The first step in the digestive process happens before we even taste food. Just by smelling that homemade apple pie or thinking about how delicious that ripe tomato is going to be, you start salivating — and the digestive process begins in preparation for that first bite.

Food is our fuel, and its nutrients give our bodies' cells the energy and substances they need to operate. But before food can do that, it must be digested into small pieces the body can absorb and use.

About the Digestive System

Almost all animals have a tube-type digestive system in which food enters the mouth, passes through a long tube, and exits as feces (poop) through the anus. The smooth muscle in the walls of the tube-shaped digestive organs rhythmically and efficiently moves the food through the system, where it is broken down into tiny absorbable atoms and molecules.

During the process of absorption, nutrients that come from the food (including carbohydrates, proteins, fats, vitamins, and minerals) pass through channels in the intestinal wall and into the bloodstream. The blood works to distribute these nutrients to the rest of the body. The waste parts of food that the body can't use are passed out of the body as feces.

Every morsel of food we eat has to be broken down into nutrients that can be absorbed by the body, which is why it takes hours to fully digest food. In humans, protein must be broken down into amino acids, starches into simple sugars, and fats into fatty acids and glycerol. The water in our food and drink is also absorbed into the bloodstream to provide the body with the fluid it needs.

How Digestion Works

The digestive system is made up of the alimentary canal (also called the digestive tract) and the other abdominal organs that play a part in digestion, such as the liver and pancreas. The alimentary canal is the long tube of organs — including the esophagus, stomach, and intestines — that runs from the mouth to the anus. An adult's digestive tract is about 30 feet (about 9 meters) long.

Digestion begins in the mouth, well before food reaches the stomach. When we see, smell, taste, or even imagine a tasty meal, our salivary glands, which are located under the tongue and near the lower jaw, begin producing saliva. This flow of saliva is set in motion by a brain reflex that's triggered when we sense food or think about eating. In response to this sensory stimulation, the brain sends impulses through the nerves that control the salivary glands, telling them to prepare for a meal.

As the teeth tear and chop the food, saliva moistens it for easy swallowing. A digestive enzyme called amylase, which is found in saliva, starts to break down some of the carbohydrates (starches and sugars) in the food even before it leaves the mouth.

Swallowing, which is accomplished by muscle movements in the tongue and mouth, moves the food into the throat, or pharynx. The pharynx, a passageway for food and air, is about 5 inches (12.7 centimeters) long. A flexible flap of tissue called the epiglottis reflexively closes over the windpipe when we swallow to prevent choking.

From the throat, food travels down a muscular tube in the chest called the esophagus. Waves of muscle contractions called peristalsis force food down through the esophagus to the stomach. A person normally isn't aware of the movements of the esophagus, stomach, and intestine that take place as food passes through the digestive tract.

At the end of the esophagus, a muscular ring or valve called a sphincter allows food to enter the stomach and then squeezes shut to keep food or fluid from flowing back up into the esophagus. The stomach muscles churn and mix the food with acids and enzymes, breaking it into much smaller, digestible pieces. An acidic environment is needed for the digestion that takes place in the stomach. Glands in the stomach lining produce about 3 quarts (2.8 liters) of these digestive juices each day.

Most substances in the food we eat need further digestion and must travel into the intestine before being absorbed. When it's empty, an adult's stomach has a volume of one fifth of a cup (1.6 fluid ounces), but it can expand to hold more than 8 cups (64 fluid ounces) of food after a large meal.

Role of the Intestines

By the time food is ready to leave the stomach, it has been processed into a thick liquid called chyme. A walnut-sized muscular valve at the outlet of the stomach called the pylorus keeps chyme in the stomach until it reaches the right consistency to pass into the small intestine. Chyme is then squirted down into the small intestine, where digestion of food continues so the body can absorb the nutrients into the bloodstream.

The small intestine is made up of three parts:

1. the duodenum, the C-shaped first part
2. the jejunum, the coiled midsection
3. the ileum, the final section that leads into the large intestine

The inner wall of the small intestine is covered with millions of microscopic, finger-like projections called villi. The villi are the vehicles through which nutrients can be absorbed into the body.

The liver (located under the ribcage in the right upper part of the abdomen), the gallbladder (hidden just below the liver), and the pancreas (beneath the stomach) are not part of the alimentary canal, but these organs are essential to digestion.

The liver produces bile, which helps the body absorb fat. Bile is stored in the gallbladder until it is needed. The pancreas produces enzymes that help digest proteins, fats, and carbs. It also makes a substance that neutralizes stomach acid. These enzymes and bile travel through special channels (called ducts) directly into the small intestine, where they help to break down food. The liver also plays a major role in the handling and processing of nutrients, which are carried to the liver in the blood from the small intestine.

From the small intestine, undigested food (and some water) travels to the large intestine through a muscular ring or valve that prevents food from returning to the small intestine. By the time food reaches the large intestine, the work of absorbing nutrients is nearly finished. The large intestine's main function is to remove water from the undigested matter and form solid waste that can be excreted.

The large intestine is made up of three parts:

1. The cecum is a pouch at the beginning of the large intestine that joins the small intestine to the large intestine. This transition area expands in diameter, allowing food to travel from the small intestine to the large. The appendix, a small, hollow, finger-like pouch, hangs at the end of the cecum. Doctors believe the appendix is left over from a previous time in human evolution. It no longer appears to be useful to the digestive process.
2. The colon extends from the cecum up the right side of the abdomen, across the upper abdomen, and then down the left side of the abdomen, finally connecting to the rectum. The colon has three parts: the ascending colon; the transverse colon, which absorb fluids and salts; and the descending colon, which holds the resulting waste. Bacteria in the colon help to digest the remaining food products.
3. The rectum is where feces are stored until they leave the digestive system through the anus as a bowel movement.

Digestive System Problems

Nearly everyone has a digestive problem at one time or another. Some conditions, like indigestion or mild diarrhea, are common; they result in mild discomfort and get better on their own or are easy to treat. Others, such as inflammatory bowel disease (IBD), can be ongoing and troublesome and should be discussed with a GI specialist or gastroenterologist (doctors who specialize in the digestive system).

Problems With the Esophagus

Problems affecting the esophagus may be congenital (present at birth) or noncongenital (developed after birth). Examples include:

• Congenital conditions. Tracheoesophageal fistula is a connection between the esophagus and the trachea (windpipe) where there shouldn't be one. In babies with esophageal atresia, the esophagus comes to a dead end instead of connecting to the stomach. Both conditions are usually detected soon after a baby is born — sometimes even before — and require surgery to repair.
• Noncongenital conditions. Esophagitis (inflammation of the esophagus) can be caused by infection, certain medications, or gastroesophageal reflux disease (GERD). With GERD, the esophageal sphincter (the valve that connects the esophagus with the stomach) doesn't work well and allows the acidic contents of the stomach to move backward up into the esophagus. GERD often can be corrected through lifestyle changes, such as dietary adjustments. Sometimes, though, it requires treatment with medication.

Problems With the Stomach and Intestines

Almost everyone has experienced diarrhea or constipation. With diarrhea, muscle contractions move the contents of the intestines along too quickly and there isn't enough time for water to be absorbed before the feces are pushed out of the body. Constipation is the opposite: The contents of the large intestines do not move along fast enough and waste materials stay in the large intestine so long that too much water is removed and the feces become hard.

Other common stomach and intestinal disorders include:

• Gastrointestinal infections can be caused by viruses, by bacteria (such as Salmonella, Shigella, Campylobacter, or E. coli), or by intestinal parasites (such as amebiasis and giardiasis). Abdominal pain or cramps, diarrhea, and sometimes vomiting are the common symptoms of gastrointestinal infections. These usually go away on their own without medicines or other treatment.
• Appendicitis, an inflammation of the appendix, most often affects kids and teens between 11 and 20 years old, and requires surgery to correct. The classic symptoms of appendicitis are abdominal pain, fever, loss of appetite, and vomiting.
• Gastritis and peptic ulcers arise when a bacterium, Helicobacter pylori, or the chronic use of drugs or certain medications weakens the protective mucous coating of the stomach and duodenum, allowing acid to get through to the sensitive lining beneath. This can irritate and inflame the lining of the stomach (gastritis) or cause peptic ulcers, which are sores or holes in the lining of the stomach or the duodenum that cause pain or bleeding. Medications usually successfully treat these conditions.
• Inflammatory bowel disease (IBD) is chronic inflammation of the intestines that affects older kids, teens, and adults. There are two major types: ulcerative colitis, which usually affects just the rectum and the large intestine; and Crohn's disease, which can affect the whole gastrointestinal tract from the mouth to the anus as well as other parts of the body. They are treated with medications and, if necessary, intravenous (IV) feedings to provide nutrition. In some cases, surgery may be necessary to remove inflamed or damaged areas of the intestine.
• Celiac disease is a disorder in which the digestive system is damaged by the response of the immune system to a protein called gluten, which is found in wheat, rye, and barley and a wide range of foods, from breakfast cereal to pizza crust. People with celiac disease have difficulty digesting the nutrients from their food and may experience diarrhea, abdominal pain, bloating, exhaustion, and even depression when they eat foods with gluten. Symptoms can be managed by following a gluten-free diet. Celiac disease runs in families and can become active after some sort of stress, such as surgery or a viral infection. A doctor can diagnose celiac disease with a blood test and by taking a biopsy of the small intestine.
• Irritable bowel syndrome (IBS), a common intestinal disorder, affects the colon and may cause recurrent abdominal cramps, bloating, constipation, and diarrhea. There is no cure, but IBS symptoms may be treated by changing eating habits, reducing stress, and making lifestyle changes. A doctor may also prescribe medications to relieve diarrhea or constipation. No one test is used to diagnose IBS, but a doctor may identify it based on symptoms, medical history, and a physical exam.

Problems With the Pancreas, Liver, and Gallbladder

Conditions affecting the pancreas, liver, and gallbladder often affect the ability of these organs to produce enzymes and other substances that aid in digestion. Examples include:

• Cystic fibrosis is a chronic, inherited illness that not only affects the lungs but also causes the production of abnormally thick mucus to block the ducts or passageways in the pancreas. This mucus also prevents digestive juices from entering the intestines, making it difficult to properly digest proteins and fats. This causes important nutrients to pass out of the body unused. To help manage their digestive problems, people with cystic fibrosis can take digestive enzymes and nutritional supplements.
• Hepatitis, a condition with many different causes, is when the liver becomes inflamed and may lose its ability to function. Viral hepatitis, such as hepatitis A, B, or C, is highly contagious. Mild cases of hepatitis A can be treated at home; however, serious cases involving liver damage may require hospitalization.
• The gallbladder can develop gallstones and become inflamed — a condition called cholecystitis. Although gallbladder conditions are uncommon in kids and teens, they can occur in those who have sickle cell anemia or are being treated with certain long-term medications.

Keeping Digestion on Track

The kinds and amounts of food a person eats and how the digestive system processes that food play key roles in maintaining good health. Eating a healthy diet is the best way to prevent common digestive problems.

Reviewed by: Yamini Durani, MD
Date reviewed: October 2012
Originally reviewed by: Stephen E. Shaffer, MD