Your digestive system is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. Inside this tube is a lining called the "mucosa." In your mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help you digest food. The digestive tract of a typical adult is about 30 feet long.
Two solid organs, the liver and the pancreas, produce digestive juices that reach the intestine through small tubes. In addition, parts of other organ systems (such as nerves and blood) play a major role in the digestive system.
Why digestion is important
Food is the body's fuel source. Nutrients in food give your body's cells the energy and other substances they need to operate.
When you eat foods, such things as bread, meat, and vegetables, they are not in a form that your body can use as nourishment. Your food and drink must be changed into very small pieces (molecules) before they can be absorbed into the blood and carried to cells throughout your body.
Each morsel of food you eat must be broken down into nutrients that can be absorbed by the body—it takes hours to fully digest food. In your digestive tract, protein is broken down into amino acids, starches into simple sugars, and fats into fatty acids and glycerol. The water in your food and drink is also absorbed into the bloodstream to provide your body with the fluid it needs.
Digestion is the process by which food and drink are broken down into their smallest parts so that the body can use them to build and nourish cells and to provide energy.
How food is digested
Digestion involves the mixing of food, its movement through your digestive tract, and the chemical breakdown of the large molecules of food into smaller molecules. Digestion begins in the mouth, when you chew and swallow, and is completed in the small intestine. The chemical process varies somewhat for different kinds of food.
Movement of Food Through the System
The large, hollow organs of your digestive system contain muscles that enable their walls to move. This movement of the organ walls propels food and liquid and also mixes the contents within each organ.
The movement of your esophagus, stomach, and intestine is called peristalsis. The action of peristalsis looks like an ocean wave moving through the muscle. The muscle of the organ produces a narrowing and then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ.
The first important muscle movement occurs when you swallow food or liquid. Although you are able to start swallowing by choice, once the swallow begins, it becomes involuntary and proceeds under the control of the nerves.
When you start to eat, the salivary glands in your mouth pump out digestive juices (saliva, or spit), which begin to break down your food chemically. The brain triggers this flow of saliva whenever you sense food or even think about eating. Together your salivary glands, which are located under the tongue and near the lower jaw, produce 1 to 3 pints of saliva a day.
Your tongue and teeth help to get the digestive process started by chewing and chopping the food so it's small enough to be swallowed. Swallowing is very complicated—when you are ready to swallow, your tongue pushes a small bit of mushed-up food (known as a bolus) toward the back of your throat and into the opening of your esophagus. The journey from the back of your throat through the esophagus to the stomach typically takes eight seconds.
Your esophagus is the organ into which the swallowed food is pushed. It connects the throat above with the stomach below. At the junction of the esophagus and stomach, there is a ringlike valve, which keeps the opening between the two organs closed. However, as food approaches the closed ring, the surrounding muscles relax and allow the food to pass.
The food then enters your stomach, which has three mechanical tasks to do:
- First, your stomach must store the swallowed food and liquid—this requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material.
- The second job is to mix up the food, liquid, and digestive juice produced by the stomach—the lower part of your stomach mixes these materials by its muscle action.
- The third task of the stomach is to empty its contents slowly into your small intestine.
Several factors affect emptying of your stomach, including the type of food you have eaten (mainly its fat and protein content) and the degree of muscle action of the emptying stomach and the next organ to receive the contents (the small intestine).
As the food passes along your small intestine, which is over twenty feet long, the nutrients are absorbed through the wall of the small intestine and passed into the bloodstream. By the time the food has reached the large intestine (colon), the nutrients have been removed and waste materials remain.
In the colon, the waste material, which includes undigested parts of food (mostly fiber) and older cells that have been shed from the mucosa, is passed along by the muscle contractions (peristalsis). Eventually the waste reaches the end of the digestive tract, the rectum.
Your colon absorbs water from the waste material, which causes the material (stool) to become firmer. Stool usually remains in the colon and rectum for a day or two, until it is expelled by a bowel movement.
There is a wide variation in normal bowel movements—the average person has a bowel movement anywhere from 3 times a day to 3 times a week.
Production of Digestive Juices
The glands that act first are in your mouth--the salivary glands. Saliva produced by these glands contains an enzyme that begins to digest the starch from food into smaller molecules.
The next set of digestive glands is in the lining of your stomach. They produce stomach acid and an enzyme that digests protein. One of the unsolved puzzles of the digestive system is why the acid juice of the stomach does not dissolve the tissue of the stomach itself. In most people, the stomach mucosa is able to resist the juice, although food and other tissues of the body cannot.
After your stomach empties the food and juice mixture into the small intestine, the juices of two other digestive organs mix with the food to continue the process of digestion.
One of these organs is the pancreas. It produces a juice that contains a wide array of enzymes to break down the carbohydrate, fat, and protein in food. Other enzymes that are active in the process come from glands in the wall of the small intestine.
The liver produces yet another digestive juice—bile, which, between meals, is stored in the gallbladder. At mealtime, bile is squeezed out of the gallbladder into the bile ducts to reach the intestine and mix with the fat in your food. The acids in the bile dissolve the fat into a watery mixture, much like detergents that dissolve grease from a frying pan. After the fat is dissolved, it is further broken down by enzymes from the pancreas and the lining of the intestine.
Absorption and Transport of Nutrients
Digested molecules of food (including carbohydrates, fats, proteins, and vitamins), as well as water and minerals from your diet, are absorbed from the cavity of the upper small intestine. Most of these absorbed materials cross the mucosa into your blood and are carried off in the bloodstream to other parts of the body for storage or use by the body’s cells for energy and nourishment.
How the digestive process is controlled
A fascinating feature of the digestive system is that it contains its own regulators. Hormones that control the functions of the digestive system are produced and released by cells in the mucosa of the stomach and small intestine. These hormones are released into the blood of the digestive tract, travel back to the heart and through the arteries, and return to the digestive system, where they stimulate digestive juices and cause organ movement.
The hormones that control digestion are:
- Gastrin, which causes the stomach to produce an acid for dissolving and digesting some foods. It is also necessary for the normal growth of the lining of the stomach, small intestine, and colon.
- Secretin, which causes the pancreas to send out a digestive juice. It stimulates the stomach to produce pepsin, an enzyme that digests protein, and stimulates the liver to produce bile.
- Cholecystokinin (CCK), which causes the pancreas to grow and produce the enzymes of pancreatic juice. It also causes the gallbladder to empty.
Additional hormones in the digestive system regulate appetite:
- Ghrelin, which is produced in the stomach and upper intestine in the absence of food in the digestive system and stimulates appetite.
- Peptide YY, which is produced in the digestive system in response to a meal in the system and inhibits appetite.
Both of these hormones work on the brain to help regulate the intake of food for energy.
Two types of nerves help to control the action of the digestive system.
Extrinsic (outside) nerves come to the digestive organs from the unconscious part of the brain or from the spinal cord. They release a chemical called acetylcholine and another called adrenaline.
- Acetylcholine causes the muscle of the digestive organs to squeeze with more force and increase the "push" of food and juice through the digestive tract.
- Acetylcholine also causes the stomach and pancreas to produce more digestive juice.
- Adrenaline relaxes the muscle of the stomach and intestine and decreases the flow of blood to these organs.
Even more important, though, are the intrinsic (inside) nerves, which make up a very dense network embedded in the walls of the esophagus, stomach, small intestine, and colon. The intrinsic nerves are triggered to act when the walls of the hollow organs are stretched by food. They release many different substances that speed up or delay the movement of food and the production of juices by the digestive organs.