Food does not simply drop down this passageway by means of
gravity. Things move through the esophagus because they are pushed by contractions (peristalsis) 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 reaches the far end of the 16-inch-long tube, the lower esophageal sphincter (LES) opens to allow the food to enter the stomach, 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 into the throat. That's where the upper esophageal sphincter comes into play. The two sphincters (upper and lower) make sure the food doesn't back up.
If the esophagus is a conduit with a valve at each end, the stomach can be likened to a storage facility where the food is prepared for digestion. This food warehouse can accommodate anything from a light snack to a five-course meal.
The stomach doesn't just hold food; muscles in the lower stomach also mix that food into a soft mush. The saliva,
hydrochloric acid, and the enzyme pepsin aid this process. Hydrochloric acid and pepsin, produced by the glands that line the stomach, help break down proteins into their constituent amino acids. Just why this potent acid doesn't dissolve stomach tissue is a mystery; the stomach mucosa clearly has some sort of defense system to protect itself. After mixing, a once identifiable meal is reduced to a thick liquid called chyme.
The other important function of the stomach, apart from storing, 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, which is similar to rolling waves depositing sand and shells on the shore, 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 gastric emptying.
The stomach is a specialized muscle designed to push its contents along by a series of
involuntary contractions that are governed by nerves in the stomach wall. These nerves transmit electrical impulses to the brain. The nerves that carry impulses from the GI tract, called visceral nerves, recognize only stretching, pulling, or expansion (distention) of the muscles of the intestinal wall and pain can result when these sensations are excessive. It typically takes about two hours to reduce a meal to a liquid form and have it ready to move along to the small intestine
. A high-protein meal can take an extra hour or two, and a high-fat meal can take up to six hours.
An empty stomach initiates a series of rhythmic contractions known as hunger pangs. They serve as a signal to the brain: "Feed me!" These contractions also produces stomach noises, which may be caused when air or fluid is rattling around inside.
The small intestine, which is actually about 21 feet long, is the next stop on the journey that food takes 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 takes about three to five hours to move through the small intestine.
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. And this is where bile and pancreatic juices join the mix. In the small intestine, the food moves along as if on a conveyor belt.
After the duodenum, the next part of the small intestine is the jejunum, which measures 8 feet in
length. In the jejunum, fats, starches, and proteins are broken down and absorbed by the cells lining the bowel. The third and lowest portion of the small intestine, the ileum, is approximately 12 feet long and absorbs water, along with vitamin B12 and bile salts.
Some digested food stays in the small intestine for several hours and 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 fats for easier processing. Enzymes secreted by the pancreas, such as trypsin, amylase, and lipase, help digest proteins, carbohydrates and fats. Enzymes are actually catalysts, which cause a chemical reaction
themselves. Enzymes and proteins are dependent upon one another. If there are too few enzymes, protein cannot be broken down. An insufficient amount of digestive enzymes can contribute to many of the same problems as inadequate HCL (stomach acid).
Colon (Large Intestine)
Finally, what's left of the food arrives in the colon, or large intestine, a 4-foot-long, muscular
tube about the diameter of your fist (2-3 inches) and the walls act like a sponge and soak up 80-90 percent of the remaining water. In fact, the colon accepts about a quart of liquid from the ileum each day. 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 methane gas. Some of these gases are consumed as nutrients by the cells of the colon, while others are expelled as waste. Many different microbes live in your intestinal tract numbering in the trillions.
Friendly bacteria assist with digestion, vitamin production and immune defense. Unfortunately,
many common activities, i.e., antibiotics, steroids, synthetic hormones, and diets high in fat and sugar can destroy the delicate balance of bacteria and create an imbalance, which is a setup for disease.
Undigested matter, such as fiber, is propelled along by contractions of the colon wall and settles as solids in the rectum, the final 6 inches of the colon. The rectum is guarded by a sphincter
muscle that helps control defecation. 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 defecation can be carried out at 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 ľ to Ĺ 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 hardly need to think about this long, complex process. But when something goes wrong somewhere in the digestive tract, it can cause untold miseries. Good health begins with good digestion!