Structure and function of small intestine
The intestine is the last and more important part of the G.I. tract, where the final act of digestion,namely absorption of nutrients into the blood stream, take place.The intestine is divided into small and large intestine, however the small intestine is the only part where digestion and absorption take place.
It's divided into 3 parts - duodenum, jejunum and ileum- that to the naked eyes bear no difference.Only when under the microscope the different structures such a duodenal glands in the duodenum, plicae circulares in the jejinum and Payer's patches in the ileum become apparent.
After the stomach turns chewed food into chyme the next step is to turn chyme into something that can pass into the blood stream and be transported to any cell that needs it. To achieve this feat the small intestine needs a helpful hand from the more sophisticate chemical factory in the human body, the pancreas.
As soon as the chyme starts to trickle into the small intestine the proteins it contains stimulates the pancreas into releasing CCK, Secretine, Pancreatic Lipase and Pancreatic Amylase.
CCK role is to stimulate the pancreas into producing enzymatic juices, open the sphincter of oddi so that those juices can enter the duodenum, slow stomach emptying and stimulate contractions of gallbladder so that bile can be push into the duodenum and make the absorption of food into blood stream possible.
Secretine's actions meanwhile tells the pancreas to produce alkaline rich juices and at the same time tells the paraietal cells in the stomach to stop producing HCL, in no time the acidity of chyme is reduced to levels that the duodenum can tolerate.
Pancreatic Lipase will attach fats after bile from the gallbladder has emulsified them and it will cut off 2 fatty acid from the original triglicerides(TG's) leaving only monoglicerides(MG's) and fatty acids(FG's).
Pancreatic Amylase will take care of carbs digestion and will reduce them into disaccarides.
While all this happens the small intestin is not just sitting around. Duodenal glands will start producing Enterocrinin which start circulating in the blood and tells the small intestine to start producing Succus Entericus.
Succus Entericus is composed by sucrase,maltase and lactase plus erepsin and nuclease. Sucrase, maltase and lactase will cut disaccarides into monosaccarides while erepsin and nuclease will cut proteins into amino acids.
After carbs, lipids and proteins are cut to the smallest possible sizes they will be absorbed into the blood stream. However while amino acids and monosaccarides are small enough to pass directly into the blood stream MG's and FG's are still way too big to get there. To pass the cellular barrier of the intestin the bile salts will have to mechanically push them inside the cellular walls where they recombine into TG's (intracellular TG's) and enter the lymphatic system which will then transport them into the blood stream via the left subclavian.
After digestion is completed and all the nutrients are passed into the blood stream they will reach the cells and upon entering them, metabolic reactions will extract H+ (hydrogen ions) that will eventually power the human machine.