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For the industrial process, see Anaerobic digestion. For the treatment of precipitates in analytical chemistry, see Precipitation (chemistry) § Digestion. For the journal, see Digestion (journal). For the term in alchemy, see Digestion (alchemy).
Digestive system
Details
Identifiers
Latin
systema digestorium
MeSH
D004063
Anatomical terminology
[edit on Wikidata]
Digestion is the breakdown of large insoluble food compounds into small water-soluble components so that they can be absorbed into the blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion. The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. Mechanical digestion takes place in the mouth through mastication and in the small intestine through segmentation contractions. In chemical digestion, enzymes break down food into the small compounds that the body can use.
In the human digestive system, food enters the mouth and mechanical digestion of the food starts by the action of mastication (chewing), a form of mechanical digestion, and the wetting contact of saliva. Saliva, a liquid secreted by the salivary glands, contains salivary amylase, an enzyme which starts the digestion of starch in the food.[1] The saliva also contains mucus, which lubricates the food; the electrolyte hydrogencarbonate (HCO−3), which provides the ideal conditions of pH for amylase to work; and other electrolytes (Na+, K+, Cl−).[2] About 30% of starch is hydrolyzed into disaccharide in the oral cavity (mouth). After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called a bolus. It will then travel down the esophagus and into the stomach by the action of peristalsis. Gastric juice in the stomach starts protein digestion. Gastric juice mainly contains hydrochloric acid and pepsin. In infants and toddlers, gastric juice also contains rennin to digest milk proteins. As the first two chemicals may damage the stomach wall, mucus and bicarbonates are secreted by the stomach. They provide a slimy layer that acts as a shield against the damaging effects of chemicals like concentrated hydrochloric acid while also aiding lubrication.[3] Hydrochloric acid provides acidic pH for pepsin. At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis, which is waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes. Pepsin breaks down proteins into peptides or proteoses, which is further broken down into dipeptides and amino acids by enzymes in the small intestine. Studies suggest that increasing the number of chews per bite increases relevant gut hormones and may decrease self-reported hunger and food intake.[4]
When the pyloric sphincter valve opens, partially digested food (chyme) enters the duodenum where it mixes with digestive enzymes from the pancreas and bile juice from the liver and then passes through the small intestine, in which digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of nutrient absorption occurs in the small intestine. Water and minerals are reabsorbed back into the blood in the colon (large intestine) where the pH is slightly acidic (about 5.6 ~ 6.9). Some vitamins, such as biotin and vitamin K (K2MK7) produced by bacteria in the colon are also absorbed into the blood in the colon. Absorption of water, simple sugar and alcohol also takes place in stomach. Waste material (feces) is eliminated from the rectum during defecation.[5]
^Avraham, Regina (1989). The Digestive System. Introduction by C. Everett Koop. New York: Chelsea House. pp. 49. ISBN 0-7910-0015-X. OL 2055854M. Retrieved 2024-03-20.
^Berne, Robert M.; Levy, Matthew N. (2000). Principles of Physiology (3rd ed.). St. Louis: Mosby. pp. 373-374. ISBN 0-323-00813-5. OL 9840795M. Retrieved 2024-03-20.
^Allen, Adrian; Flemström, Gunnar (January 2005). "Gastroduodenal mucus bicarbonate barrier: protection against acid and pepsin". American Journal of Physiology. Cell Physiology. 288 (1): C1–19. doi:10.1152/ajpcell.00102.2004. ISSN 0363-6143. PMID 15591243.
^Miquel-Kergoat, Sophie; Azais-Braesco, Veronique; Burton-Freeman, Britt; Hetherington, Marion M. (2015-11-01). "Effects of chewing on appetite, food intake and gut hormones: A systematic review and meta-analysis". Physiology & Behavior. 151: 88–96. doi:10.1016/j.physbeh.2015.07.017. ISSN 1873-507X. PMID 26188140.
^Maton, Anthea; Jean Hopkins; Charles William McLaughlin; Susan Johnson; Maryanna Quon Warner; David LaHart; Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, NJ: Prentice Hall. ISBN 978-0-13-981176-0. OCLC 32308337.
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cellular enzymes called proteases, but may also occur by intra-molecular digestion. Proteolysis in organisms serves many purposes; for example, digestive...
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sequential stages. The gastric digestion stage, the hepatic digestion stage, the vascular digestion stage, and the tissue digestion stage. Each stage digests...
wings (if present), is specialized for locomotion; and the abdomen is for digestion, respiration, excretion, and reproduction.: 22–48 Although the general...
primitive ones: sponges have small pores (ostia) throughout their body for digestion and a larger dorsal pore (osculum) for excretion, comb jellies have both...
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