Denaturation by the pepsin enzyme and hydrochloric acid in the stomach
Initially stops in the duodenum due to nearly neutral ph value; Admixture by the enzymes trypsinogen and chymotrysinogen (from the pancreas). These enzymes are activated by enteropeptidase in the duodenum and split up polypetides into short-chain peptides.
Carboxypeptidase and aminipeptidase (from the pancreas) break these down further into di and tripeptides and into individual amino acids.
Di and tripeptides are rapidly absorbed in the jejunum, and there are active transport systems for the individual amino acids.
Enzymatic splitting of polysaccharides in the mouth – with the help of alpha-amylase – into polysaccharide fragments (dextrins).
Carbohydrate digestion stops in the stomach.
In the duodenum alpha-amylase (from the pancreas) is added again. Oligosaccharides and disaccharides are formed (maltose, lactose, sucrose). Villi of the jejunum (microvilli) add the enzymes maltase, lactase and sucrase for the splitting into the monosaccharides glucose, galactose and fructose. These diffuse into the bloodstream and are transported to the liver.
90% triglycerides, 10% phospholipids, cholesterol and fat-soluble vitamins E, D, K, A
Initial / pre-splitting of the triglycerides in the stomach with the help of the lipase enzyme.
The largest and final part takes place in the jejunum, after bile acids and pancreatic juices have been added in the duodenum. The pancreatic lipase enzyme splits tryglycerides into monoglycerides and free fatty acids. Additionally, pancreatic enzymes partly split the cholesterol and fatty acid compounds as well as of the phospholipids.
Monoglycerides, fatty acids, cholesterol, phospholipids and fat-soluble vitamins (E, D, K, A) are, under the influence of bile acids, deposited on the villi in the form of “tiny droplets” (micelles).
Uptake of fats and their split-up fragments predominantly takes place in the duodenum and at the beginning of the jejunum; short and medium chain fatty acids are transported to the liver by diffusing into the portal vein system. Long chain fatty acids or large fat molecules in the epithelial cells are surrounded by a protein coat ( chylomicrons). Lymphatic vessels of the villi pass them on via larger lymphatic vessels and the thoracic duct, past the liver, into the bloodstream and they increase blood fat levels.
The electrolytes (Na, K, Mg, Cl) found in the small intestine mainly come from the digestive juices added in the intestine and only a small part comes from food and drink
Are predominantly reabsorbed in the jejunum (partly actively, partly passively)
Water passively follows the reabsorbed electrolytes
Fat-soluble vitamins (E, D, K, A) are only absorbed via the formation of micelles when other fats are present
Water-soluble vitamins are absorbed in the jejunum via passive diffusion; Vitamin B12 can, however, only be absorbed in the (terminal) ileum with the intrinsic factor enzyme produced in the stomach.