Control of Digestion Flashcards
regions of the small intestine
- duodenum : digestion
- jejunum : absorption
- ileum : absorption
structure of the small intestine
mucosa
submuscosa
muscularis externa
serosa
has villi at the surface (increase SA)
–> where absorption/digestion occur –> secretion by endocrine cells in crypts
enterocytes occur on the villi and they transport nutrients and ions + goblet cells on the villi secrete mucus
microvilli (brush border) on enterocytes incease SA for digestion and absorption
GIP and GLP-1
- glucose-dependent insulinotropic
polypeptide/gastric inhibitory peptide (GIP) - glucagon-like peptide-1 (GLP-1)
–> carbs in chyme triggers the release of the two hormones (incretins = potentiate insulin secretion)
–> release from duodenal K and L cells
* inhibit acid production and decreases gastric motility
==> signal to stomach there already is chyme in SI
==> ensure duodenum has enough time to digest chyme
* stimulates pancreatic insulin secretion due to carbs
secretin
chemoreceptors senses the acidic chyme to release secretin
–> release from S cells in duodenum
* inhibits acid
production and decreases gastric motility
* triggers bicarbonate release from pancreas (duct cells) to neutralise acid
==> secretin binds to receptors on duct cells to release bicarbonate in pancrease
Pancreatic duct cells secrete bicarbonate
exocrine pancrease secretes digestive enzyme and sodium bicarbonate
via carbonic anhydrase (CA)
duct cells filled with cells secrete NaHCO3 that enters the digestive tract
–> similar to stomach except H+ is transported into the lumen in the stomach but HCO3- is transported in the lumen instead
= HCO3- transported into lumen => down pancreatic duct => duodenum => neutralise acid
(A small amount of bicarbonate can also be produced by duodenal cells, but the majority produced by pancreas)
CCK
cholecystokinin (CCK; hormone)
–> release from I cells in the duodenum
* inhibits acid production and decreases gastric motility
stimulates pancreatic acini cells in pancreas to secrete enzymes
stimultes gallbladder to contract and release bile (bile travels down ducts to duodenum)
Relaxes sphincter of Oddi = for contents of pancreatic juice + bile to enter duodenum
acini cells
filled with vesicels containing zymogens and + enzymes that are secreted in response to CCK
acini cells lead to channels which flow into pancreatic duct network
Activation of zymogesn
pancreatic duct released pancreatic secretions (include inactive zymogens) into lumen of small intestine (SI)
–> zymogens (also known as proenzymes) + trypsinogen
trypsinogen is activated into => trypsin by enterpeptidase (type of protease) in the brush border
trypsin now in active form activates other zymogens
=> zymogens (inactive forms) ==> Prevents autodigestion in pancreas
Motility in the small intestine
Some peristalsis occurs in the SI to propel food through the GIT
-> Segmental contractions (alternating sections contract and relax)
= mulitple regions contracting silmutaneously, mainly mix and not to propel
= no/little net forward movement
o Contracting segments: circular muscles contract, longitudinal muscles relax
o Responsible for mixing chyme with digestive enzymes, bicarbonate
o Assist with digestion and absorption
Digestion of Carbohydrates
glucose polymers (starch/glycogen)
1. digested by amylase (begins in the mouth. Salivary amylase is inactive in the stomach so no carb digestion in stomach) into disaccharides
==> Pancreatic amylase will continue digesting polysaccharides into disaccharides
2. disaccharides digested by enzymes
= Intestinal brush border enzymes (maltase, sucrase, lactase) will digest further into
monosaccharides
3. monosaccharides ready for absorption
Digestion of Proteins
Protein digestion starts in the stomach (pepsin) = endoprotease : clears internal peptide bonds
* In duodenum, proteins digested by trypsin, chymotrypsin and carboxypeptidase = released by pancreatic juice
o Trypsin and chymotrypsin are endopeptidases =.release mono/dipeptides
o Carboxypeptidase is an exopeptidase – releases peptide thats one residule shorter and a single amino acid from C-terminus (attacks ends or proteins)
Digestion of Fats – bile secretion
- The liver produces and secretes bile which facilitates lipid digestion
- Bile is comprised of bile salts (emulsify fats), cholesterol, pigments such as bilirubin (from haemaglobin degradation)
gall bladder stores and CONCENTRATES bile (removing H2O and dehydrating it) in response to CCK
bile travels down cystic duct and common bile duct (from gall ballder to lumen of SI) which joins pancreatic duct
1. bile salts emulsify fat into smaller fat droplets
2. colipase displaces bile salt and recruites lipase to access fats + phospholipase digests phosphlipids
3. sequential mixing plus action of lipase and colipase + phospholiase = micelles (monoglycerides + free FA forms these micelles structures)
–> trypsin activates procolipase => colipase and prophospholiase => phospholipase
