It is important to know exactly what the digestive system is composed of and how it works.
This integral part of the human body includes the digestive tract and the organs which serve as accessories to it. The digestive system processes food into molecules which can be absorbed by the body and utilized by the cells. When a person eats, the food consumed is broken down bit by bit until the various molecules are finally tiny enough to be absorbed by the body and the resulting waste products are eliminated.
Other names for the digestive tract are gastrointestinal (GI) tract and the alimentary canal. It consists of a tube which is long and continuous and extends all the way from the mouth to the anus. The areas of the body which are part of this tract include the mouth, pharynx, stomach, esophagus, and the small and large intestines. The teeth and tongue are considered to be accessory structures of the mouth. The gallbladder, pancreas, liver and salivary glands are not considered to be part of the digestive tract but are known as major accessory organs which have a role in digestion due to the fluids they secrete into the digestive tract.
Once in the body, food undergoes three different processes in the body: digestion, absorption and metabolism. Both digestion and absorption take place in the digestive tract. Once the nutrients are absorbed, they become available to all of the body's cells and are used for metabolism.
There are six functions or activities which the digestive system used to prepare nutrients for use by the cells in the body. They include:
- Ingestion - Taking in food is the first activity of the digestive system. This process must occur before anything else can happen.
- Mechanical Digestion - When a person eats large pieces of food, these pieces must be broken down into small particles which can then be acted upon by several different enzymes. This process is called mechanical digestion. It starts in the mouth with mastication (or chewing) and continues with mixing and churning actions in the stomach.
- Chemical Digestion - Proteins, carbohydrates and fats contain complex molecules which are transformed by chemical digestion into smaller molecules which can be absorbed and used by the body's cells. Through a chemical process called hydrolysis, chemical digestion uses water to break apart these complex molecules. Digestive enzymes help to speed up the otherwise slow process of hydrolysis.
- Movements - The ingested food is masticated and moves from the mouth into the pharynx, and then on to the esophagus. This movement is known as deglutition, or more commonly as "swallowing." Smooth muscles in the stomach contract in small segments of the digestive tract and mix the food particles with fluids and other enzymes. Peristalsis is the name given to the movements that propel the particles of food through the digestive tract. These rhythmic contractions move the particles of food through the various areas of the body where chemical and mechanical digestion can occur.
- Absorption - This process takes place when the simple molecules which are produced from chemical digestion pass through the lining in the small intestine and move into the lymph capillaries or the blood.
- Elimination - Some of the food molecules cannot be digested but still need to be eliminated from the body. The act of removing wastes which are indigestible through the anus in the form of feces is called defecation.
Why is digestion important? Because the foods a human eats (such as meat, vegetables and bread) are not in a form which the body can immediately make use of as nutrients. Such foods must be changed into much smaller molecules before they can be absorbed into the bloodstream and thus be carried to various cells throughout the body.
It is easy to think of food which is eaten as going on a "journey" through the body. This journey can be quite slow, taking up to 24 hours to cover a distance which is roughly 9 meters (or 30 feet) through a variety of muscular chambers and tubes. This process begins when food is put into the mouth and ground down and crushed by the teeth during chewing. The resulting ball of food is called a "bolus," which continues down the throat or pharynx. The bolus then travels through the esophagus to the stomach and into the small intestine, the large intestine and finally to the anus.
Chemicals break down food in the small intestine into small molecules which can be absorbed into the blood. Several glands are in the digestive system, including salivary glands which make spit; the pancreas, which makes powerful digestive juices; and the liver, which is the body's major processor of nutrients.
There are three pairs of salivary glands which make saliva. They are the parotid glands, which are located in front of and just below each ear; the submandibular glands, which are found on the inner sides of the mandible, or lower jawbone; and the sublinguals, located in the mouth's floor (below the tongue). While 99.5% of saliva is simply water, it also contains solutes such as amylase, which is a digestive enzyme that begins the process of breaking down salts and starches. The body produces saliva to lubricate food which has been eaten in order to make chewing and swallowing easier. Saliva also helps the mouth to remain moist between periods of drinking and eating.
The first digestive glands are found in the mouth, and are called the salivary glands. Saliva contains an enzyme that starts to digest the starch from food into smaller molecules. The next digestive glands are in the lining of the stomach. A mucous layer coats the stomach to prevent the digestive juices there (which are acidic) from dissolving the stomach's tissues.
The stomach empties the food and digestive juice mixture into the small intestine, where the juices of the pancreas and the liver mix with the food. The liver produces bile, which is stored until needed by the gallbladder. When a person eats, the bile seeps into the intestine to mix with and dissolve fat.
The whole digestive process is controlled by hormone regulators and nerve regulators. This complex system allows the body to obtain the nutrients needed to survive.
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