When we talk about the digestive system, we should start with the brain because even before the food comes into the mouth, we're thinking about it — we're planning what we want to eat, smelling its aroma as it simmers on the stove, looking at it on the plate. We do, in a very real sense, eat with our eyes, or more specifically, with our heads. When we see or smell food or even if we think about a food we love, the brain sends signals to the nerves that control the gastrointestinal tract.
These signals put the digestive system on alert, as it were — our mouth begins to water, the stomach starts to contract to be ready to receive the food, and the pancreas, a glandular organ that releases enzymes essential to digestion, starts to secrete chemicals that will break down the food.
Inside the mouth the food is ground and broken down by the teeth while the saliva excreted there lubricates the food. Although we tend to secrete saliva more during meals or when thinking about food, we secrete small amounts of saliva to moisturize the mouth throughout the day. But saliva does more than simply moisten the food. It contains enzymes that start the chemical breakdown of the food, a process that will continue in the stomach and the intestines.
Once the food is chewed and moisturized by the saliva, it is pushed back by the tongue into the throat, where muscles propel the food into the food pipe, or the esophagus. The esophagus pushes the food downward by an action that we call peristalsis, which is basically an orderly sequence of contractions like the wave motion moving across stadium bleachers.
These contractions, which push the food down into the stomach, are powerful enough to allow us to swallow even if lying down — or upside down. Astronauts, for example, have no trouble swallowing in space, where no gravity forces food from the mouth to the stomach. Between the esophagus and the stomach a sphincter ensures that the passage normally opens only one way — from the esophagus into the stomach.
The Stomach The stomach breaks down the food not only physically with its powerful contractions but also chemically through the action of enzymes originally mixed into the food in the mouth and the stomach's own powerful acids and enzymes.
Although most of the enzymes, which chemically break down the food, are secreted in the small intestine, the small amounts secreted with the saliva and in the stomach juices jump-start the process. By the time the food leaves the stomach it has the consistency of porridge.
Pylorus and Small Intestine
Before the food leaves the stomach for the small intestine it passes through another sphincter, called the pylorus, which acts like a policeman directing rush-hour traffic down to a single-lane road. This powerful ring-like muscle is critical in the digestive process because it joins two organs that are very different in terms of size, shape, purpose and chemical environment.
The stomach is really a big storage bag but the small intestine is a narrow tube in which the major part of the digestive process takes place. The pylorus ensures that the small intestine is not over-filled by too much food entering all at once and that there is enough time for the digestive enzymes in the small intestine to break down the food chemically.
The Small Intestine
The contractions of the small intestine continue to break down the food physically, but more importantly, the small intestine secretes enzymes that break down the food chemically and absorbs the resulting nutrients. In the duodenum, the first part of the small intestine, the food is mixed with large amounts of digestive enzymes secreted by the pancreas, a nearby gland, and bile manufactured by the liver.
The chemical activity of digestion is critical to our ability to use the food we eat. Protein, for example, is a long chain of building blocks called amino acids. Although the body cannot absorb the entire chain, it can absorb amino acids, either singly or in pairs. Similarly, the complex fat molecules that we eat are broken down into fatty acids, and complex carbohydrates, long chains of sugars, are broken down into absorbable single sugars.
The body produces different enzymes to digest each type of nutrient. Some require intermediate steps for successful breakdown. Fats, for example, cannot mix with the watery contents of the duodenum, which contains the fat-breaking enzymes. The bile secreted by the liver first emulsifies the fat, breaking it down into small droplets that the enzymes can then break down into fatty acids that can be absorbed.
Some easily digested foods are completely broken down in the upper small intestine and other components of the food that don't need to be digested, such as iron, calcium, or zinc, are directly absorbed here. The rest of the absorption takes place in the remainder of the small intestine, the jejunum (about 40 percent of the small intestine) and the ileum (about 60 percent of the small intestine), which links to the large intestine.
Absorption of Nutrients
Despite its name, the small intestine stretches to about fifteen feet, depending on a person's size. While an impressive length, the intestine's surface area would be insufficient to absorb what we eat each day if it were simply a length of hose. Instead, it is structured to increase its surface area tremendously through fingerlike projections along the lining called villi that are in turn covered with hairlike projections called microvilli.
These tiny projections increase the surface area of the small intestine by a factor of 600. Along this rugged surface the component nutrients are transferred to the blood stream through very complex absorption mechanisms and carried to the bloodstream, which distributes them throughout the body.
Large Intestine and Gas
Not all of the components of the food we eat can be completely broken down and absorbed. Dietary fiber, for instance, passes through the stomach and the small intestine intact. What remains goes into the large intestine, or colon, where most of the fluid as well as some sodium, calcium and a tiny amount of zinc are absorbed. About two quarts of liquefied material reaches the colon each day. Most of it is absorbed.
Billions of bacteria play a critical role in the large intestine. The bacteria that reside here decompose carbohydrates (mostly fiber) that were not digested by the small intestine and synthesize vitamin K and certain B vitamins. The fermentation process by which the bacteria decompose the carbohydrates produces intestinal gas.
By the end of the passage through the colon — which measures about 5 feet — only a small amount of what you originally consumed remains, which constitutes the feces. The bulk of the feces — about 75 percent — is water and the remainder is solids made up of bacteria, undigested organic matter, and dead cells that are shed from the lining of the intestines. The feces travels to the very end of the colon, the rectum, where it accumulates until the urge comes, and then it is excreted.
It's a Gas
Scientists estimate that the average adult releases between 12 and 122 cubic inches of intestinal gas each day. Most of that gas is made up of hydrogen and methane produced by the bacteria as they ferment the fiber that was not digested in the stomach or small intestine.
Many fruits and vegetables, especially beans, contain these polysaccharides, which cannot be broken down by the enzymes in the stomach or small intestine. As the bacteria decompose these complex sugars, gas is released. Most of the gas is passed without notice.
Common wisdom tells us that beans give us a lot of gas — more than could pass without notice. Ever questing for eternal answers, scientists in one study measured the intestinal gas produced by people who ate a control diet and compared them to those who ate a meal that was half pork and beans. The folks in the control group released .9 cubic inches per hour of flatus (intestinal gas) while the pork and beans group released a whopping 10.7 cubic inches.
For more information about the digestive system, see the next page.