The whole process of digestion involves many different organs, which are called the digestive system, and include the mouth, esophagus, stomach, small intestines, large intestines, rectum and anus. Other organs are involved in supporting the digestive process as well, but are not technically considered part of the digestive system. These organs are the tongue, the glands in the mouth that produce saliva, the pancreas, liver and gallbladder.

Saliva is secreted by the salivary glands in your mouth and moistens the food to improve the chewing and grinding. Saliva also contains some enzymes that begin the breakdown of starches and fats. For example, carbohydrate digestion begins with the salivary enzyme alpha-amylase, and fat digestion begins with the secretion of the enzyme lingual lipase by glands under your tongue.

The esophagus, sometimes called the gullet, connects the mouth to the stomach. It delivers the saliva-mixed food from the mouth to the stomach and serves as an air lock between the outside world and the digestive tract. The importance of the esophagus' ability to separate the mouth and stomach can be seen in the condition known as GERD (gastroesophageal reflux disease), in which the esophageal barrier is not effective, so the acid contents of the stomach can escape into the esophagus. Everyone experiences some gastroesophageal reflux, and the esophagus, with the help of another helpful component of saliva, salivary bicarbonate, has the ability to clear any stomach acid that escapes. In many people, however, this reflux occurs more frequently than it should, causing pain and affecting healthy digestion. This situation is called GERD and is one of the most commonly seen conditions in medicine today.

The esophagus opens into the stomach, which is a large chamber consisting of the fundus, the body and then the antrum. The entire involvement of the stomach in digestion is called the gastric phase of digestion. The stomach is the primary place where proteins are disassembled and broken down into small peptides. Due to its acidic environment, the stomach is also a decontamination chamber for bacteria and other potentially toxic microorganisms that may have entered your gastrointestinal system through your mouth.

The fundus and body of the stomach, which are usually referred to together and constitute the majority of the stomach in size, are where the stomach stores food before it is delivered to the intestine. When the food enters the fundus and body of the stomach, the lining of the fundus (called the gastric fundal mucosa) produces hydrochloric acid (HCl). This acidic environment is critical for destroying toxins in foods, such as bacteria, as well as for untwisting the complex three-dimensional protein chains, a process called denaturation of the proteins.

The gastric fundus mucosa also secretes the enzyme pepsinogen, which is present in the stomach much of the time but is inactive until the acid is present, when it becomes activated as pepsin. Pepsin acts on the denatured proteins by hydrolyzing, or cutting, the bonds between amino acids in the protein chain, resulting in several smaller chains, or peptides.

Fat hydrolysis is very active in the stomach. The fats have already been exposed to lipase in the saliva, which begins the hydrolysis, but it is the gastric lipase, secreted by the stomach, that is primarily responsible for fat hydrolysis in humans.

The antrum, or lower part of the stomach, is the site for the stomach's grinding action and contains a sensor mechanism, called gastrin, for regulating the level of acid produced in the body of the stomach. The antrum also controls the emptying of food into the intestine through the pyloric sphincter. This way the food can be delivered into the intestine in a controlled manner. Once the food-acid-enzyme mixture leaves the stomach, it is called chyme. The movement of chyme through the pyloric sphincter stimulates the intestine to release the hormones secretin and cholecystokinin, which signal the pancreas to release its contents, the pancreatic juice, inside the lumen (the lining) of the duodenum (the first segment of the small intestine).

The small intestine, which is specifically designed to maximize the digestion and absorption process, has an expanded surface area with inner folds, called plicae, villi and microvilli, to increase its surface area and enhance its ability to absorb nutrients. All together, this surface is called the brush border of the small intestine. Some enzymes are present on the surface of the brush border, such as disaccharidases like sucrase, maltase, and lactose, which hydrolyze disugars (sugars composed of two monosaccharides) to their two individual sugar molecules.

The duodenum, the part of the small intestine that is closest to the stomach, is a neutralization chamber in which the chyme from the stomach is mixed with bicarbonate, which appears again, this time in the pancreatic juice. Bicarbonate lessens the chyme's acidity, thus allowing more enzymes to function and furthering the breakdown of macromolecules still present. The pancreatic juice also contains many of the enzymes necessary for digestion of proteins, such as trypsin and chymotrypsin, enzymes that cut proteins and peptides down into one-, two-, and three-amino acid chains; and amylase, an enzyme that continues the hydrolysis of starch.

A few nutrients, like iron and calcium, are taken up most efficiently in the duodenum; however, the jejunum, the middle section of the small intestine, is the place where most nutrients are actively absorbed. The amino acids as well as most vitamins and minerals are absorbed in the jejunum. The process of absorption used by the jejunum is called active absorption since your body uses energy to select the exact nutrients it needs. Protein carriers or channels hook-up to these nutrients and take them through the cell wall of the jejunum and into the portal vein, which carries them to the liver.

Active fat absorption also occurs in the duodenum and the jejunum, and requires that the fat be put into small aggregates that can be transported into your body directly. The body uses bile as a detergent to solubilize the fat. Bile is produced by the liver, stored in the gall bladder, and released into the duodenum and jejunum after a meal. It then can form miscelles, small fat droplets, for fat absorption. This process is particularly important for the absorption of the fat-soluble vitamins (vitamins A, D, E, and K), and for cholesterol absorption.

The majority of starch is also digested in the duodenum and jejunum, the first and second segments of the small intestine. The monosaccharide products of carbohydrate digestion, glucose and galactose, are actively absorbed through the intestine by a process that requires energy. Fructose, another common monosaccharide product of carbohydrate digestion, and also a common sweetener for many processed foods, is absorbed more slowly by a process called facilitated transport. Facilitated transport does not require energy.

The ileum is the final part of the small intestine. The ileum is responsible for completing the digestion of nutrients and for reabsorbing the bile salts that have helped to solubilize (keep in solution), the fats. Although most nutrients are absorbed in the duodenum and jejunum, the first two segments of the small intestine, the ileum is the place where vitamin B12 is selectively absorbed into your body.

At the end of transport through the small intestine, the chyme has been depleted of around 90 percent of its vitamins and minerals and the majority of its other nutrients. In addition, around eight to 10 liters of fluid is also absorbed in the small intestine each day. Complex carbohydrates that resist the enzyme degradation, such as fiber and resistant starch, remain, as do a small amount of other food molecules and nutrients that have escaped the digestion process. For example, about 3-5% of ingested protein normally escapes digestion and continues to the large intestine.

The large intestine is not designed for enhancing absorption but is particularly specialized to conserve the sodium and water that escape absorption in the small intestine, although it only transports about one liter of fluid per day. The large intestine is about five feet long, including its final segments, the colon and the rectum.

It is interesting, given that most digestion and absorption occurs prior to the large intestine, that food, which at this point is primarily fiber, will spend more time in your large intestine than anywhere else during digestion. On average, food travels through the stomach in 1/2 to two hours, continues through the small intestine over the next two to six hours, and spends six to 72 hours in your large intestine before final removal by defecation.

One reason food stays longer in the large intestine may be that the large intestine is capable of generating nutrients from food. The food that makes it into the large intestine is primarily fiber, and the large intestine contains an ecosystem of bacteria that can ferment much of this fiber, producing many nutrients necessary for the health of the colon cells. Colonic fermentation also produces a series of short-chain fatty acids, including proprionate, acetate, and butyrate, which are required for healthy colonic cell growth and have many other health promoting functions in your body.

The friendly bacteria that are responsible for the primary amount of healthy colonic fermentation are called the probiotics (pro-life) and include the Bifidobacteria and Lactobaccillus genuses. Along with providing beneficial fermentation products, probiotic bacteria keep pathogenic, or disease-promoting bacteria, from colonizing your colon. Certain fibers in food, called prebiotics, specifically support these probiotic bacteria. Prebiotics include such molecules as inulin and fructooligosaccharides, which are found in chicory and Jerusalem artichoke, and may include some other carbohydrates such as galactooligosaccharides, arabinogalactans, and arabinoxylans, which are found in soy and rice fibers, and in larch tree extracts.

Some fiber isn't fermented, but it is also important because it provides bulk for stool excretion, and can bind toxins and waste products for their removal through the stool. Finally, the rectum and the anus allow for controlled elimination of stool.

The pancreas can be thought of as a protein factory. It produces and secretes many of the enzymes necessary for digestion, which include the enzymes that digest protein (trypsin, chymotryosin, carboxypeptidase, and elastase), enzymes that digest fat (lipase and phospholipase), and the enzyme that digests carbohydrate (alpha-amylase). The pancreas releases these enzymes in a pancreatic juice, which is enriched with bicarbonate. The bicarbonate is used to neutralize the acid in chyme. More than a liter of pancreatic juice is released per day in response to signals from eating a meal.

Since your body's tissues are made of protein, the pancreatic enzymes that digest protein have the ability to digest your own tissues. Your body has an intricate protection from self-digestion by these enzymes. The stomach and intestinal tract lining have a mucous layer protecting the tissue from direct digestion by these enzymes. The pancreas uses other mechanisms for protection. Primarily, it produces the enzymes in an inactive form, called zymogens or proenzymes. For example, trypsin is produced as the inactive proenzyme trypsinogen. Trypsinogen is transported to the intestine where it is activated to trypsin by a protease enzyme on the brush border of the intestinal cells. All pancreatic enzymes except lipase and alpha-amylase are secreted as proenzymes, and are therefore inactive within the pancreas.

The liver is one of the most active organs in your body. The liver is the clearinghouse for all nutrient absorption through the gastrointestinal system. The liver reviews the compounds that have been taken in and has the ability to distinguish toxins and other molecules. It has a detoxification system, in which drugs and toxins are chemically converted to molecules that can be eliminated through the kidneys (urine) or the intestine (stool). The liver is also responsible for synthesizing most of the proteins that circulate in your blood, and it produces bile, which is important for the digestion of fats and is used for the excretion of cholesterol and other fat-soluble molecules.

The liver is the major organ involved in maintaining healthy blood sugar (glucose) levels. It monitors your body's glucose needs and provides glucose from digestion, or obtains glucose by breaking down glycogen, the form in which glucose is stored in your liver. The liver has only about a 24-hour supply of glycogen. In prolonged fasting, when glucose is not provided in the diet and glycogen stores have been used, your liver will synthesize glucose from amino acids and other molecules.

The liver is also the primary organ in which fats are metabolized. The liver can make cholesterol and is the primary place where cholesterol is removed from the blood. The liver eliminates cholesterol in the form of bile acids. Every day, your liver secretes about 500 milliliters of bile acids, which are used during digestion to solubilize fats.

The gallbladder is the storage site for the bile acids produced by the liver. After a meal is consumed, the gallbladder is signaled to release its contents into the duodenum and jejunum, where they are available for fat digestion.