10.8: Small Intestine—Where the Action Is

The first 12 inches of the small intestine is the duodenum [from the Latin “twelve (fingers breadth) each”]. It’s given a separate name, distinguishing it from the rest of the intestine, because so much happens here.

As the stomach churns the digesting food, the contractions squirt a little of the now-liquefied food (called chyme) down into the duodenum. The muscular ring between the stomach and small intestine only opens when the food has been liquefied into chyme and only allows it to pass through in small and steady squirts. The duodenum isn’t as well protected as the stomach against acid, but it does get some chemical protection from some nearby alkaline secretions.

The duodenum is a common site of painful ulcers, which can bleed. But pain isn’t a dependable warning sign. Many people diagnosed with duodenal ulcers don’t report pain as a symptom. Sometimes the discomfort is misinterpreted as hunger, which can unwittingly lead to overeating and weight gain. When there isn’t pain, a bleeding ulcer is often discovered by blood in the stool, or anemia resulting from blood loss.

Surgical removal of part of the stomach was once a common treatment for duodenal ulcers. (When part of the stomach is removed, less acid is produced.) Drugs (e.g., cimetidine/Tagamet) that reduce the stomach’s acid production have replaced surgery as the standard treatment.

Most duodenal ulcers are caused by H. pylori bacteria. (Most of the other cases are caused by “nonsteroidal anti-inflammatory agents,” like ibuprofen and aspirin.) H. pylori infection can be diagnosed by a blood or breath test, and treated with a precise combination of medications that includes antibiotics.

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H. pylori infection is thought to occur by close person-to-person contact and by exposure to vomit of an infected person. A vaccine is being developed.⁵

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Ulcers can also occur in the esophagus or stomach, but are much less common. H. pylori-related stomach ulcers raise the risk of stomach cancer, but this doesn’t seem to be the case with duodenal ulcers.

From a meal, carbohydrates and proteins leave the stomach first (after two hours or more). Fats leave last—not leaving completely for perhaps four or five hours. (Fats tend to be the last to leave the stomach on the downward, digestive journey because characteristically they float to the top of the watery mixture.)

This doesn’t mean that all carbohydrates or all fats follow tidy, distinct schedules. Remember that the stomach has worked hard to make an intimate mix of small particles. The entry of chyme into the duodenum triggers a flurry of activity and a medley of individual digestive processes.

Digestive fluids from the pancreas, liver, and gallbladder enter by way of small tubes (ducts) that converge into a single duct (the bile duct) which empties into the duodenum. Like vicious animals awaiting their prey, the digestive enzymes, bile, and bicarbonate rush through the bile duct and into the duodenum to literally pounce on the incoming chyme.

Digestive Secretions from the Pancreas

The pancreas is about 6 inches long and about 1 inch thick and lies horizontally behind the stomach. It’s a lumpy sort of organ, resembling blobs of whitish flesh. In fact, its name comes from two Greek words which mean “all flesh.” If you’ve ever eaten sweetbreads, you’re acquainted with the pancreas of the cow. (A good many people who get a look at the lumpy cow pancreas go to some lengths to avoid eating it.)

The digestive duty of the pancreas is to make sodium bicarbonate (“baking soda”) and digestive enzymes, and secrete them into the duodenum. The bicarbonate is alkaline and neutralizes the acid chyme coming from the stomach. Pancreatic enzymes include those necessary to digest starch, protein, and fat (see Table 10-2).

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As discussed in Chapter 5, the pancreas also makes insulin and glucagon, the hormones that regulate blood sugar.

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Digestive Secretions from the Liver

The liver lies just below the diaphragm and weighs about three pounds. Its role in digestion is to make and secrete bile. Bile isn’t an enzyme but, rather, an emulsifier that divides dietary fat into very small particles (see Fig. 10-3).

From the moment that fat enters the digestive tract, it presents a special problem because it doesn’t dissolve in water—a characteristic that’s not compatible with much of body chemistry. The digestive system, along with most of the body’s chemistry, is a watery system.

We have a similar problem when we wash dishes. We can’t deal with the grease of dirty dishes with water alone. So we add detergent. Detergents don’t really make fats dissolve in water. They divide the fat into very small particles so that they can be carried away.

Emulsification is the process of finely dividing the fat so that it’s suspended in a water-based fluid. It’s analogous to the homogenization of whole milk, in which the fat (cream) is finely divided so that it stays suspended in the fat-free portion of milk, rather than rising to the top. But in homogenization, the fat particles are made smaller by mechanical means (i.e., forced through a small nozzle), whereas an emulsifier such as bile has chemical properties that finely divide the fat (see Fig. 10-3).

Finely dividing the fat greatly increases its surface area. This is crucial for fat digestion because the fat-digesting enzymes are in a watery solution and can’t penetrate the fat. The fat-digesting enzymes can only reach and digest the fat that lies exposed on the surface of a fat particle.

Bile is made up of bile acids, bile pigments, cholesterol, and other substances—all dissolved together in an alkaline solution. The bile acids are made from cholesterol, and the bile pigments are made from breakdown products of hemoglobin (the oxygen-carrying molecule in red blood cells). Bile contains both red and green pigments, and the two can form a color range from yellow to green to varying shades of brown, effectively changing the color of the feces.

Lacking true explanations, changes in the color of bile were used for centuries to explain illness and changes in mood. Recall a part of the 12th century poem quoted in Chapter 1:

“Peaches, apples, pears…engender black bile and are enemies of the sick.”

Abraham Lincoln’s young son Willie died from what was diagnosed as “bilious fever” (probably typhoid fever). Even today, we describe an ill-tempered person as bilious.

Bile is made steadily by the liver—about a quart a day. But since the need for it isn’t constant, the bile is shunted aside into the gallbladder, where it’s stored until needed.

The Gallbladder

The gallbladder is a pear-shaped organ about 3 or 4 inches long that serves as a reservoir for the bile produced by the liver. Here the bile is concentrated and sits at-the-ready, to be released into the intestine to do its job of emulsifying fat.

The release of this bile is triggered by the entry of chyme into the small intestine. The intestine responds to this chyme by producing a hormone, which travels via the bloodstream to the gallbladder and causes the gallbladder to contract and squirt bile into the duodenum.

The gallbladder isn’t an essential organ. Many people have their gallbladder removed without serious consequences. Without a gallbladder, bile enters the duodenum directly from the liver in a steady and less concentrated amount. Even after a gallbladder has been removed, most fatty foods are still tolerated, unless they’re eaten in particularly large amounts.

The usual reason for removing the gallbladder is the painful presence of gallstones. In the United States, gallstones are common, and their incidence increases with age. Cholesterol is a normal component of bile and is the major component of most gallstones. Cholesterol tends to form stones because it doesn’t readily dissolve in bile fluid.

Figure 10-3: Bile salts emulsify fats into small droplets.

Precisely why gallstones form isn’t known, but they are more common among women, the obese, Native-Americans, Mexican-Americans, those eating a western diet, and those with a family history of gallstones. A stone can cause pain and inflammation if it blocks the flow of bile in the gallbladder itself or in the bile duct leading to the duodenum. The gallbladder is removed if this stone can’t be passed, broken up by ultrasound, or dissolved by medication.

The Incredible Lining of the Small Intestine

The adult small intestine is about 1 1/2 inches wide and about 20 feet long when relaxed (about 10 feet long when contracted). It’s the main site of digestion and absorption.

The ancients were puzzled by its great length. If the stomach, as they believed, was where digestion took place, what use was all this coiled tubing? Today we understand that the stomach does relatively little digesting and absorbing of nutrients. The stomach is more of a churning, acid place of preparation for digestion and absorption, and as a muscular reservoir of food that squirts out chyme onto the luxurious carpet of the small intestine.

The lining of the small intestine is truly remarkable. In much the same way as a wadded terry-cloth towel is used to quickly absorb a spill, the absorptive surface of the small intestine is expanded by folds and loops (see Fig. 10-4). The folds in the lining are covered with tiny upraised fingers (villi) which are densely packed, like very expensive carpet.

Figure 10-4: Inner Surface of the Small Intestine

Each of the villi, in turn, is covered with a brush-like set of projections of its own—the microvilli. It’s been estimated that the inner surface area of a person’s small intestine (including all the villi and microvilli) is about 1800 square feet—about the floor space of a three-bedroom house. This lining is only one cell thick and is continually renewed. There’s a completely new lining about every three days. New intestinal cells are born at the base of the villi, and they migrate to the tips of the villi where—at the “old age” of three days—they come off.

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The microvilli also have digestive enzymes such as lactase (which splits the “milk sugar” lactose) and sucrase (which splits the “table sugar” sucrose).

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Imagine the chyme squirted into the small intestine, being immediately mixed with the rich digestive fluid coming from the bile duct, and continually pushed against an active and extensive absorptive surface that aids and abets rapid digestion and absorption. This process is so effective that virtually all the nutrients have been completely absorbed by the time the digestive material reaches the lower part of the small intestine. But not all: Vitamin B₁₂ combined with intrinsic factor is absorbed in this lower segment. Bile acids are also absorbed here so they can be recycled to the liver and made into bile again.

The remarkable efficiency of the small intestine is exemplified by surgically bypassing most of the small intestine so that a person can overeat and still lose weight. Mainly in the 1970s, this desperate procedure was performed on extremely obese patients. To achieve the malabsorption necessary for rapid weight loss, all but about two or three feet of the small intestine was shunted aside. Rapid weight loss was a result—along with massive amounts of smelly stool.

Once the desired weight loss was achieved, the intestinal bypass could be undone. But, unless the old dietary habits had changed, the lost weight returned. The use of this procedure faded when serious side-effects, such as liver failure, became a recognized complication. There are now safer surgeries for the extremely obese (“bariatric surgery”); most limit the amount of food entering the stomach (e.g., gastric bypass).