The small intestine is in the gastrointestinal tract.
It isan organ between the stomach and the large intestine. It is approximately 23ftlong and is made up of three structural parts; the duodenum, jejenum and ileum.The small intestine is mainly involved in the digestion andabsorption of nutrients.
It receives pancreatic secretions andbile through the hepatopancreatic duct which also supports with its functions.Digestion is a process in which ingested food is chemicallybroken down into absorbable molecules. During absorption there is movement ofnutrients, water and electrolytes from the lumen of the small intestine intothe cell, then into the blood.CARBOHYDRATESDigestionThe three carbohydrate products which are absorbedby the small intestine include; glucose, galactose and fructose.Digestion of starch begins in the mouth, by salivaryamylase.
Much of carbohydrate digestion occurs in the stomach and duodenum. Themain enzyme is pancreatic amylase, which produces disaccharides fromstarch by digesting the 1-4 glycosidic bonds. The disaccharides produced areall converted to glucose by brush border enzymes.
The disaccharides that naturallyoccur in food do not require amylase to break them down. Brush border enzymes(lactase, sucrase, trehalase) hydrolyse these compounds into molecules ofglucose, galactose and fructose. AbsorptionGlucose and galactose are both absorbed across the apicalmembrane by secondary active transport (along with Na+) through theSodium-Glucose cotransporter (SGLT1). Both glucose and galactose exit the cellvia GLUT2 receptors across the basolateral membrane into the blood.Fructose enters the cell by facilitateddiffusion via GLUT5 and is transported into the blood via GLUT2receptors.PROTEINDigestionDigestion of protein begins in the stomach with the actionof pepsin, it breaks down protein into amino acids and oligopeptides.
The process of digestion is completed in the small intestine with brush borderand pancreatic enzymes. They split the oligopeptides into amino acids,dipeptides and tripeptides.AbsorptionAmino acids are absorbed through a Sodium cotransporter, ina similar mechanism to the monosaccharides. They are then transported acrossthe basolateral membrane through facilitated diffusion. Di andtripeptides are absorbed via separate H+ dependent cotransporters andonce inside the cell are hydrolysed to amino acids. LIPIDSDigestionLipids are hydrophobic in nature, and therefore are poorlysoluble in the aqueous environment of the digestive tract. Lipid digestion isstarted by lingual and gastric lipases, but this only digest 10% ofingested lipids.
The remaining lipids are digested in the small intestine.The fat goblets are emulsified by bile into smaller chunks, calledmicelles, which have a much larger surface area.Pancreatic lipase, phospholipaseA2 and cholesterol ester hydrolase (3 major enzymes involved inlipid digestion) hydrolyse the micelles, breaking them downinto fatty acids, monoglycerides, cholesterol and lysolecithin.AbsorptionThe products from digestion are released at the apicalmembrane and diffuse into the enterocyte. Inside the cell, the products are re-esterified toform the original lipids, triglycerides, cholesterol and phospholipids.
Thelipids are then packaged inside apoproteins to form a chylomicron.The chylomicrons are too large to enter circulation, so they enter lymphaticsystem via lacteals.