Linda Crampton is a writer and former teacher with a first-class honors degree in biology. She writes about the scientific basis of disease.
A Vital System in the Body
Our digestive system is a vital part of our body and our life. The nutrients needed by our organs and cells are supplied by digestion. During this process, the food that we eat is broken down into small molecules that can be absorbed through the lining of the intestine. The molecules are then transported to where they are needed via the bloodstream. Disorders of the digestive, gastrointestinal, or GI tract can be annoying, uncomfortable or painful, and in some cases, potentially dangerous.
In this article, I describe a healthy GI tract. I also give an overview of some health disorders that can develop in the different sections of the tract. The disorders are listed below.
- acid reflux and GERD
- dyspepsia (indigestion)
- peptic ulcers
- celiac disease
- Crohn's disease
- ulcerative colitis
- irritable bowel syndrome,
Digestive System Overview
The digestive tract is a hollow tube of about 30 feet (9 meters) in length. It extends through the body from the mouth to the anus. It's wider in some places than in others and is folded into loops in certain sections.
As food travels along the digestive tract, it’s gradually broken down by chewing, churning, and the action of digestive enzymes. The food is eventually converted into particles that are small enough to be absorbed by the blood and lymph vessels in the lining of the digestive tract. Undigested food passes out of the body in the stool, or feces.
The parts of the digestive tract in order are as follows:
- small intestine
- large intestine (or colon, the longest part of the large intestine)
- rectum (which is technically the last part of the large intestine)
- anal canal and anus
The liver and pancreas help the process of digestion in the digestive tract, but food never enters them. The liver produces a green fluid called bile. The bile emulsifies fats in the small intestine, breaking them up into small droplets that digestive enzymes can attack more easily. The pancreas makes digestive enzymes and sends them into the small intestine.
1. The salivary glands in the mouth generally make between 0.5 and 2 liters of saliva each day.
2. We make more saliva when we see, smell, taste, or even think about food. All of these factors make our mouth “water.”
3. Saliva contains an enzyme called salivary amylase (or ptyalin) which begins the digestion of starch. Chewing also helps to break up food.
4. Saliva contains several antimicrobial enzymes, including lysozyme. Lysozyme damages the cell walls of bacteria, thereby killing the microbes.
5. The flow of saliva helps to remove food trapped in the mouth. It also helps to neutralize acids. These actions reduce the chance of tooth decay.
6. Chemicals in saliva remineralize tooth enamel, strengthening it after acids have removed minerals from the enamel.
7. Saliva also lubricates food, making it easy to swallow.
Xerostomia, or dry mouth, is a condition in which little or no saliva is made. It’s sometimes known as cottonmouth. Xerostomia makes speaking and swallowing difficult and leads to increased tooth decay. It also causes halitosis (bad breath).
There are many causes of xerostomia. Some of these causes are dehydration, anxiety, certain medications, alcohol, radiation therapy to the head or neck, and breathing through the mouth instead of the nose. Aging also results in reduced saliva production, although this may be due to the fact that older people often take more medications than younger people.
Sjogren’s syndrome is an autoimmune disease in which the body’s immune system destroys the glands that make saliva and tears, resulting in xerostomia. Other diseases may include xerostomia as a symptom. These diseases include diabetes, rheumatoid arthritis, hypertension (high blood pressure), and thyroid problems.
The pharynx is a passageway at the back of the nose and mouth. It leads to the esophagus and passes behind the entrance to the larynx on the way, as shown in the illustration above. The larynx, or voicebox, is the upper section of the trachea, or windpipe. The epiglottis closes the entrance of the trachea as we swallow.
1. The function of the esophagus is to transport the swallowed food to the stomach.
2. The esophagus opens at the back of the throat like the trachea (windpipe) but is located behind the trachea.
3. During swallowing, a flap of tissue called the epiglottis covers the opening to the trachea so that food doesn't enter it.
4. At the end of the esophagus is a circular muscle called the lower esophageal sphincter (LES), also known as the cardiac sphincter. This muscle acts like a valve and normally closes off the entrance to the stomach when the stomach is digesting food.
5. Peristalsis is a series of wave-like muscle contractions in the walls of the digestive tract that propel food along the esophagus, stomach, and intestines.
We can't breathe and swallow at the same time. As we swallow, a set of coordinated, rapid, and quickly reversible actions occur to prevent choking. These actions include the covering of the tracheal opening by the epiglottis and the inhibition of breathing by the brain.
Acid Reflux and GERD
In some people, the lower esophageal sphincter doesn’t close the exit of the esophagus properly. This allows the acidic stomach contents to move upwards into the stomach, producing a burning sensation known as heartburn or acid reflux. Many people experience this condition occasionally. If it occurs frequently, however, a doctor may diagnose GERD, or gastroesophageal reflux disease.
Certain foods and drinks may affect the function of the lower esophageal sphincter and trigger the stomach contents to enter the esophagus. Foods that may have this effect in some people include fatty foods, spicy foods, onions, garlic, peppermint, tomatoes, citrus fruits, chocolate, caffeine, coffee, and alcohol.
Smoking also weakens the lower esophageal sphincter and can produce heartburn. A very full stomach or being pregnant or overweight can put pressure on the stomach and the lower esophageal sphincter, forcing stomach contents into the esophagus and causing heartburn.
1. The stomach is a hollow sac that has a volume of about 1 liter when empty. It can expand to about 4 liters when it's full.
2. The stomach lining produces hydrochloric acid, which produces a pH of 1 to 3 in the cavity of the stomach, or the lumen. The pH scales runs from 1 to 14, with 1 being the most acidic pH and 14 being the most basic (alkaline) pH.
3. The lining of the stomach also produces an inactive enzyme called pepsinogen. In the stomach cavity, hydrochloric acid converts pepsinogen into pepsin, an enzyme that partially digests proteins. The acid also kills many (but not all) bacteria in the stomach.
4. In addition, the stomach lining produces a mucus layer which protects the organ from damage by pepsin and hydrochloric acid.
5. At the bottom of the stomach is a circular muscle called the pyloric sphincter. This sphincter allows the partially digested food from the stomach, known as chyme, to gradually enter the small intestine.
Dyspepsia and Peptic Ulcers
Indigestion, or dyspepsia, is a term that refers to abdominal discomfort. Symptoms include pain, bloating, and nausea. Indigestion is generally not serious and is short-lived. It may be caused by overeating or eating too fast, drinking too many carbonated drinks, anxiety, certain medications, and many of the same foods that can cause heartburn.
Indigestion that lasts for a long time or occurs repeatedly requires advice from a doctor. Indigestion with additional symptoms such as shortness of breath, chest pain or pressure, arm pain, and sweating requires immediate medical attention, since these can be symptoms of a heart attack. Indigestion accompanied by vomiting, a yellow color in the skin or eyes (jaundice), black feces (which may be due to blood in the feces), or weight loss also requires medical attention.
One bacterium can hide in the stomach mucus, where it's protected from the acidic contents in the lumen of the stomach. This bacterium is called Helicobacter pylori, or H. pylori. In some people, this bacterium causes no apparent problems. In others, an H. pylori infection can result in painful inflammation of the stomach lining (gastritis) and sores called stomach, gastric, or peptic ulcers. A peptic ulcer can also form at the start of the small intestine.
Small Intestine Facts
1. The small intestine is about 20 feet (6 meters) long and is the longest part of the digestive tract. It has three sections, which are called the duodenum, the jejunum, and the ileum.
2. The small intestine is folded into loops in order to fit into the abdominal cavity.
3. Bile made by the liver is stored in the gall bladder until it’s needed. Bile is sent to the small intestine when fat is eaten to help break up the fat.
4. The pancreas sends several digestive enzymes into the small intestine. Two of these are pancreatic amylase, which digests starch into maltose, and lipase, which digests fats into glycerol and fatty acids.
5. The pancreas also sends trypsinogen to the small intestine. The inactive trypsin is turned into active trypsin in the intestine. Trypsin then digests proteins (which are made of long chains of amino acids) into peptides (short chains of amino acids).
6. The pancreatic enzymes are transported to the small intestine in a basic fluid, which helps to neutralize the acidic material from the stomach.
7. Glands in the lining of the small intestine secrete additional enzymes to finish digesting the food. These enzymes include a group called peptidases, which digest peptides into amino acids, and maltase, which converts maltose molecules into two glucose molecules.
The pancreas is an unusual organ that secretes hormones (including insulin) as well as digestive enzymes.
The lining of the small intestine contains tiny folds or projections called villi. The folds increase the surface area available for absorption of digested food. The surface cells of the villi contain tiny projections called microvilli, which further increase the surface area.
The glucose, other simple sugars, and amino acids produced in digestion travel into the capillary of the villus. Glycerol and fatty acids pass into the lymph vessel, which is called a lacteal. In people with celiac disease, the villi in the small intestine are damaged or destroyed. This means that the surface available for the absorption of nutrients is greatly reduced.
Celiac disease develops in people who are intolerant to gluten, a protein complex found in certain grains, including wheat, rye, and barley. Gluten is also used as a food additive. As long as gluten is eaten, the destruction of villi continues. Gluten can cause problems outside the digestive tract as well as in the digestive tract in sensitive people.
In most people with celiac disease, the villi will heal and regrow if a gluten-free diet is followed. It's important that someone diagnosed with the disease does stop eating gluten, since its continued consumption increases the risk of intestinal cancer.
Crohn’s disease is a condition in which part of the digestive tract is inflamed. Any part of the digestive tract may be affected, but the most commonly affected sites are the small and large intestine, especially the ileum. Crohn’s disease is classified as a type of inflammatory bowel disease, or IBD. "Bowel" is an alternate name for intestine. The inflamed areas may be located in patches that are surrounded by healthy tissue.
Possible symptoms of Crohn’s disease include abdominal pain and diarrhea, which may be bloody. Ulcers may be present. The affected person may also experience nausea, vomiting, and weight loss. People with Crohn’s disease may experience flare-ups, when their symptoms are present, alternating with periods of remission, when symptoms are weaker.
The cause of Crohn’s disease is not known, but scientists do know that the immune system reacts abnormally in the digestive tract during Crohn’s disease. The disease can sometimes be so severe that it's debilitating.
1. The cecum is a pouch that is located where the last section of the small intestine (the ileum) connects to the large intestine, on the right side of the abdomen (from the body owner's point of view).
2. The appendix is a close-ended, worm-like tube extending from the cecum.
3. Scientists have assumed for a long time that the appendix has no function in modern humans, although it is believed to have been used in our distant ancestors to help break down plant matter.
4. Some scientists have suggested that the appendix plays a role in our immune system, which is the system that fights disease.
The full name of the appendix is the vermiform appendix. Vermiform means "having the shape of a worm".
Inflammation of the appendix is called appendicitis. The inflammation is produced by a bacterial infection inside the appendix, which causes the structure to swell and become painful.
Scientists are not sure how the appendix infection arises. One idea is that bits of stool from the large intestine block the opening to the structure. Bacteria in the stool may then infect the appendix. Another idea is that mucus made by the appendix blocks the opening of the structure instead of flowing into the large intestine. The opening may also be blocked by food material. A third idea is that bacteria from a stomach or small intestine infection enter the appendix and multiply.
The membrane that lines the abdominal cavity and covers the organs in the cavity is called the peritoneum. If a swollen and infected appendix bursts, the peritoneum may become inflamed. This is an extremely serious condition called peritonitis.
Most people with appendicitis are treated by removal of the appendix. People without the structure appear to live a normal life.
Large Intestine Facts
1. The large intestine is about 5 feet long (1.5 meters). Although it's shorter than the small intestine, it’s also wider.
2. The large intestine consists of the cecum, the colon (which is the longest part and has several sections), and the rectum.
3. The main function of the large intestine is to absorb excess water from the intestinal contents and send it into the bloodstream. If too much water is removed, a person will experience constipation. If too little water is removed, the person will experience diarrhea.
4. Although water is the main substance absorbed in the large intestine, salts, vitamins, and other helpful chemicals are also absorbed through its lining.
Scientists believe that between 500 and 1000 different species of bacteria live in the large intestine. Some yeasts are also normal inhabitants of the intestine. Bacteria live on the surface of the body and in body passageways, but the majority are found in the large intestine. Bacterial cells are much smaller than human cells.
Most of the bacteria in the large intestine are harmless. Some—and perhaps many—are very helpful. Some of the bacteria produce vitamin K, which we absorb and use in the blood clotting process. Some produce B vitamins, including biotin, which we also absorb. It's unclear how significant this absorption is with respect to our daily requirement for vitamins, however. Large intestine bacteria may also decrease the level of pathogenic (disease-causing) organisms in the intestine.
Some intestinal bacteria ferment soluble fiber into short-chain fatty acids. The short-chain fatty acids (SCFA) are thought to have a number of health benefits, including helping to maintain blood glucose levels, reducing the level of LDL cholesterol (the “bad” cholesterol) in the body, and stimulating the activity of the immune system.
The latest analysis estimates that only 43% of our body consists of human cells. The rest of the cells come from the organisms that live inside us, including bacteria.
Ulcerative colitis is a type of inflammatory bowel disease that is similar to Crohn’s disease but affects only the large intestine. The linings of both the colon and the rectum may become inflamed and develop ulcers.
In ulcerative colitis, the surface layer of the large intestine lining is inflamed. Crohn’s disease causes both surface and deeper damage to the intestine. Another difference between the two types of inflammatory bowel disease is the fact that the inflammation in ulcerative colitis is generally continuous instead of occurring in patches.
People suffering from ulcerative colitis may experience the same symptoms as people with Crohn’s disease—pain, diarrhea, bleeding from the rectum, nausea, and weight loss. They may also experience alternating flare-ups and remissions.
The cause of ulcerative colitis is unknown, but the immune system reacts abnormally in the large intestine during the disorder just as it does during Crohn’s disease.
Irritable Bowel Syndrome
Irritable bowel syndrome (IBS) involves abdominal pain accompanied by diarrhea (in diarrhea-predominant IBS, or IBS-D), constipation (in constipation-predominant IBS, or IBS-C), or alternating diarrhea and constipation.
There is no damage to the large intestine in irritable bowel syndrome. Instead, the colon is unusually sensitive to certain triggers, which are often substances in the diet. The triggers may be different in different people. People suffering from IBS need to discover their personal triggers—if this is possible—and then avoid them.
Common IBS triggers include fatty foods, sorbitol, concentrated fructose, caffeinated or carbonated beverages, and alcohol. Dairy is also a frequent cause of IBS, although some people find that fermented dairy is safe to eat. Some people with diarrhea-predominant IBS find that raw vegetables and too much insoluble fiber trigger an IBS attack, while cooked vegetables and soluble fiber do not. People with constipation-predominant IBS may find that insoluble fiber helps their condition.
People with irritable bowel syndrome should check that they don't have the somewhat similar celiac disease, Crohn’s disease, or ulcerative colitis. Although IBS is a much less serious disorder in terms of damage to the body, if a way to control the problem isn't discovered, it can be very detrimental to well-being.
The anus is the circular opening at the end of the GI tract. Its name comes from the Latin word anus, which means circle or ring.
Rectum and Hemorrhoid Facts
1. The rectum is the last five to eight inches of the large intestine.
2. The colon sends its contents into the rectum, which stretches as it fills up.
3. The walls of the rectum contain stretch receptors. When the rectum has expanded by a certain amount, the receptors are stimulated and send a message to the brain. The brain gives us the sensation that we need to defecate (release feces from the body). Feces is then sent into the short anal canal and then out of the body through the anus.
4. Hemorrhoids, sometimes known as piles, are swollen veins in the anal canal (internal hemorrhoids) or close to the anus (external hemorrhoids). They may be painful or itchy and may bleed.
5. Large hemorrhoids may protrude through the anus.
6. It's not always clear why hemorrhoids develop. Some possible causes are straining during defecation and any condition that puts pressure on the area, such as being overweight or pregnant. Aging may weaken the tissue that supports the veins in the anal canal and increase the likelihood of hemorrhoid development. Genetics may also play a role.
7. Any bleeding from the anus should be investigated by a doctor. A person shouldn't assume that it's caused by a hemorrhoid.
8. The time between ingestion of food in the mouth and release of the undigested food in the feces is generally between 24 and 72 hours.
Our digestive tract is fascinating to explore and normally serves us well. It's such an important part of our body that any problems that develop may have a major effect on our life. Fortunately, medical attention can help many of the problems.
- Dry mouth or xerostomia information from the Mayo Clinic
- Acid reflux and GERD facts from WebMD
- Fact about peptic ulcers from the U.S. National Library of Medicine
- Celiac disease information from the NIH, or National Institutes of Health
- Crohn's disease facts from the Mayo Clinic
- Information about appendicitis from the NHS (National Health Service)
- Ulcerative colitis information from WebMD
- Irritable bowel syndrome recommendations from HealthLinkBC (a Government of British Columbia site)
- Hemorrhoid facts from the NHS
- "More than half of your body is not human" from the BBC
This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.
© 2011 Linda Crampton
Linda Crampton (author) from British Columbia, Canada on May 13, 2012:
Thank you very much, Infonolan. It's nice to meet you!
infonolan on May 13, 2012:
Excellent information, may help me resolve some of my recent digestive complaints!!! :-)
Linda Crampton (author) from British Columbia, Canada on July 30, 2011:
Hi, chanroth. Thank you for the visit and the comment!
chanroth from California, USA on July 30, 2011:
Reading your hub remind me of school. I just love reading about anatomy and biology stuff.
Linda Crampton (author) from British Columbia, Canada on February 23, 2011:
Thank you very much for your kind comment and the rating, toknowinfo.
toknowinfo on February 23, 2011:
Excellent and informative hub. You made a technical subject very easy to read. I appreciate learning from you. Thanks for sharing your knowledge. Well done. Rate up and useful
Linda Crampton (author) from British Columbia, Canada on February 15, 2011:
Thank you for your comment, Purple Perl.
Esther Shamsunder from Bangalore,India on February 15, 2011:
Good for Biology students. Great explanation.