The connection between the digestive system and the mouth is that the mouth is where the first phase of digestion takes place. A component of the foregut, the mouth is where mastication, or the physical breakdown of food by chewing, occurs. It is also where digestive enzymes, via saliva, begin the chemical breakdown of food. Therefore, the terms digestive system and the mouth can be used interchangeably when discussing where food enters the gastrointestinal tract.
In the digestive system, food that is consumed is converted into a usable unit of energy known as a calorie through a combination of physical and chemical processes. Physical processes break the food into smaller pieces that can travel more easily through the gastrointestinal tract, and chemical processes break down the food at the molecular level so that its nutrients may be absorbed by the body. In the various organs of the digestive system and the mouth, both of these processes tend to take place concurrently.
The first portion of the digestive tract is known as the foregut. It includes the mouth, esophagus, stomach, and a part of the first segment of the small intestine known as the duodenum, as well as the accessory digestive organs like the liver, pancreas, and gallbladder, organs that further contribute to food breakdown by secreting fluids or transporting nutrients. The mouth is portion of the foregut in which the physical and chemical processes of digestion begin.
When food is placed in the mouth, the salivary glands immediately secrete saliva, the clear lubricating fluid of the mouth. Among its multiple functions in the digestive system and the mouth, including protecting both from harmful bacteria, saliva contains compounds known as enzymes. These enzymes begin a process known as catabolism, chemically breaking down nutrients in the food such as starch and fat into their molecular components. Catabolism begins in the mouth, but most of the process is not completed until the partially digested food reaches the small intestine.
Physical breakdown of food, which occurs both throughout the digestive system and the mouth, is initiated by chewing. Also known as mastication, using the teeth to break the food into smaller pieces renders it easier to swallow. This process also creates a greater surface area for the enzymes in saliva to act on the food. By tearing, grinding, or crushing, the teeth break apart the food, which then mixes with saliva in the mouth to form a substance known as bolus that can be swallowed and delivered via the esophagus to the stomach.