How Does Your GI Tract Work?

In order for you to understand what happens to the food after you eat it, it is useful to review some anatomy and physiology of the GI tract.

The Esophagus



When we eat, the food first enters the mouth and then passes down our esophagus which is just a tube about a foot long that carries the food from our mouth into our stomach. It has a sphincter muscle at its entrance and another one at its exit into the stomach. These two sphincters remain constricted (like the anus) unless we are swallowing. The act of swallowing opens the upper sphincter, and the food is forced into the esophagus by the contraction of the muscles of the mouth, tongue and pharynx. The passage through our esophagus is not a passive one. The esophagus actually drives the food down through a process called “peristalsis.” After all, the astronauts in the Space Shuttle can swallow just fine in zero gravity. You can swallow on your back or even if you are standing on your head. The lower sphincter then relaxes and allows the food to pass into the stomach. It then closes so that the stomach contents cannot pass back into the esophagus. If this sphincter is weak, acid reflux occurs.

The Stomach



The stomach is an expandable bag that has the capacity to hold about six cups (42 ounces) at any given time. It is like an accordion, having the ability to collapse on itself or expand with the introduction of food. It is a myth that you can shrink your stomach. You really can’t. You can stretch it though! In addition to its reservoir function, the stomach initiates digestion which is the process whereby food is broken down from the form in which we eat it into the basic chemicals of which it is composed. To do this, the stomach produces acid and enzymes which dissolve the food. At the same time, the stomach is very effective at grinding the food into a more simplified state that can be transferred into the small intestine, a little at a time. Thus, the stomach acts as a reservoir so we can eat at convenient times instead of having to eat constantly. It begins the digestive process and presents the food a little at a time to the small intestine where the rest of the digestive process occurs.

The other major process that is essential for us to process our food is absorption. This is the actual passage of the nutrients in their simple state across the lining of the intestine and then into the circulation system (bloodstream). This occurs in the small intestine. We have to learn about a couple of very important organs first, the duodenum and pancreas.

The Duodenum and Pancreas



The first part of the small intestine is called the duodenum which is a C-shaped organ that receives the food from the stomach. The food coming into the duodenum is very acidic as it comes out of the stomach. The small intestine cannot handle it this way or ulcers would occur. That’s why the duodenum is the most common place to develop an ulcer. To neutralize this acid, the pancreas produces sodium bicarbonate and empties its contents early into the duodenum right as the food enters before it travels on to the small intestine.

The pancreas also produces very strong enzymes, which further digest (break down) the food into its component parts: amino acids, fatty acids and simple sugars. Finally, the bile that comes from the liver is emptied into this same place which mixes with the fat in our diet to make it easier to digest and absorb. As you can see, the duodenum is a very important structure. What an ingenious design.


Digestion is the process of breaking down food into its component parts:
Protein is broken down into 20 different amino acids.
Fat which is ingested as triglycerides is broken down into fatty acids and glycerol.
Carbohydrates are broken down into simple sugars. Some of the structure of carbohydrates is not digestible. This is where fiber comes from.

The Small Intestine (SI)



The digested food passes out of the duodenum and into the small intestine which is 18 feet long and has a “furry” lining which allows for it to absorb the digested food. The furry lining of the small intestine is due to the fact that it is like a rug with many individual folds called villi. If you flatten out the villi in your intestine, it would cover a football field. As you can imagine, it is almost impossible for us to not absorb something we eat if it is in its absorbable form. You cannot overwhelm the absorptive capacity of the small intestine. That is one main reason that obesity is so common. There is not much other structure to the small intestine. It’s just a very long tube that has an incredible absorptive surface. The first half of the small intestine is called the jejunum, and the second half is called the ileum. At the end of the ileum is a specialized section where we absorb our fat-soluble vitamins like A, D, E and K. There is another sphincter here which slows down the passage of the food into the next segment, the colon.

The Large Intestine: Colon




Whatever is left in the small intestine at the end of the ileum that is not absorbable (e.g.: fiber) passes into the colon in the lower right hand side of your abdomen down by your appendix. The colon is five feet long and surrounds our abdomen like a large question mark. It has two jobs, the first of which is to absorb water. About a half-gallon of water enters the colon every day. This has to be absorbed back into our bodies and the colon does this very well. The second job of the colon is to package the stool in a form that can be conveniently eliminated. We all know how important this is.

The packaging process of stool is a complex one. When our food enters the colon on the right side, the contents are very liquid. We already said that the colon absorbs about a half-gallon of water every day from the stool. This is a continuous and progressive process as the stool courses around the five-foot colon towards the left side of the body. The contents of the colon on the right side are liquid whereas they are solid on the left, which results in a different pressure situation on the right vs. the left side of the colon.

Why is this important? Well, since the colon has a low-pressure zone on the right and a high-pressure zone on the left, different things can happen to the colon as a result. The left side of the colon is where we develop diverticulosis, a condition where pockets form in the lining of the colon. These pockets are very thin-walled sacs, which can rupture. To best understand what they are, think of a bicycle tire which has an outer tire and an inner tube. Imagine you have a crack in the outer tire and a bubble of the inner tube is sticking out. This is what a diverticulum is. We want to do everything we can to decrease the pressure in the left side of our colons to avoid rupturing a diverticulum.

Fiber help avoid developing diverticuli. Eating fiber lowers the pressure in the left side of the colon. This makes diverticulosis less likely to develop and if you already have diverticulosis, it makes them less likely to rupture.

Our colons are also home to trillions of bacteria. We have them from birth and provide them a home throughout our lives. In return, these bacteria produce vitamins and other nutrients for us which they extract from the contents of our colon. Some of these can be absorbed and used as nutrition for us. There are actually more cells in the bacteria in our colons than there are in the rest of our bodies. The science of the fecal microbiome is in its infancy and is the source of much research funding today. There are over 400 different species of bacteria in humans. Each of us has his/her own mixture of them. As you will learn in a later chapter, these bacteria can also be a cause of obesity.


The Rectum



The final organ in the GI Tract is the rectum, which is about four inches long and controls how we move our bowels. The rectum is very sensitive, almost with the sensitivity of the tip of your finger. It can tell if you have solid, liquid or gas in it. Imagine if you couldn’t tell the difference. You would never be able to pass gas without having an accident. Needless to say, the rectum is very sensitive.

The emptying of the rectum is dependent upon its sphincter, the anus. Unfortunately, we were probably not meant to stand upright. As a result, hemorrhoids develop here in the distal rectum.

That’s it, you now are experts on the GI Tract. Let’s use this knowledge to help us understand the PVC Diet.