As digestion , digesting (from Old High German firdouwen "melt, liquefy," related to the Middle High German döuwen / Douwen , "digest", and High German "thaw") or digestion (from the Latin digestio ) refers to the digestion of food in the digestive tract with the help of digestive enzymes . Chemical cleavage (more precisely: hydrolysis ) of high molecular weight carbohydrates , fats and proteins creates low molecular weight compounds (e.g. mono- and disaccharides , fatty acids , amino acids , di- and tripeptides ), some of which are converted into energy or otherwise in the Production of new body substance can be used by the living organism incorporating it into various cell structures after a chemical conversion.
Many nutrients ingested with food are insoluble in water. Therefore, they cannot be absorbed from the small intestine into the blood and lymph . The nutrients become water-soluble when they are broken down into smaller basic building blocks. Enzymes that are contained in digestive juices catalyze different cleavage reactions many times over. In this way, the food ingested can be digested in a relatively short time.
Digestion in humans
In humans, digestion takes place mainly in the mouth , stomach (Gaster), duodenum (duodenum) and the rest of the small intestine (jejunum and ileum) instead. However, nutrients are almost exclusively absorbed in the duodenum and small intestine.
Digestion and intake of food
The saliva is produced by three salivary glands :
- Parotid gland (glandula parotidea),
- Sublingual gland (sublingual gland) and
- Lower jaw salivary gland (submandibular gland).
The aqueous saliva contains the enzyme ptyalin , an α- amylase . This splits the starch (polysaccharides) in the food into malt sugar ( maltose ), maltotriose and oligosaccharides - this is why bread tastes sweet after a longer period in the mouth. Physiologically, however, this does not always play a role, as the time from ingestion of food to inactivation of the amylase due to the low pH value of the stomach is too short for actual "digestion". The further breakdown of starch takes place later in the small intestine. Furthermore, “animal starch” ( glycogen ) is also broken down into maltose. During the chewing process, the tongue mixes the chew, which is then pressed against the palate and the swallowing reflex is triggered. The epiglottis is briefly lowered, the windpipe is closed so that no food can get into it, breathing is stopped and access to the nose is blocked off.
After the oral cavity, the food enters the esophagus. The esophagus is a muscular tube that lies behind the windpipe and carries food to the stomach. If food remains in the windpipe, this leads to strong coughing and in the worst case to suffocation. The food is pressed into the stomach in a few seconds by muscle movements ( peristalsis ), which run in waves from the throat to the stomach. This is an active transport process within the digestive system. Because of this process, swallowing is possible while lying down as well as in a handstand.
The chyme is collected over a long period of time in the stomach , which has a capacity of approx. 1.5 to 2 liters. At first, the amylase continues to digest starch in the stomach. At the entrance is the stomach mouth ( cardia ). The gastric mucosa , which lines the inner wall of the stomach, is heavily folded and interspersed with numerous gland cells. These cells can be divided into three types: secondary cells , main cells and parietal cells . Hydrochloric acid is produced by the parietal cells . After half an hour to an hour, this has acidified the entire contents of the stomach. The main cells secrete the inactive enzyme pepsinogen , which is activated to pepsin by the hydrochloric acid . The pepsin breaks down proteins into smaller peptides , which are later broken down further. Since pepsin can also convert collagen - the main component of connective tissue - and the pH value in the stomach is around 0.9 due to the hydrochloric acid, it is necessary to protect the gastric mucosa in particular. A tough mucus, rich in hydrogen carbonate , is therefore constantly secreted in the adjoining cells , which covers the gastric mucous membrane and forms a buffer for local neutralization of gastric acid. Another protein- splitting substance is cathepsin . These enzymes and other substances are contained in gastric juice, of which 1.5–2 liters are produced daily. In addition, the intrinsic factor is formed in the parietal cells of the stomach , which is important for vitamin B 12 absorption in the ileum . The peristalsis prevailing in the stomach mixes the chyme and pushes it through the porter (M. sphincter pylori), if the latter is relaxed, into the duodenum .
In the case of carbohydrates , it is directly linked to digestion by the saliva in the mouth. By Ptyalin is strength to oligosaccharides and maltose processed. These are broken down into their individual components ( glucose , fructose , galactose and mannose ) by the enzymes lactase , sucrase and maltase , which can then be absorbed into the cells of the intestinal mucosa through an Na + -glucose symport .
The also pre-digested proteins, which are now only available as peptides (poly-, di- and tripeptides), are broken down into amino acids by peptidases (mainly trypsin , chymotrypsin and carboxypeptidases ). They are taken up into the cell through a Na + amino acid symport. 90% of the di- and tri-peptides are transported directly into the cell via special transport mechanisms and there are broken down into amino acids by a cytoplasmic peptidase.
The not yet digested fats (lipids) are present as fat droplets. These are first reduced to a fat emulsion using lecithin and bile acid (primary micelles ). Then they are broken down into free fatty acids and 2-monoglyceride by the enzymes pancreatic lipase and bile salt-activated lipase . The salts of the bile acid now form so-called secondary micelles with the fatty acids, in which the 2-monoglyceride is enclosed. These passively diffuse into the intestinal mucosa, since the cell membranes of the intestinal mucosa are lipophilic . The salts of the bile acid remain, 90% of which are later taken up in the ileum.
Finally, about 80% of the water ingested with food and given off by the body through secretion is withdrawn from the chyme. That's about 9 liters of water every day, 2 liters from food and 7 liters from digestive secretions. This happens in connection with the absorption of salts from the chyme. These passively diffuse, following the concentration gradient, into the intercellular spaces. Potassium channels and sodium-potassium pump in the cell membrane to ensure a uniform concentration of sodium - and potassium - ions in the cell. The water follows the osmotic pressure created by the salts and diffuses from there into the bloodstream.
As a result, the nutrient components are absorbed in the small intestine. After the nutrients (carbohydrates, fats and proteins) have been broken down into their individual components, these are absorbed (transferred) into the blood and lymph via the small intestine villi. This process provides most of the energy-rich substances for our organism.
In the large intestine (intestinum crassum, colon) another 19% of the water still present in the chyme is absorbed. Substances that can neither be fermented by digestive enzymes in the small intestine nor by the microorganisms in the large intestine are excreted unchanged through the rectum.
The pressure in a human rectum is between 55 Torr (at rest) and over 100 Torr (extreme pressing). Significantly higher pressures were found in penguins; pressures of up to half an atmosphere (450 Torr) ensure that bowel movements are accelerated below the polar lowest temperatures .
The entire digestive process takes different lengths of time depending on the type of food consumed; the times vary due to the composition of the food (e.g. the proportion of carbohydrates and fats).
|Length of stay|
|esophagus||1-10 seconds||approx. 10 seconds|
|stomach||0.5-6 hours||4-6 hours|
|Small intestine||7–9 hours||about 6 hours|
|Large intestine||25-30 hours||6–8 hours|
|Rectum||30-120 hours||about 6 hours|
During sleep, digestion is particularly supported by the somatotropin hormone system , which leads to a faster absorption of nutrients.
Digestion in animals
"Digestion" of feelings and problems
Many idioms associate the "processing" of feelings, stress and problems with the digestive system, for example: "I chewed on it for a long time", "I have to swallow that first", "He eats everything in", "That lies (heavily) in my stomach ”,“ my stomach turns around ”, a problem“ hits my stomach ”,“ it pisses me off ”,“ I'm thoroughly fed up with that ”. Or also: "The bile is coming up", "Something went down his liver".
The fact that mental stress can have a disruptive effect on digestion is widely recognized in medicine (see Psychosomatics ). Together with irritable bowel syndrome , the irritable stomach is one of the most common digestive disorders in industrialized countries. Around 25 percent of the population have to deal with it at least once in a lifetime. Usually this diagnosis is made when no organic disorder can be found. For many people, stress affects the stomach, resulting in nausea, heartburn , diarrhea or constipation .
"Digestion" in alchemy
In the preparation of pharmaceutical and alchemical preparations, digestion (Latin digestio ) was understood to mean a slow transformation and mutual penetration of the corresponding ingredients when left standing in moderate heat.
"Digestion" in humoral physiology
In medieval physiology, based on ancient humoral pathology , the stomach was imagined as a kind of saucepan over the hot liver. Due to the heat generated by the liver, supplemented by the warmth of the heart, the solid food components should accordingly be converted into a beer-like juice through the digestio (digestion). A subdivision of this digestive performance in the sense of a three-stage scheme ( digestio prima , d. Secunda and d. Tertia ) can be found in Isaak ben Salomon Israeli .
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