready for the last step in digestion.

After leaving the small intestine, things get a little less constricting. What's left of you has entered the large intestine, and it's a bizarre place. The large intestine doesn't secrete much, only a bit of mucus to help move you along. It mostly absorbs water--about 1.6 gallons (6 liters) of it every day--along with a few electrolytes left behind by the small intestine. But that doesn't mean there isn't quite a bit brewing in here.

You quickly get the feeling you're not alone, because the large intestine is teeming with microbial life, and it's been waiting for you. Many different species begin to attack, breaking down parts of you that the stomach, well, couldn't stomach. Some even synthesize key nutrients like thiamin (vitamin B1) and vitamin K.

Tougher stuff like cellulose is actually fermented in the process. The result is gas--lots of gas. You don't smell so sweet either. After roughly a day of slow percolating, you're a mass of bacteria-infested undigested fiber and water. One last squeeze from those digestive muscles and you're released. The end result: a healthy, growing body for the eater and a burial at sea for you.

Christopher Call
Updated November 22, 2006