When you see (or smell) something appetizing, your digestive organs leap into action. Your mouth waters. Your stomach contracts. Intestinal glands begin to secrete the chemicals that turn food into the nutrients that build new tissues and provide the energy you need to keep zipping through the days, months, and years.

This article introduces you to your digestive system and explains exactly how your body digests the many different kinds of foods you eat, all the while extracting the nutrients you need to keep on truckin’.

Digestive System

Your digestive system may never win a Tony, Oscar, or Emmy, but it certainly deserves your applause for its ability to turn complex food into basic nutrients. Doing this requires not a cast of thousands but a group of digestive organs, each designed specifically to perform one role in the two-part process.

Digestive organs

Although exceedingly well-organized, your digestive system is basically one long tube that starts at your mouth, continues down through your throat to your stomach, and then goes on to your small and large intestines and past the rectum to end at your anus.

In between, with the help of the liver, pancreas, and gallbladder, the usable (digestible) parts of everything that you eat are converted to simple compounds that your body can easily absorb to burn for energy or to build new tissue. The indigestible residue is bundled off and eliminated as waste.

Figure 1 shows the body parts and organs that comprise your digestive system.

Digestion: A two-part process

Digestion is a two-part process — half mechanical, half chemical:

• Mechanical digestion takes place in your mouth and your stomach. Your teeth break food into small pieces that you can swallow without choking. In your stomach, a churning action continues to break food into smaller particles.

• Chemical digestion occurs at every point in the digestive tract where enzymes and other substances, such as hydrochloric acid (from stomach glands) and bile (from the liver), dissolve food, releasing the nutrients inside.

How Body Digests Food

Each organ in the digestive system plays a specific role in the digestive drama. But the first act occurs in two places that are never listed as part of the digestive tract: your eyes and nose.

The eyes and nose

When you see appetizing food, you experience a conditioned response. In other words, your thoughts — “Wow! That looks good!” — stimulate your brain to tell your digestive organs to get ready for action. What happens in your nose is purely physical.

The tantalizing aroma of good food is transmitted by molecules that fly from the surface of the food to settle on the membrane lining of your nostrils; these molecules stimulate the receptor cells on the olfactory nerve fibers that stretch from your nose back to your brain.

When the receptor cells communicate with your brain — “Listen up, there’s good stuff here!” — your brain sends encouraging messages to your mouth and digestive tract. In both cases — eyes and nose — the results are the same: “Start the saliva flowing,” they say. “Warm up the stomach glands. Alert the small intestine.”

In other words, the sight and scent of food has made your mouth water and your stomach contract in anticipatory hunger pangs. But wait! Suppose you hate what you see or smell?

For some people, even the thought of liver is enough to make them want to barf — or simply leave the room. At that point, your body takes up arms to protect you: You experience a rejection reaction — a reaction similar to that exhibited by babies given something that tastes bitter or sour.

Your mouth purses and your nose wrinkles as if to keep the food (and its odor) as far away as possible. Your throat tightens, and your stomach turns — muscles contracting not in anticipatory pangs but in movements preparatory for vomiting up the unwanted food. Not a pleasant moment. But assume you like what’s on your plate. Go ahead. Take a bite.

The mouth

Lift your fork to your mouth, and your teeth and salivary glands swing into action. Your teeth chew, grinding the food, breaking it into small, manageable pieces. As a result:

• You can swallow without choking.

• You break down the indigestible wrapper of fibers surrounding the edible parts of some foods (fruits, vegetables, whole grains) so that your digestive enzymes can get to the nutrients inside.

At the same time, salivary glands under your tongue and in the back of your mouth secrete the watery liquid called saliva, which performs two important functions:

• Moistening and compacting food so that your tongue can push it to the back of your mouth and you can swallow, sending the food down the slide of your gullet (esophagus) into your stomach.

• Providing amylases, enzymes that start the digestion of complex carbohydrates (starches), breaking the starch molecules into simple sugars.

No protein digestion occurs in your mouth, though saliva does contain very small amounts of lingual lipases, fat-busting enzymes secreted by cells at the base of the tongue; however, the amount is so small that the fat digestion that occurs in the mouth is insignificant.

The stomach

If you were to lay your digestive tract out on a table, most of it would look like a simple, rather narrow tube. The exception is your stomach, a pouchy part just below your gullet (esophagus).

Like most of the digestive tube, your stomach is circled with strong muscles whose rhythmic contractions — called peristalsis — move food smartly along and turn your stomach into a sort of food processor that mechanically breaks pieces of food into ever smaller particles.

While this is going on, glands in the stomach wall are secreting stomach juices — a potent blend of enzymes, hydrochloric acid, and mucus. One stomach enzyme — gastric alcohol dehydrogenase — digests small amounts of alcohol, an unusual nutrient that can be absorbed directly into your bloodstream even before it’s been digested.

Other enzymes, plus stomach juices, begin the digestion of proteins and fats, separating them into their basic components — amino acids and fatty acids. Stop! If the words amino acids and fatty acids are completely new to you and if you are suddenly consumed by the desire to know more about them this instant. Stop again!!

For the most part, digestion of carbohydrates comes to a screeching — though temporary — halt in the stomach because the stomach juices are so acidic that they deactivate amylases, the enzymes that break complex carbohydrates apart into simple sugars. However, stomach acid can break some carbohydrate bonds, so a bit of carb digestion does take place.

Back to the action. Eventually, your churning stomach blends its contents into a thick soupy mass called chyme (from cheymos, the Greek word for juice). When a small amount of chyme spills past the stomach into the small intestine, the digestion of carbohydrates resumes in earnest, and your body begins to extract nutrients from food.

The small intestine

Open your hand and put it flat against your belly button, with your thumb pointing up to your waist and your pinkie pointing down. Your hand is now covering most of the relatively small space into which your 20-foot-long small (20 feet? small?) intestine is neatly coiled.

When the soupy, partially-digested chyme spills from your stomach into this part of the digestive tube, a whole new set of gastric juices are released. These include:

• Pancreatic and intestinal enzymes that finish the digestion of proteins into amino acids

• Bile, a greenish liquid (made in the liver and stored in the gallbladder) that enables fats to mix with water

• Alkaline pancreatic juices that make the chyme less acidic so that amylases (the enzymes that break down carbohydrates) can go back to work separating complex carbohydrates into simple sugars

• Intestinal alcohol dehydrogenase, which digests alcohol not previously absorbed into your bloodstream

While these chemicals are working, contractions of the small intestine continue to move the food mass down through the tube so that your body can absorb sugars, amino acids, fatty acids, vitamins, and minerals into cells in the intestinal wall.

The lining of the small intestine is a series of folds covered with projections that have been described as “finger-like” or “small nipples.” The technical name for these small fingers/nipples is villi. Each villus is covered with smaller projections called microvilli, and every villus and microvillus is programmed to accept a specific nutrient — and no other.

Nutrients are absorbed not in their order of arrival in the intestine but according to how fast they’re broken down into their basic parts:

• Carbohydrates — which separate quickly into single sugar units — are absorbed first.

• Proteins (as amino acids) go next.

• Fats — which take longest to break apart into their constituent fatty acids — are last. That’s why a high-fat meal keeps you feeling fuller longer than a meal such as chow mein or plain tossed salad, which are mostly low-fat carbohydrates.

• Vitamins that dissolve in water are absorbed earlier than vitamins that dissolve in fat.

After you’ve digested your food and absorbed its nutrients through your small intestine:

• Amino acids, sugars, vitamin C, the B vitamins, iron, calcium, and magnesium are carried through the bloodstream to your liver, where they are processed and sent out to the rest of the body.

• Fatty acids, cholesterol, and vitamins A, D, E, and K go into the lymphatic system and then into the blood. They, too, end up in the liver, are processed, and are shipped out to other body cells.

Inside the cells, nutrients are metabolized, or burned for heat and energy or used to build new tissues. The metabolic process that gives you energy is called catabolism (from katabole, the Greek word for casting down). The metabolic process that uses nutrients to build new tissues is called anabolism (from anabole, the Greek word for raising up).

The large intestine

After every useful, digestible ingredient other than water has been wrung out of your food, the rest — indigestible waste such as fiber — moves into the top of your large intestine, the area known as your colon. The colon’s primary job is to absorb water from this mixture and then to squeeze the remaining matter into the compact bundle known as feces.

Feces (whose brown color comes from leftover bile pigments) are made of indigestible material from food, plus cells that have sloughed off the intestinal lining and bacteria — quite a lot of bacteria. In fact, about 30 percent of the entire weight of the feces is bacteria. No, these bacteria aren’t a sign you’re sick. On the contrary, they prove that you’re healthy and well.

These bacteria are good guys, microorganisms that live in permanent colonies in your colon, where they:

• Manufacture vitamin B12, which is absorbed through the colon wall

• Produce vitamin K, also absorbed through the colon wall

• Break down amino acids and produce nitrogen (which gives feces a characteristic odor)

• Feast on indigestible complex carbohydrates (fiber), excreting the gas that sometimes makes you physically uncomfortable — or a social pariah

When the bacteria have finished, the feces — perhaps the small remains of yesterday’s copious feast — pass down through your rectum and out through your anus. But not necessarily right away: Digestion of any one meal may take longer than a day to complete. After that, digestion’s done!