The whole process of digestion involves many different organs. The digestive system includes 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.
Digestion begins in the mouth with the chewing of food (mastication). Chewing not only breaks down very large aggregates of food molecules into smaller particles and allows saliva and enzymes to enter inside the larger food complexes, but also sets off a signaling message to the body to start the entire digestive process. Research has shown that the activation of taste receptors in your mouth and the physical process of chewing signal the nervous system. For example, the taste of food can trigger the stomach lining to produce acid, a process called the cephalic phase of digestion; therefore, your stomach begins to respond to food even before any food leaves your mouth.
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 America today.
The esophagus opens into the stomach. 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 hydrocholoric 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.
