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Digestion begins when food enters the mouth (ingestion) where it is broken down into smaller particles. This task is carried out by the teeth, which are hard pieces anchored onto the gums, and the tongue, a muscular organ that helps place food between the teeth for cutting and grinding it up.

In addition, food is mixed with saliva (produced by the salivary glands), which contains a digestive enzyme dubbed salivary amylase or ptyalin. This enzyme triggers the process of digesting starch. Saliva also acts as a lubricant and it destroys a proportion of the bacteria ingested along with food.

Thus, after approximately one minute, the mixture is converted into a soft mass called the food bolus. This mass arrives to the esophagus pushed by the pharynx by means of a process dubbed swallowing, which, through reflex contractions, allows food and liquids to pass smoothly on to the esophagus. Once in the esophagus, the food bolus moves towards the stomach through a wave-like motion produced by involuntarily muscle contractions called peristalsis or peristaltism. The peristaltic motion involves the relaxing of the muscles in front of the bolus and their contraction behind it, pushing the food bolus on from the esophagus to the stomach. This action is eased by the mucus secreted from the mucus glands located at the esophageal walls.

When the food bolus is located within the stomach, it is mixed with great amounts of gastric juices becoming a mass dubbed chyme. The chyme contains hydrochloric acid and pepsin which is an enzyme in charge of decomposing some proteins.
Also, in this part of the process, starches and sugars contained in the food are mixed with the gastric juices, but final digestion takes place in the small intestine.

Sometimes, these two incompletely digested carbohydrates ferment, producing gases that are expelled by the mouth or enter the intestine. On the other hand, lipids advance through the stomach practically unchanged.

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