The Digestive Journey - How The Gut Works: Digestive Disorders

Content provided by the Faculty of the Harvard Medical School

The digestive journey

Pop a grape, a chocolate, or a 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. This activity, aided by the chemical composition of your diet, is catalyzed by salivary glands, which secrete saliva, releasing an enzyme that starts changing some starches into simple sugars and softening 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 you begin by using your tongue to push 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.

Esophagus

Food does not simply drop down the esophagus by means of gravity. Things move through this passageway because they are pushed by contractions of the esophageal muscles.

Think of the esophagus (and the intestines) as an empty tube surrounded by coats of muscle that contract in a succession of waves. As the ball of food, called a bolus, travels toward the far end of the 16-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 the other "gatekeeper" — the upper esophageal sphincter — comes into play. The two sphincters (upper and lower) make sure the food doesn't back up in the wrong direction.

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, apart from storing and then grinding and mashing the food, is to deliver the resulting chyme to the small intestine in amounts it can handle. Too large a load could overwhelm the intestine's ability to absorb nutrients. Peristaltic contractions drive this mixture through the pyloric sphincter — another muscular gate in the digestive tract — and into the duodenum, the first part of the small intestine. The process of delivering chyme to the intestine occurs over time and is affected by numerous factors: a variety of hormones, what's been ingested (fluids move more quickly than solids), and external considerations such as emotions and physical exercise. All can either delay or stimulate stomach emptying.

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 (distention) of the muscles in the walls of the digestive tract. Intestinal pain can result when these sensations are excessive.

Figure 3: The stomach wall The stomach lining is not a smooth, balloon-like surface. Instead, it has several layers that contain neuron connections to the brain as well as glands that secrete juices to help digest food.

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.

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, involves the foot-long duodenum, which is 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.

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%–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 groin), and out of the body. The time required for food to move through the colon varies widely, generally in the range of 4–72 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 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 an 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.

When all goes according to nature's plan, you're hardly aware of this long, complex process. But when something goes wrong — when the gut acts up — it can cause untold miseries.

Last updated:	August 21, 2007