Your Digestive System

Your Gastrointestinal System: What It Is and How It Works

by Mark B. Pochapin, MD

Human digestive system shown within framework of semi-transparent skeleton. The body's gastrointestinal system—also referred to as the GI system or digestive system—is a fascinating system of organs, responsible for the digestion and absorption of nutrients…and much more.

The gastrointestinal system is responsible for the entry, absorption, and exit of food, water, and nutrients, but it also plays a key role in other important functions. These include energy production, blood flow, nerve signaling, hormone control, metabolism, and detoxification. In addition, the gastrointestinal system is one of the largest disease-battling systems, or immune systems, in your body.

To help you understand the different parts of the gastrointestinal system and what they do, let’s follow a tuna salad sandwich as it winds its way from your mouth down the 25-foot tunnel commonly known as your digestive tract:

The Mouth
Transparent human head showing salivary glands

The tuna salad sandwich you've just had for lunch begins its digestion in your mouth. While you chew your sandwich, chemicals (called enzymes) in your saliva help break it down into forms the body can more easily absorb. For example, these enzymes help to turn the carbohydrates in the bread into simple sugars.

The Esophagus
Human esophagus and stomach located in context of transparent skeleton. Once a few bites of your sandwich are chewed, moistened, and broken down, the partially digested food moves on to the esophagus, the food pipe connecting your mouth to your stomach. When your food arrives at the lower end of the esophagus, there is a valve, or sphincter, that acts as a gate between the esophagus and stomach. This sphincter helps ensure that the food can pass easily to the stomach, but cannot back up into the esophagus.

Beginning in the esophagus, food moves smoothly through the entire digestive tract by a process called peristalsis, a coordinated, rhythmic wave of muscular contraction that travels in a single direction. Peristalsis works independently of gravity. This means you could eat while standing on your head, and the food would still move properly from your esophagus to your stomach and through your digestive system.

The Stomach
Human stomach and small intestine in context of transparent skeleton.Once your tuna salad sandwich travels through the esophagus, its next destination is the stomach. Your stomach stores the food material for hours and churns it into a liquid-like form called chyme. Enzymes in the stomach continue the work of breaking down your sandwich. For example, these enzymes chop the proteins from the tuna into microscopic fragments called amino acids. Proteins are also digested in other areas of the digestive system.

The stomach also produces hydrochloric acid. This acid is so corrosive that it can eat its way through metal. Fortunately, the inner lining of your stomach has a protective layer of mucus, to prevent the acid from burning right through your stomach wall. Hydrochloric acid is in your stomach for a good reason. It activates digestive enzymes and sterilizes the food you eat. This sterilization process may not be such a big deal today, because the food we eat is fairly clean and often cooked. However, it was a great advantage ages ago when early humans ingested bug-infested tree bark and rotting dead animals. Thank goodness for refrigeration and supermarkets!

Back to that tuna salad sandwich: In its now partially digested form, it sits in your stomach for about 2 to 4 hours.

The Small Intestine

At this stage in the digestion process, the stomach empties your now liquefied tuna salad sandwich into your small intestine through a valve or sphincter, called the pyloric valve. This sphincter serves as a gate, and prevents the passage of food into the small intestine until it has been properly processed by your stomach. The small intestine is made up of three segments: the duodenum, jejunum, and ileum. It is roughly 21 feet in length and coiled loosely in the part of your body commonly called the abdomen. When you feel as though food and gas are moving in your "stomach," often this sensation is actually the result of movement in your small intestine.

The upper-most part of the small intestine, the duodenum, acts to continue the breaking down of your food. The other two parts of the small intestine, the jejunum and ileum, work to absorb the nutrients from your food to support the health and energy needs of your body. The lining of your small intestine is filled with closely packed, finger-like projections, called villi. These villi greatly increase the amount of surface area available for absorbing nutrients. In fact, if these villi were spread out flat, their surface area would span the length of a tennis court, or about 200-square feet.

The Pancreas, Liver, and Gallbladder
Human liver, pancreas, and gall bladder within transparent skeleton.Other organs in the gastrointestinal system also aid in the digestion process. Your pancreas is a flask-shaped organ situated just below your stomach toward the back. Its job is to secrete digestive enzymes into the small intestine to help break down proteins, carbohydrates, and fats. Apart from its digestive function, your pancreas also produces two hormones, insulin and glucagon, that help regulate your body's blood sugar.

All of the absorbed nutrients from the gastrointestinal tract eventually pass through your liver, which is the largest organ in the body. For example, the carbohydrates from the bread of the tuna salad sandwich arrive at the liver in the form of simple sugars. The liver converts these sugars to glucose, the body's primary fuel. Any glucose not used for fuel is stored in your liver or in your muscles as glycogen. The liver can also turn proteins and fats into glucose if your body needs additional energy sources.

Among its many other functions, your liver also manufactures and secretes a green liquid, called bile. Bile contains bile salts that break up (or emulsify) dietary fat so that digestive enzymes can break it down even further.

The gallbladder is a pear-shaped organ, situated just under the liver. Its job is to receive bile from the liver and store it. During a meal, your gallbladder contracts and squirts bile into your duodenum (first part of your small intestine) through a tube called the common bile duct.

The Colon
Human colon in context of transparent skeleton.Once the nutrients have been absorbed by your small intestine and processed by your liver, what is left of your tuna salad sandwich moves on by peristalsis to your colon, a long muscular tube, between 4 to 6 feet in length. Sometimes, the term colon is used interchangeably with the word large intestine. However, the small intestine is actually much longer than the colon.

By the time the tuna salad sandwich reaches your colon, the remaining material consists of undigested food particles (such as fiber), water, and secretions from your small intestine.

The colon is made up of four sections: the ascending (right colon); the transverse (across) colon, which hangs like a necklace down to as low as your belly button; the descending (left) colon, which moves down toward your pelvic area; and finally the sigmoid colon.

The colon is also constructed of four layers of tissue. The innermost layer, the mucosa, is smooth, thin, and has no villi (finger-like projections). The mucosa has direct contact with the material that passes through the colon. The cells of the mucosa are in a constant state of replenishment—dying off, sloughing off, and being replaced by new cells about every 4 to 6 days. Underneath the mucosa is the submucosa, a layer of tissue that provides support for the mucosa. The submucosa also harbors the white blood cells (lymphocytes, monocytes and neutrophils) that keep bacteria from the colon out of the bloodstream. The third layer is the muscularis propria, made up of muscle cells that assist in peristalsis (movement of food through the gastrointestinal tract). Finally, the fourth and outermost layer is the serosa, which provides added strength to the colon and serves as a protective barrier.

The primary duties of the colon are to absorb water and electrolytes, such as sodium and potassium, from what was your food and to compact solid waste so that it can be eliminated from the body. As water is removed in the colon, the material becomes more solid. In this state, it is called stool or feces. Stool moves into the ascending colon, across the abdomen through the transverse colon, and down the left side of your abdomen in the descending and sigmoid colon, where it is stored until being emptied into the rectum once or twice a day.

In addition, your colon harbors an enormous colony of bacteria. It is true that there are disease-causing bacteria in our environment, but most bacteria that we encounter are friendly and assist in the functioning of digestion and protection from disease. The helpful bacteria in the colon, known as the normal flora, promote health and immunity in a variety of ways. First, they help stimulate the immune system's production of disease-fighting white blood cells. Second, they form a protective barrier to keep levels of bad bacteria from attaching to the colon walls and being absorbed. Third, they produce certain types of acid that discourage harmful organisms, such as yeast, from proliferating. Fourth, some normal flora synthesize certain B vitamins for proper metabolism, as well as vitamin K, which is essential to normal blood clotting. Finally, these bacteria help change fecal matter into a form that can be properly eliminated. These friendly bacteria help to make the colon an important organ in the body's immune system.

The Rectum
Illustration showing sigmoid colon, rectum and anus.Once the colon processes the digestive material, the remaining content moves into the rectum. The rectum is one of the most important organs in the digestion process, functioning to store stool temporarily, until the body is ready to eliminate it.

The Anus
The rectum works in concert with the anus, located at the very end of the digestive tract. There, anal sphincter muscles work to control the movement of stool and help in its elimination. The time it takes for that tuna sandwich to enter at the mouth and exit at the anus is called transit time. If you eat a healthy diet, and include plenty of water and fiber, your transit time should be just over a day.

With all of its essential functions, the gastrointestinal system is responsible for providing the body’s fuel and is truly is one of the largest disease-fighting components in the human body. This means that, by taking care of your gastrointestinal system and helping it to work better, you and your healthcare team can help you to be as healthy as you can be—from the inside out.

Source: Adapted from What Your Doctor May Not Tell You About Colorectal Cancer, Warner Books, 2004.
Copyright: Mark B. Pochapin, MD.

All medical illustrations are copyrighted by BioDigital, Inc.