As I had said (to most of you) before the actual start of this newsletter series, the vast majority of illnesses we get enter our bodies through the things we eat, drink, or breathe. Others come through the skin by contact with absorbable liquids or gases, or through critter bites or cuts, where germs can go directly into your bloodstream. For right now, let’s focus on your digestive system.

We’re going to start this with reasonably simple explanations of the main parts of your digestive tract. Below is a picture that also contains some glands, but we won’t address those at this time. They are important, but we just want a basic overview at this point. Note that there is also a liver, spleen, gall bladder, etc., which are also part of your digestive system, but we’ll cover those later also.

Mouth

The function of your mouth and its associated structures is to form a receptacle for food, to begin mechanical digestion through chewing, to swallow food, and to form words in speech. It can also assist the respiratory system in the passage of air. Note that because both food and air go through the mouth, this accounts for the vast majority of the “bad” critters that enter your body.

Esophagus

The esophagus is a muscular tube that carries food and liquids from the back of the mouth /throat to the stomach for digestion after it has been chewed and chemically softened in the mouth. Food is forced downward to your stomach (or upwards, if you are standing on your head) by powerful waves of muscle contractions passing through the walls of your esophagus. Because these contractions are so strong, you can swallow in any position - even upside-down! If the food is bad, poison, or more than you can “stomach,” it might travel back by the same force to be thrown out through the mouth (barfing).

Your esophagus has a ring of muscle at the top and at the bottom. These rings contract (attempting to close off any backwash) after the food passes through and enters the stomach, where there is an abundance of churning acid waiting to digest the food. If your bottom muscle weakens, your stomach’s contents, along with the stomach acid, might return to the esophagus and cause an uncomfortable burning sensation known as “heartburn,” although it has absolutely no connection with the heart at all.

Stomach

Your stomach is a hollow, sac-like organ connected to the esophagus (input from the mouth) and the duodenum (output - the first part of the small intestine). Your stomach consists of layers of muscle and nerves that continue the breakdown of food that starts in the mouth (chewing).

Your stomach is also a storage compartment that enables you to eat only two or three large meals a day. If the stomach was not capable of acting as a storage device, you would have to eat about every twenty minutes all day long. The average adult stomach stretches to hold from two to three pints and produces approximately the same amount of gastric juices every twenty-four hours.

Your stomach has several functions: (1) as a storage bin, holding a meal in the upper portion and releasing it a little at a time into the lower portion for processing; (2) as a food mixer, the strong muscles contract and mash the food into a sticky, slushy mass; (3) as a sterilizing system, where the cells in the stomach produce an acid that is supposed to kill germs in “bad” food; (4) as a digestive tub, the stomach produces digestive fluid that splits and cracks the chemicals in food to be distributed as fuel for the body.

Note that the process of digestion is triggered by the sight, smell, or taste of food, so that the stomach is prepared when the food arrives. Note the connection to nose and eyes? And the connecting point is your brain. Every time you pass a bakery shop or smell some good cooking, your stomach begins the digestive process. If you don’t put something in your stomach, these gastric juices can begin eroding the stomach lining itself. We’ll talk more about that later too, so don’t let this be an excuse to eat everytime you see or smell food.

Small Intestine

If your small intestine were not looped back and forth upon itself, it could not fit into the abdominal space it occupies. It is held in place by tissues that are attached to the abdominal wall and measures eighteen to twenty-three feet in the average adult, which makes it about four times longer than the person is tall. It is a three-part tube of about one and one-half to two inches in diameter and is divided into three sections: (1) the duodenum, a receiving area for chemicals and partially digested food from the stomach; (2) the jejunum, where most of the nutrients are absorbed into the blood and (3) the ileum, where the remaining nutrients are absorbed before moving into the large intestine.

Your intestines process about 2.5 gallons of food, liquids, and bodily waste every day. In order for enough nutrients to be absorbed into the body, it must come in contact with large numbers of intestinal cells that are folded like gathered skirts. Each of these cells contain thousands of tiny finger-like projections called “villi,” and each villus contains microscopic “microvilli.” In one square inch of small intestine, there are about 20,000 villi and ten billion microvilli. Each villus brings in fresh oxygenated blood and sends out nutrient-enriched blood. The villi sway constantly to stir up liquefied food and remove the nutrients so that they can be absorbed and then passed through the membranes of the villi into the blood and lymph vessels. The fatty nutrients go to the lymph vessels, and glucose and amino acids go to the blood and on to the liver.

The muscles that encircle this tube constrict about seven to twelve times a minute to move the food back and forth, to churn it, knead it, and to mix it with gastric juices. Your small intestine also makes waves that move the food forward, but these are usually weak and infrequent to allow the food to stay in one place until the nutrients can be absorbed. If a toxic substance enters the small intestine, these movements may be strong and rapid to expel the poisons quickly (you’ve heard those groaning sounds, right?).

Large Intestine

Your large intestine, or colon, consists of ascending, transverse, descending, and sigmoid portions. The ascending portion connects to the output of the small intestine and moves upward along the right abdominal wall near the surface of the liver, then bends sharply at a right angle to the left. From there, it crosses the abdominal cavity (must be why they call it the transverse colon, huh?) to the left abdominal wall and begins the descending colon that goes down the left abdominal wall to the pelvic region. The colon then forms an angle to form an s-shaped curve called the sigmoid colon. The last few inches of the colon is the rectum.

Substances that have not been absorbed in the small intestine enter the large intestine in the form of liquid and fiber. The large intestine, or “bowel,” is sometimes called the “garbage dump” of the body, because the materials that reach it are of very little use to the body and are sent on to be disposed of. The first half of the colon absorbs fluids and recycles them into the blood stream. The second half compacts the wastes into feces, secretes mucus that binds the substances, and lubricates it to protect the colon and ease its passage. Of the two to two and one-half gallons of food and liquids taken in by the average adult, only about twelve ounces of waste enters the large intestine. Feces are made up of about 75% water. The remainder is protein, fat, undigested food roughage, dried digestive juices, cells shed by the intestine, and dead bacteria.

Rectum

Your rectum is a short muscular tube that forms the lowest portion of the large intestine and connects it to the anus. Feces collects here until pressure on the rectal walls cause nerve impulses to pass to the brain, which then sends messages to the voluntary muscles in the anus to relax, permitting expulsion. That might seem way too simple because, at some point in our lives, we all have a problem here. More on this later.