Digestion can be either chemical or mechanical. Mechanical digestion is the breaking down of food into smaller particles so that it can more easily be processed by the digestive system. The best example of this is mastication, which is the term for chewing. The teeth chop the food up into smaller pieces which then pass through the digestive system. Chemical digestion is the breaking down of food particles through chemical reactions. Chemical and mechanical digestion combine to process food, absorbing nutrients and minerals and discarding the waste.
Very little mechanical digestion occurs outside of the mouth. The act of chewing food breaks it down into more easily-digested pieces. The stomach performs a little mechanical digestion as the muscles expand and contract to move the food around in the stomach. This is in order to expose the food to more of the chemical secretions in the stomach and thus digest it easier. The movement of food in the same way, through muscle contractions, also occurs in the small and large intestines.
The expansion and contraction of muscles that occurs throughout the gastrointestinal tract during mechanical digestion is called peristalsis. There are two groups of muscles that work simultaneously to move the food. Circular muscles and longitudinal muscles work in conjunction. First one group contracts, then the other. An earthworm moves through a similar series of muscle contractions.
The process of chemical digestion begins in the mouth, which secretes saliva, a chemical that works to begin digesting food as soon as it is eaten. Saliva, also called spit or drool, comes from the salivary glands which are located in the mouth and is 98% water. There are three major pairs of salivary glands and hundreds of minor ones.
Chemical digestion that occurs in the stomach is mainly via enzymes and hydrochloric acid. The stomach secretes pepsinogen which turns into pepsin and breaks down proteins. The hydrochloric acid provides a low pH level in which the enzymes can thrive. The combination of enzymes and stomach acid also help to kill any bacteria that may be lurking within the food. Chewed food that enters the stomach is called bolus and it is called chyme after it leaves the stomach.
After food leaves the stomach, it enters the small intestine where three more liquids are added to further digest it. The liver produces bile and stores it in the gallbladder until needed. The pancreas secretes pancreatic juice which is also used to break down food. Finally, food is further digested by enzymes secreted from the mucous membranes within the small intestine. Still moving by means of peristalsis, the food leaves the small intestine and enters the large intestine, where many nutrients are absorbed, and then the waste exits the body via the anus.