GI tract absorption Flashcards
What are the 3 main sections of the small intestine ?
- duodenum
- jejunum
- ileum
Describe the structure of the small intestine
- it has circular folds which increase the surface area and allow the spiralling of chyme (this allows chyme to come into contact with membrane bound enzymes)
- it contains lymph nodules known as Peyer’s patches
Describe the structure and function of the duodenum
- 20 - 25 cm long
- absorbs 9 litres of water per day
Describe the structure and function of the jejunum
- 1 m long
- absorbs 3 - 4 litres of water per day
Describe the structure and function of the ileum
- 2 - 3 m long
- absorbs 1 litre of water per day
What are the main cell types found in the small intestine ?
- enterocytes
- goblet cells
- paneth cells
- I cells
- S cells
- D cells
What is the structure and function of enterocytes ?
- columnar epithelial cells
- have microvilli on their ends which creates a brush border
What is the function of goblet cells ?
they secrete mucus to reduce friction
What is the function of paneth cells ?
they have an immunological function and produce lysozymes
What is the function of I cells ?
release CCK
What is the function of S cells ?
release secretin
What is the function of D cells ?
release somatostatin
Describe the structure of intestinal villi
- they have microvilli creating a brush border
- contain lacteals which are involved in the absorption of fats
- there is a capillary network which has a high blood flow and a countercurrent flow which maximises absorption
- found over intestinal crypts
- brush border enzymes are anchored on the apical membrane of enterocytes
Describe the state of the food in the duodenum
- carbohydrates have been partly converted by salivary amylase
- proteins have been hydrolysed by pepsin
- fats are now mostly in the form of triglycerides
How is food release from the stomach regulated ?
- fats have an important role in the regulation of food release from the stomach
- they increase the motility of the stomach
- fats are the last to leave the stomach
Describe the digestion of carbohydrates
- carbohydrates are hydrolysed by salivary and pancreatic amylase into disaccharides
- brush border enzymes hydrolyse the disaccharides into monosaccharides
Which brush border enzymes are involved in the digestion of disaccharides ?
- maltase hydrolyses maltose into 2 glucose molecules
- lactase hydrolyses lactose into glucose and galactose
- sucrase hydrolyses sucrose into glucose and fructose
Describe the effects of a mutation in the lactase enzyme
a mutation in the lactase enzyme means there will be no production of glucose and galactose and so the person will be lactose intolerant
Describe how glucose and galactose is absorbed
1) glucose and galactose will enter the enterocyte via SGLT1
2) Na+ will enter simultaneously
3) the Na+ concentration is maintained via Na+/K+ ATPase
4) a high glucose concentration will result in the insertion of GLUT 2 into the brush border membrane
5) GLUT 2 is also present on the basolateral surface
6) glucose will diffuse into the blood
Describe how fructose is absorbed
1) fructose will enter the enterocyte via a GLUT 5 carrier - this is a Na+ independent transporter
2) a high glucose concentration will result in the insertion of GLUT 2 into the brush border membrane and fructose can also enter the enterocyte via GLUT 2
Which enzymes are involved in the hydrolysis of proteins ?
- pancreatic enzymes : trypsin, chymotrypsin and carboxypeptidase
- brush border enzymes : aminopeptidases
- stomach : pepsin
Describe how proteins are digested
- pancreatic enzymes break down proteins into small peptides
- aminopeptidases break down proteins into amino acids
Describe how proteins are absorbed
1) small peptides enter enterocytes via a H+ dependent carrier
2) amino acids enter via a Na+ dependent carrier
3) small peptides are then hydrolysed into amino acids by intracellular peptidases
4) amino acids exit the cells via imports and Na+ linked antiports
5) they diffuse into the blood
Describe how fats are digested and absorbed
1) fats form large fat globules
2) they get coated with bile salts to break them into smaller globules
3) phospholipids bind to smaller globules
4) this forms an emulsion
5) pancreatic lipase will act on this emulsion and form micelles
6) the monoglycerides and fatty acids within the micelles will enter the enterocytes via diffusion
7) the smooth endoplasmic reticulum will resynthesise the fats
8) this forms chylomicrons which diffuse into lacteals
Describe the function of the liver
- largest organ in the abdomen
- it has a dual blood supply : oxygen rich through the hepatic artery and nutrient rich through the portal vein
- it secretes bile salts to emulsify fats
- 94% of bile slats are reabsorbed back into the liver through the portal vein
Describe the function of the gallbladder
- it is a reservoir for the storage of bile
- concentrates bile
- CCK will cause contraction of the gallbladder to release bile
How much bile is lost from the body each day ?
0.8g of bile is lost through faeces each day
How is bile regulated ?
- through the return of bile
- secretion of bicarbonate rich watery fluid enhanced by secretin
