Inside the gut

 | January 1, 2008

Inside the gut

The "gut." It's an ancient Anglo-Saxon word that refers to the human digestive system. Think of this marvel of nature's engineering as a perpetual food processor, constantly mixing, grinding, and transforming the meats, vegetables, fruits, and snacks that people eat into biologically useful molecules.

Nearly 30 feet long if stretched out straight, the gut is a series of hollow organs linked to form a long, twisting tube that runs from the mouth to the anus. This string of organs is known as the alimentary canal, gastrointestinal (GI) tract, or digestive tract (see Figures 1 and 2). It comprises the esophagus (or gullet), stomach, small intestine, and colon (which includes the rectum). These organs break down food and liquids — carbohydrates, fats, and proteins — into chemical components that the body can absorb as nutrients and use for energy or to build or repair cells. What's left is expelled by a highly efficient disposal system.

The organs of the gut are almost always moving, driven by muscles in their walls. These muscles consist of an outer longitudinal layer and an inner circular layer. The coordinated contractions of these layers push food and fluids the length of the canal, just as rolling waves deposit sand and shells on the shore. This dynamic movement along the gastrointestinal tract is known as peristalsis.

Helping with the job of digestion is the mucosa, or lining, of the mouth, stomach, and small intestine, which harbors glands that produce digestive enzymes. The salivary glands, liver, and pancreas also secrete juices that help make food soluble (dissolvable in water) so that nutrients can pass easily into the bloodstream.

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The digestive journey

Pop a grape, chocolate, or shrimp into your mouth. Immediately, digestion begins. In the mouth itself, the tongue and teeth help to get the process started by chewing and chopping the food so it's small enough to be swallowed. Salivary glands secrete saliva, releasing an enzyme that changes some starches into simple sugars and softens the food for swallowing. The saliva also allows the taste buds of the tongue to sense the flavors of your foods.

Swallowing is a complicated, coordinated act that begins when your tongue pushes food back into your throat or pharynx. This voluntary action sets off an involuntary chain of events that transports the food from the throat into the esophagus and down into the stomach, a journey that typically takes eight seconds.

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Esophagus

Food does not simply drop down the esophagus by means of gravity. Matter moves through this passageway because it is pushed by contractions of the esophageal muscles.

Think of the esophagus (along with the intestine) as an empty tube surrounded by layers of muscle that contract in a succession of waves. As the ball of food, called a bolus, travels toward the far end of the 10- to 13-inch-long tube, the lower esophageal sphincter (LES) opens to allow the food to exit, then closes again. This esophageal tube is quite elastic, stretching to nearly two inches across to accommodate foods of various sizes.

While the esophagus is moving things along, it also has to keep things from backing up (regurgitating) and re-entering the throat. That's where another gatekeeper, known as the upper esophageal sphincter (UES), comes into play. The two sphincters, upper and lower, make sure the food doesn't back up in the wrong direction.

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Stomach

If the esophagus is a conduit with a valve at each end, the stomach can be likened to a storage and processing facility, where the food is prepared for digestion.

This food warehouse can accommodate anything from a light afternoon snack to a five-course meal. Without this large storage capacity, people would have to eat small, frequent meals, and they'd be unable to drink large quantities of liquids at any given time.

But the stomach doesn't just hold food: muscles in the lower stomach also mix that food into a soft mush. This process is aided by the liquids we drink and by saliva, hydrochloric acid, and the enzyme pepsin. Hydrochloric acid and pepsin, produced by the glands that line the stomach, help break down proteins into their constituent amino acids. The stomach mucosa has a defense system, including an overlying layer of mucus and bicarbonate, to protect itself. After mixing, a once-palatable meal is reduced to a thick liquid called chyme.

The other important function of the stomach is delivering the resulting chyme to the small intestine in amounts it can handle. The involuntary contractions that push stomach contents along are governed by nerves in the stomach wall (see Figure 3), which transmit electrical impulses to the brain. The nerves that carry impulses from the GI tract, called visceral nerves, recognize stretching, pulling, or expansion (distension) of the muscles in the walls of the digestive tract. Pain can result when these sensations are excessive.

When you haven't eaten for a while and your stomach is empty, it initiates a series of rhythmic contractions known as hunger pangs. They serve as a signal to the brain: "Feed me!" These contractions explain stomach noises, which also can be caused when air or fluid is moving around inside. Once you've eaten, it takes about two hours for the muscular stomach to reduce a typical meal to a liquid and have it ready to move along to the small intestine. A high-protein meal can take an extra hour or two. A high-fat meal can take up to six hours. That's why eating foods with healthy fats (such as those in nuts) helps you feel full longer than eating high-carbohydrate foods like sugar.

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Small intestine

The small intestine, which is a remarkable 21 feet long, is the next stage on the journey that food makes through the digestive tract. The main work of digestion takes place in this long tube. The small intestine breaks down fats, starches, and proteins into fatty acids, simple sugars, and amino acids, which it can then absorb.

The food you eat generally takes three to five hours to move through the small intestine. During this time, the food is bathed in digestive enzymes and juices that flow into the intestine through ducts from the liver and pancreas. Bile, produced by the liver and stored in the gallbladder, emulsifies fat, enabling its absorption. Enzymes secreted by the pancreas, such as trypsin, amylase, and lipase, help digest proteins, carbohydrates, and fats. Once reduced to products the body can manage, the nutrients from digested food are absorbed by the intestine's thin lining and sent to cells throughout the body by way of the bloodstream and lymphatic system.

Food moves through the small intestine as if on a conveyor belt. The first step, after the stomach empties food through the pyloric sphincter, is the foot-long duodenum, located a few inches above the navel. Many minerals, such as iron and calcium, are absorbed in the duodenum. This is also where bile and pancreatic juices join the mix.

After the duodenum, the next part of the small intestine is the jejunum, which measures eight feet in length. In the jejunum, fats, starches, and proteins are further broken down and absorbed.

The third and lowest portion of the small intestine, the ileum, is approximately 12 feet long. The ileum absorbs water, as well as vitamin B₁₂ and bile salts.

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Colon (large intestine)

Finally, what's left of the food arrives in the colon, or large intestine, a four-foot-long muscular tube about the diameter of your fist, where the walls act like a sponge and soak up 80% to 90% of the remaining water. In fact, the colon accepts about a quart of liquid from the ileum each day. Once inside the colon, food residue travels up the right side (the ascending colon), across the transverse colon, down the left side (the descending colon), through the sigmoid colon to the rectum (behind the left side of the groin), and out of the body. The time required for food to move through the colon varies widely, but is generally in the range of four to 72 hours.

Bacteria that reside in the colon help in the digestive process, feeding off whatever remains of your meal. The bacteria produce fatty acids as well as hydrogen, carbon dioxide, and, in some people, methane gas. Some of these gases are consumed as nutrients by the cells of the colon, while others are expelled as waste. Undigested matter, such as fiber, is propelled along by contractions of the colon wall and settles as solids in the rectum, the final six inches of the colon.

The end of the rectum is guarded by sphincter muscles that help control what goes out. The waste accumulates until the rectal wall becomes so distended that it signals the internal anal sphincter to relax, triggering the urge for a bowel movement. Fortunately, the external anal sphincter, which is under voluntary control, keeps the rectal contents in place until a convenient time.

What comes out is primarily water and colon bacteria, plus bile, mucus, and cells normally shed from the intestinal lining. Undigested food makes up very little of the average quarter- to half-pound stool. The exception is fiber: The more fiber you ingest, the greater the quantity of your stool.

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The aging GI tract

Aging takes a toll on the GI tract. Aging muscles, including the digestive muscles, contract more slowly, take plenty of time relaxing, and move their contents along at a more leisurely pace. For the most part, that's fine — unless you become impatient, take drastic measures to hurry things along, or develop a condition that needs a doctor's attention. Many of the aging GI system's failures can be prevented or corrected.

The mouth. The changes begin at the top, in the mouth, where the number of taste buds begins to decline with age. So does the sensitivity of those that remain. The chewing muscles also begin to weaken. As a result, some older folks lose interest in food, begin to lose weight, and develop nutritional deficiencies. Losing teeth may also reduce interest in eating. Good dental care is important so that eating doesn't become a problem.

The esophagus. Swallowing can also become more difficult as people age. Such problems are usually the result of neurological or muscular disorders. Very old people may experience a weakening of the muscles of the esophagus, which contract less vigorously around food after swallowing. Acid reflux is often a problem in the elderly, the result of the decline in esophageal contractions and in the function of the lower esophageal sphincter muscle. However, since the esophagus may be less sensitive to acid with age, acid reflux may not result in heartburn. Instead, patients may complain of nausea or vague chest discomfort. Any new onset of difficulty in swallowing should be evaluated by a doctor because the problem could be related to cancer of the esophagus or to a motor disorder (achalasia), more common in those who are older.

The stomach and duodenum. As people age, the stomach continues to make acid, but in many older people, acid production declines because of years of carrying Helicobacter pylori infection in the stomach, leading to long-term gastritis and to atrophy of the stomach lining. While a reduction in gastric acid does not usually interfere with digestion, it can lead to two disorders that are common in the elderly — vitamin B₁₂ deficiency, which can result in anemia and nerve damage, and excessive bacterial growth in the small intestine, resulting in malabsorption and poor digestion. Both problems can be treated.

The colon. Moving one's bowels may be the most frequent gastrointestinal challenge associated with aging. The problem is usually the result of a poorly functioning or diseased large intestine. Problems with this organ can also result in diarrhea and hemorrhoids. In addition, the risk for colon cancer and polyps increases with age. In fact, one in three senior citizens has one or more polyps in the colon. That's why a screening exam called a colonoscopy is recommended on a regular basis after age 50. Since colon cancer evolves from polyps, removal of polyps will keep colon cancer from getting started. In general, people pass less stool after they reach age 65. In part, this may be the result of a change in diet to softer foods, a decreased appetite, or diminished muscular activity of the colon. Constipation may also be the result of a neurological problem (see "Constipation").

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Review Date: 2008-01-01

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