In current paper, a short review of the digestive tract and system will be presented. Focus will be made on new discoveries, while more common knowledge will be discussed omitting unnecessary details.
The breakdown of food already starts in the oral cavity. First, it is influenced mechanically, increasing the area for further chemical reactions to occur. Second, saliva contains two enzymes – amylase and lipase. The former can break down sugars and starch, which is a sugar polymer into monomers. Lipase, on the other hand starts breakdown of fats, which will be further continued in the stomach and duodenum. Also, saliva helps the traversing of food down the intestines, and finally it has some antimicrobial properties, because it contains IgA, lactoferrin, lysozyme and peroxidase (Tenovuo, 2002). These along with lipase, help prevent bacterial build up on teeth, while the latter fat digestive enzyme also acts like a cleanser. An interesting experiment available to anyone can demonstrate the action of amylase – if some pastry is chewed and held in oral cavity slightly longer, than usual, a sweet taste builds up and can be sensed, from the sugars that were broke down by this enzyme.
Further traversal of food through the esophagus brings it to a temporary stopping point – the stomach. It contains a layered mucous environment, filled with HCl acidic liquid phase, that also contains some protein-digesting enzymes – proteases, which work well in acidic environments. The control of stomach secretion and movements (motility) relies entirely on an autonomous nervous system, and is controlled through hormones – gastrin, cholecystokinin, secretin, gastric inhibitory peptide and enteroglucagon. Mostly these hormones are produced by small intestine, and represent the pathway for controlling the flow of food into lower parts of the digestive tract, from the stomach. Some of the nutrients, like water or medications and drugs like aspirin or caffeine, or simple chemicals like amino acids (building blocks of proteins) and ethanol (or other alcohols) can be readily absorbed from the stomach into blood. Moreover, a neural link exists between the stomach and hypothalamus and limbic system (Uematsu et al., 2010), while the stomach itself has numerous receptors for glutamates, glucose, carbohydrates, proteins and fats. These obviously add up to our conscious and unconscious experience from food ingestion, and probably present some learning mechanisms for food preferences.
The lower gastrointestinal tract consists of small and large intestines. Inside small intestine, food goes subsequently through duodenum, jejunum and ileum, while the latter opens into the large intestine – cecum (including the appendix), colon and rectum. Most of the food breakdown after the stomach happens inside the duodenum, while the later stages' functions are concentrated on absorption. First of all, the duodenum neutralizes the acid coming from the stomach. Bile, secreted by the liver into all smaller intestine, along with pancreatic juice which contains digestive enzymes break down proteins (continuing the process which started in the stomach). Bile also emulsifies fats. In the jejunum, sugars, amino acids, fatty acids are absorbed by numerous villi. Ileum absorbs some …