The digestive system, also known as the gastrointestinal or GI system, is responsible for digestion, one of the most important functions of the body. Digestion breaks down food into nutrients that can be absorbed by the body for fuel, growth and cell repair.
Digestion takes place in the alimentary canal, also called the digestive or gastrointestinal tract, over a period of several hours. The alimentary canal consists of a series of organs that runs from the mouth to the anus and includes the esophagus, stomach, intestines, colon and rectum.
- Propels food from the mouth through the esophagus and eventually out the rectum. During this process, the food is continuously mixed and broken down into a fluid.
- Releases secretions of fluids and chemicals that help in digestion and absorption. These secretions are produced in the salivary glands (located underneath the tongue), pancreas and liver as well as the linings of organs such as the stomach and small intestines.
- Breaks down ingested foods into absorbable molecules. Although there are many varieties of foods that people eat, the ingredients in them are broken down into similar components by the digestive system. Proteins are broken down into amino acids. Carbohydrates (starches and sugars) are broken down into simple sugars (e.g., glucose). Fats are broken down into fatty acids and glycerol (a type of sugar alcohol).
- Absorbs nutrients, water and electrolytes from the intestinal lumen (inner open cavity) into the bloodstream.
The functions of the digestive system are controlled by hormones that are produced and released by cells in the lining of the stomach and small intestine. The hormones that control digestion include:
- Gastrin. This hormone directs the stomach to produce an acid for dissolving and digesting foods (stomach acid). In addition, gastrin also helps the normal growth of the stomach lining, small intestine and colon.
- Secretin. A hormone that prompts the pancreas to send out a digestive juice that is rich in sodium bicarbonate, a substance that neutralizes stomach acid. It also stimulates the stomach to produce pepsin, an enzyme that helps digest proteins as well as stimulate the liver to produce bile.
- Cholecystokinin (CCK). This hormone causes the pancreas to grow and produce the enzymes of pancreatic juice. CCK also causes the gallbladder to empty.
Hormones in the digestive system also regulate appetite. In the absence of food, ghrelin is produced in the stomach and stimulates hunger. After eating, the hormone peptide YY is produced in the intestines and acts as an appetite suppressant. The combination of these hormones helps the body regulate food intake.
Other chemicals and hormones coordinate the digestive system with the brain and nervous system to regulate digestive activity. The neurotransmitter acetylcholine causes the muscle of the digestive organs to contract with more force and increase the push of food and juice through the alimentary canal. This chemical also causes the stomach and pancreas to produce more digestive juice. The hormone adrenaline relaxes the muscles of the stomach and intestines and decreases the flow of blood to these organs. A network of nerves embedded in the walls of the esophagus, stomach, small intestine and colon also sends messages to regulate the movement of food and the production of juices by the digestive organs.
Food is digested and absorbed in the alimentary canal, a tube of organs from the mouth to the anus that is about 30 feet (9 meters) long in the average adult. The main features of the alimentary canal and the role of its organs during digestion include the following, from top to bottom:
The opening in the head through which food enters the body. Even before a person takes an initial bite of food, the sensations provided by sight, smell and even imagination of appetizing food prompts the salivary glands under the tongue to secrete saliva. This prepares the body for the meal to come.
As the teeth grind food into smaller bits, saliva moistens it for easier swallowing. A digestive enzyme in the saliva known as amylase breaks down carbohydrates (sugars and starches). Muscle movements in the tongue and mouth provide the swallowing necessary to push the food into the throat (pharynx). This is a passageway for food and air that is about 5 inches (12.7 centimeters) long. A flexible flap of tissue called the epiglottis reflexively closes over the windpipe during swallowing to prevent choking and to prevent food from entering the lungs.
As the food travels down the throat, it arrives at a junction just above a long, muscular tube known as the esophagus. This junction, called the upper esophageal sphincter, is a muscular valve that opens to allow food into the esophagus. A series of rhythmic contractions in the esophageal wall moves the food down the tube. This process is called peristalsis, and it takes between four and 10 seconds in the esophagus. The muscular movement of peristalsis continues throughout the digestive system. At the end of the esophagus, a muscular ring called the lower esophageal sphincter opens and allows food to enter the stomach. Then it quickly closes to keep the stomach’s digestive juices from flowing back into the esophagus and damaging the tube’s walls.
A large, bean-shaped, hollow muscular organ consisting of three regions: the cardia, the body (fundus) and the antrum. The upper stomach serves as a storage area for food. Here, the cardia and fundus of the stomach relax to make room for food that enters the stomach. Once food enters the stomach, muscles begin to churn and mix the food with acids and enzymes (gastric juices). This breaks down the food into smaller, digestible pieces. Cells in the stomach lining secrete mucus, hydrochloric acid and pepsin, which aid in this process.
Mucus protects the cells of the stomach lining from damage by the acid and enzymes. Any disruption to this layer of mucus, such as from a bacterial infection for example, can result in damage that leads to a stomach ulcer. Hydrochloric acid, also called stomach acid, is produced by parietal cells in the stomach lining and provides the highly acidic environment necessary for pepsin to break down proteins.
At this point, some substances (e.g., water, salt, sugars and alcohol) are ready to be absorbed into the body directly through the stomach wall. However, most substances still require further digestion. Once the food has been processed into a thick liquid called chyme, it is stored in a walnut-sized muscular tube at the bottom of the stomach called the pylorus. Once the chyme achieves the right consistency, a muscle between the stomach and small intestine called the pyloric sphincter opens to allow its passage into the small intestine.
Chyme secreted from the stomach moves down into the small intestine, which is made up of three parts:
- Duodenum. The C-shaped first part.
- Jejunum. The coiled midsection.
- Ileum. The final section leading into the large intestine.
Digestion of chyme continues as it is further broken down in the small intestine until the nutrients can be absorbed into the bloodstream. Millions of microscopic, finger-like projections (villi) cover the wall of the small intestine. Nutrients are absorbed through these villi.
Undigested food and some water moves into the large intestine through another muscular ring that prevents food from returning to the small intestine. The large intestine removes water from the undigested matter and solid waste that can be excreted. The large intestine is made up of three parts:
- Cecum. Pouch that joins the small intestine to the large intestine. The appendix is an even smaller pouch attached to the cecum. The appendix has no known function, but can become inflamed and require surgical removal.
- Colon. A long tube that connects the cecum to the rectum. Bacteria here help digest any remaining food. The ascending colon and transverse colon absorb fluids and salts. The descending colon receives the resulting waste. The sigmoid colon empties the waste into the rectum.
- Rectum. Area below the colon where feces are stored until they are ready to be expelled from the body through the anus as a bowel movement.
It takes between six and eight hours after a person consumes food before it passes through the small intestine and stomach. Elimination of food residue from the colon usually begins after 24 hours, with complete elimination taking up to several days.
Accessory digestive organs are not part of the alimentary canal, but play an essential role in the digestive process. These include:
- Pancreas. Located beneath the stomach, it produces enzymes that help digest proteins, fats and carbohydrates. The pancreas also makes sodium bicarbonate, which neutralizes stomach acid.
- Liver. Located just beneath the rib cage in the right upper part of the abdomen, it produces bile, which helps the body absorb fat. The liver also regulates substances in the blood cells.
- Gallbladder. Located just below the liver, it stores bile until it is needed.
The enzymes and bile produced by these organs move through ducts into the small intestine, where they help break down food. In addition, nutrients from the small intestine travel through the blood to the liver, which helps process the nutrients.
Many conditions or disorders are associated with the digestive system. These include:
A mild condition marked by unpleasant abdominal sensations after eating. Symptoms of indigestion include heartburn, nausea and gas and bloating.
Repeated, loose, watery and often uncontrolled bowel movements occurring more than three times a day.
Inability or difficulty passing feces from the body, resulting in too few bowel movements.
Caused by swallowing air or the breakdown of certain undigested foods by bacteria in the colon, gas in the digestive tract may result in belching, flatulence, bloating and abdominal pain or discomfort.
Inflammation in the stomach lining caused by either a bacterial infection or the chronic use of certain medications such as nonsteroidal anti-inflammatory drugs (NSAIDs), which weaken its protective mucous coating.
Painful, bleeding sores in the lining of the stomach that occur when the protective mucous coating of these organs is weakened. Ulcers may also occur in the esophagus and part of the small intestine.
A condition in which the contents of the stomach flow back (reflux) into the esophagus and damage its lining. GERD occurs when the lower esophageal sphincter, which separates the esophagus and stomach, does not close properly.
Many bacteria, viruses and parasites, sometimes in food or water, may invade the body through the digestive system. They may cause foodborne illnesses and infections such as gastroenteritis.
A common disorder resulting from improper functioning of the muscles in the colon. Symptoms of IBS include abdominal cramping, bloating, constipation and diarrhea.
Chronic inflammation of the intestines. Examples of inflammatory bowel disease include ulcerative colitis, which often affects just the rectum and the colon, and Crohn's disease, which can affect the whole alimentary canal as well as other parts of the body.
A condition characterized by the presence of small, abnormal sacs (diverticula) in the wall of the colon, which may become infected. The sacs occur most frequently in the last section of the large intestine just above the rectum. There may be only a single sac or there may be hundreds.
Inflammation of the esophagus caused by infection or the prolonged use of certain medications, such as NSAIDs.
A condition characterized by inflammation of the gallbladder and usually associated with gallstones.
Inflammation of the liver, which can result in its impairment.
Inflammation of the appendix, a pouch in the large intestine that has no known function. An inflamed appendix can be life-threatening if it bursts, so appendicitis is usually treated with surgical removal, or appendectomy.
Inflammation of the pancreas, most often caused by alcohol abuse or gallstones.
A digestive disorder caused by an abnormal response of the immune system to a protein called gluten that is found in foods containing wheat, oats, rye and barley.
Patients with celiac disease have difficulty digesting nutrients from the foods they eat because gluten damages the lining of the intestines over time. Symptoms of the disorder include diarrhea, bloating and stomach pain.
A congenital condition in which there is a gap between the upper and lower sections of the esophagus, preventing the passage of food and liquids.
A chronic, inherited condition characterized by the overproduction of thick, abnormal mucus, which blocks the ducts or passageways in the pancreas and prevents its digestive juices from entering the intestines. CF patients have difficulty digesting proteins and fats. As a result, important nutrients pass out of the body unused and patients are underweight.
An inability to break down lactose (the main sugar found in dairy products) due to the lack of lactase, a specific digestive enzyme. Patients with lactose intolerance experience symptoms such as gas, bloating and abdominal pain after consuming a food or beverage that contains lactose.
An inflammation of the abdominal cavity and its lining (peritoneum) that typically involves infection. Symptoms of peritonitis include abdominal pain and tenderness, fever and vomiting.
A bulging of part of the intestine that pushes into the groin through a weakness in the abdominal wall. Characterized by pain and feelings of pressure or burning in the groin, inguinal hernia often requires surgical treatment.
A protrusion of the stomach into the chest cavity through an opening in the diaphragm. Hiatal hernia often results in reflux.
A swelling and stretching of the veins in the wall of the anus and rectum. Some people with hemorrhoids experience bleeding, pain and a feeling of fullness in the rectum while others have no symptoms at all.
A condition in which the stomach muscles function improperly, causing food and liquid to pass too slowly into the small intestine. Common symptoms include nausea, vomiting and feeling full early in a meal (early satiety).
A precancerous condition in which the cells lining the lower part of the esophagus are replaced with abnormal cells. The condition is frequently associated with GERD.
The digestive system is responsible for breaking down food into nutrients that can be absorbed by the body for fuel, growth and cell repair. Good nutrition is essential for maintaining proper function of the body’s systems.
Eating a well-balanced, nutritious diet and maintaining healthy habits are the best ways to prevent digestive problems. Generally, health experts recommend the following:
- Eating fiber. Fruits, vegetables and whole grains pass more easily through the alimentary canal. Fiber gives stool bulk and softens it, which helps prevent constipation and diarrhea. A diet rich in fiber may also help prevent hemorrhoids, diverticular disease and colorectal cancer.
- Reducing fats and oils. Excessive consumption of foods high in fats, especially saturated fats (foods that contain high levels of saturated fatty acids) and trans fatty acids (unsaturated fatty acids formed when vegetable oils are processed and made more solid) can slow digestion and lead to heartburn, bloating and constipation. A high-fat diet can also worsen conditions such as irritable bowel syndrome, pancreatitis and Crohn's disease as well as increase the risk of colon cancer.
- Drinking water. People should consume at least eight 8–ounce servings of water throughout the day. Water lubricates food waste so that it passes more easily through the alimentary canal. It also helps soften stool, which may prevent constipation. In addition, water helps dissolve minerals, vitamins and other nutrients, thereby facilitating their absorption by the body’s tissues.
- Reducing or avoiding alcohol consumption. Health experts recommend that women consume no more than one alcoholic beverage a day and men no more than two. Alcohol can inflame the lining of the stomach and relax the lower esophageal sphincter (the valve that prevents stomach acid from backing up into the esophagus), which can cause heartburn and gastrointestinal bleeding. In addition, alcohol can worsen symptoms of nausea and diarrhea.
- Eating moderate portions at regular times. Large meals cause abdominal distension and bloating. Excessive fat, carbohydrate and fluid can cause diarrhea. Skipping meals can lead to excessive hunger, which often leads to overeating and puts a person at risk for weight gain.
- Chewing food slowly and completely. Fast eating increases the likelihood of swallowing excess air, which can lead to heartburn, belching and gas and bloating.
- Maintaining a healthy weight. Extra weight increases pressure within the abdomen and stomach, which can lead to bloating, constipation and gastroesophageal reflux disease, conditions that are more common among people who are overweight or obese.
- Getting regular exercise. Sustained physical activity (aerobic exercise) stimulates functioning of the intestinal muscles, thereby helping push more food waste through the intestines faster.
- Minimizing stress. Stress can either slow down digestion, which can trigger heartburn, bloating and constipation, or it can speed it up and lead to abdominal pain and diarrhea.
- Avoiding smoking. In addition to increasing a person’s risk for various types of cancer, cigarette smoking can contribute to heartburn and peptic ulcers. Health experts also believe that smoking can increase the risk of Crohn's disease.
The nicotine in tobacco can lead to excessive stomach acid and not enough sodium bicarbonate, a substance that neutralizes stomach acid. Air swallowed during smoking can produce belching, bloating and gas. Smokeless tobacco (e.g., spit tobacco, chew, snuff, dip) is just as hazardous.
- Consulting a physician before using any medications. The prolonged use of any medication can affect a person’s digestion over time. For instance, narcotics taken for pain relief may result in nausea or constipation. High blood pressure drugs may cause diarrhea or constipation, and some antibiotics may cause nausea or diarrhea. Nonsteroidal anti-inflammatories (e.g., aspirin, ibuprofen, naproxen) can result in stomach pain, diarrhea, ulcers and gastrointestinal bleeding when taken regularly or if the recommended dosage is exceeded.
Although healthy eating and lifestyle habits are strongly encouraged for a healthy digestive system, not all digestive disorders are preventable. Some conditions are hereditary and not related to an infection or another specific cause (e.g., celiac disease) and still others occur for unknown reasons. Patients who experience chronic gastrointestinal problems (e.g., pain, diarrhea, constipation) should consult their physician, especially a gastroenterologist, for a health evaluation.
Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions regarding the digestive system:
- Am I at risk for any digestive conditions?
- What changes in the digestive system may indicate an illness?
- For what digestive problems should I consult a physician?
- Why is it important to take care of my digestive system?
- How can I keep my digestive system healthy?
- Can any of the medications I am currently taking affect my digestive system?
- Does my condition require a visit to a gastroenterologist?
- Adamska, Edyta, Lucyna Ostrowska, Maria Górska, and Adam Kretowski. “The Role of Gastrointestinal Hormones in the Pathogenesis of Obesity and Type 2 Diabetes.” Przeglad gastroenterologiczny. Termedia Publishing House, 2014. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4108747/.
- “Digestive System.” Johns Hopkins All Children's Hospital. Accessed May 24, 2020. https://www.hopkinsallchildrens.org/Patients-Families/Health-Library/HealthDocNew/Digestive-System.
- Domalgalski, Jason. “Selected Disorders of the Digestive System.” Family Medicine, 2016, 1233–42. https://doi.org/10.1007/978-3-319-04414-9_101.
- Dowshen, Steven. “Digestive System: Nemours.” Children's Health Care, August 14, 2015. https://www.nemours.org/patientfamily/khlibrary/articles/digestive.html.
- Durani, Yamini. “Body Basics.” Rady Children's Hospital-San Diego, October 2019. https://www.rchsd.org/health-articles/digestive-system-2/.
- “GI Disorders Functional GI Disorders Motility Disorders Upper GI Disorders Lower GI Disorders Other Disorders Kids & Teens.” IFFGD. Accessed May 24, 2020. https://www.iffgd.org/the-digestive-system.html.
- Hobsley, Michael. Disorders of the Digestive System. London: Edward Arnold, 1982.
- M., Van De Graaff Kent, R. Ward Rhees, and Sidney L. Palmer. Schaum's Outlines Human Anatomy and Physiology. New York: McGraw-Hill Education, 2013.
- Mcfadden, Roger. “Disorders and Drugs of the Digestive System.” Introducing Pharmacology, 2019, 199–221. https://doi.org/10.4324/9781351001403-7.
- Mukherjee, Sandeep. “Functional Anatomy of the Digestive System: Diagram & Organs.” eMedicineHealth, April 17, 2020. https://www.emedicinehealth.com/anatomy_of_the_digestive_system/article_em.htm.
- Pelino, Carlo J. “The Digestive System: Part I.” Review of Optometry, February 7, 2011. https://www.reviewofoptometry.com/article/the-digestive-system-part-i.
- Ruiz, Atenodoro R. “Stomach - Digestive Disorders.” MSD Manual Consumer Version. MSD Manuals, October 2019. https://www.msdmanuals.com/home/digestive-disorders/biology-of-the-digestive-system/stomach.
- “Your Digestive System & How It Works | NIDDK.” Accessed May 24, 2020. https://www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works.