Most of your meal is absorbed in a three-part tube
The small intestine sits between stomach and colon, taking bile and pancreatic juice while absorbing most nutrients. Duodenum, jejunum, and ileum divide the labor along a living length typically three to five meters, though measures range from about three to over ten meters depending on method.
Estimates of its length vary wildly with method and body size, from about 3 metres to 10.5, with around 3 to 5 metres typical in a living person. In adults it is roughly 2.5 to 3 cm wide, and radiologists treat anything above 3 cm on an X-ray as abnormally swollen. Food enters from the stomach past the pyloric sphincter into the first section, the duodenum, a C-shaped loop of 20 to 25 cm curled around the head of the pancreas.
The duodenum receives acidic chyme along with pancreatic enzymes and bile, released in response to the hormone cholecystokinin. Pancreatic trypsin and chymotrypsin chop proteins into peptides, which other enzymes trim to amino acids. Bile salts emulsify fats so pancreatic lipase can split triglycerides into fatty acids and monoglycerides, and pancreatic amylase breaks starch into short chains that enzymes on the gut lining finish off. Some carbohydrates escape digestion entirely and pass on to bacteria in the large intestine.
The jejunum, about 2.5 metres long, handles most absorption. Circular folds and finger-like villi multiply its surface, and each villus holds capillaries that take up sugars and amino acids and a lymph vessel called a lacteal that collects fats. Folate is absorbed higher up, while the ileum, around 3 metres long, specialises in bile salts and vitamin B12, which it will take up only after the vitamin binds a protein called intrinsic factor. Water crosses by osmosis along the whole length, and fructose by facilitated diffusion. The ileum then empties into the cecum of the large intestine.
The gut also guards the body. Peyer's patches in the ileum sample antigens from passing microbes and present them to the immune system. In the embryo the intestine grows so fast from the fifth week that it bulges out through the umbilicus before retreating. Around 70 percent of the roughly 20,000 human protein-coding genes are active in the normal duodenum. Blood arrives through the celiac trunk and superior mesenteric artery, and the jejunum and ileum hang from the mesentery.
Source: Small intestine
