gastrointestinal tract L27-29 Flashcards
saliva functions
lubrication
digestion
solution
moisture
protection
3 types of salivary glands
parotid
sublingual
submandibular
*1.5L secretion per day
salivary gland structure
acinus> primary secretion
duct> seconadry modification/ low H2O permeability
final saliva
isotonic HCO3 rich
salivation control
cephalic
parasympathetic
sympathetic
cephalic phase
unconditioned/ conditioned
sight/ thought/ smell/ taste of food
vagus efferent nerves direct effect on histamine/ gastrin release
parasympathetic salivary phase
cholinergic
cranial nerves VII / IX
(not vagus)
large volume
sympathetic salivary phase
smaller volume
rich in enzymes and mucus
gastric functions
reservoir
mixing food w gastric secretions
digestion
controlled gastric emptying
protection
gastric juice composition
HCl
pepsinogen> pepsin
mucus/ bicarbonate
intrinsic factor
(2L/day)
parietal cells
secrete intrinsic factor/ gastric acid
stimulated by histamine/ ACh/ gastrin
gastric acid
kills bacteria
activates pepsin
denatures protein
intrinsic factor
complexes w vitamin B12 to permit absorption
parietal cell secretion of HCl
carbonic anydrase catalyses H+ generation
apical membrane has proton pump/ K+ channels/ Cl- channels
basal membrane has Na+/K+ ATP-ase and Cl-/HCO3 exchange
chief cells
secrete pepsin and gastric lipase
stimulated by ACh/ acid/ secretin
pepsin
digests proteins
inactivated at pH >3.5 and denatured at pH >7.2
gastric lipase
digests fats
mucous neck cells/ surface cells
secrete mucus > stimulated by tonic secretion
secrete bicarbonate> stimulated by mucus
mucus in gastric juice
physical barrier between lumen and epithelium
bicarbonate in gastric juice
buffers gastric acid to prevent epithelium damage
autodigestion prevention
gastric mucosal protection
mucus-HCO3 barrier
parietal cell function control
direct/ indirect chemical regulators:
histamine from ECL cells
somatostatin from D cells
gastrin from G cells
ACh from vagus and enteric neurones
enterochromaffin-like cells
secrete histamine
stimulated by ACh/ gastrin
histamine in gastric juice
stimulates gastric acid secretion
D cells
secrete somatostatin
stimulated by acid in stomach
somatostatin in gastric juice
inhibit gastric acid
G cells
secrete gastrin
stimulated by ACh/ peptides/ amino acids
gastrin
stimulates gastric acid secretion
gastric phase
food in stomach
gastrin release
negative feedback inhibition by acid w somatostatin release
ampulla of vater
common entrance of bile duct and pancreatic duct into duodenum
pancreatic juice fluid and electrolytes
wash out enzymes
~pH 8 alkaline to neutralise gastric acid in duodenum
primary secretion from acinar cells
modified to HCO3- rich juice by duct cells
1.5L / day
pancreatic enzymes
amylolytic (amylase)
proteolytic (trypsin, chymotrypsin, elastase)
lipolytic (lipase, phospholipase)
nucleolytic (DNAase, RNAase)
inactive precursors (zymogens)
pancreatic proteases
secreted as zymogens
enterokinase activation in intestine
trypsin can auto-activate
endo/exoproteases
amino acid specificity
protein digestion intestinal peptidases
pancreatic HCO3- secretion
acinar cell primary secretion (isotonic NaCl secretion)
duct HCO3- secretion (isotonic via apical HCO-Cl exchange and CFTR channels)
cystic fibrosis
lack of CFTR channels and fluid secretion
lack of washout enzymes
pancreatic damage
poor nutrition
pancreatic secretion control
secretin > duodenal S cells
release stimulated by acid ^HCO3- secretion
CCK (cholecystokinin-pancreozymin) > duodenal I cell
release stimulated by fat/protein
gall bladder contraction/ pancreatic enzyme secretion stimulation
vagus nerve> stimulates acinar and duct cells
bile functions
elimination of waste products (cholesterol/ bile pigments/ minerals/ lipophilic drugs and metabolites/ heavy metals)
promotion of lipid digestion and absorption
bile
hepatocytes secrete bile into canaliculi
secrete HCO3- rich fluid
enters duodenum by common bile duct
stored and concentrated in gall bladder
bile composition
biliary lipids (bile acids/ phospholipids/ cholesterol)
bile pigments (bilirubin)
bile acids
primary> liver synthesis from cholesterol
secondary > modification by intestinal bacteria
conjugation> to amino acids/ ^solubility/
enterohepatic circulation of bile acids
synthesis of primary bile acids in liver (0.6g synthesised per day/ conjugated)
secretion into duodenum in bile
lipid digestion/ absorption
reabsorption in terminal ileum
recirculation through hepatic portal vein to liver
taken up by liver and secreted into bile
emulsification of dietary lipids
^sa exposed to lipases/ digestion promotion
bile salts coat lipids
gall bladder function
concentrates bile
absorbs electrolytes/ water
isotonic
Na+ w bile salts
biliary secretion control
bile acid- dependent
ductal > secretin
gall bladder contraction> cholecystokinin/ vagal nerves/ integrated w pancreatic enzyme secretion
gallstones
failure to keep cholesterol in solution
small intestine 3 sections
duodenum
ileum
jejunum
functions of small intestine
digestion
nutrient absorption
electrolyte absorption
water absorption
secretion
carb digestion/ absorption
luminal digestion of starch-type polysaccharides
membrane digestion by disaccharides
absorption of monosaccharides in small intestine
amylase
endo enzyme that can only break internal bonds
can’t produce monosaccharides
carb absorption
monosaccharides
apical membrane > SGLT1 (glucose/ galactose)- Na+ coupled, secondary active transport
GLUT5 (fructose) -facilitated diffusion
basolateral membrane (GLUT2- basolateral diffusion)
protein digestion and absorption
gastric/ pancreatic protease luminal digestion
brush-border peptidase membrane digestion
cytosolic digestion in epithelium of small peptides
small intestine absorption > amino acids/ peptides
antigenic quantities of intactv protein
protein digestion enzymes
gastric pepsin ~20%
pancreatic peptidases
membrane and cytosolic peptidases
peptide absorption
acid microclimate utilisation
followed by cytosolic peptide digestion/ export
peptide carrier for drugs (ACE inhibitors, beta lactam antibiotics, prodrugs)
amino acid absorption
neutral/ cationic/ anionic transporters
apical membrane, coupled to ion uptake/ secondary active transport/ epithelial accumulation
facilitated diffusion on basolateral membrane
lipid digestion and absorption steps
low water solubility
solubilisation
digestion
absorption
re-esterification of fatty acids
transport to blood
dietary lipids
triglycerides (triesters of glycerol and long-chain fatty acids)
phospholipids (
short chain fatty acids
lipid digestion stages
- emulsification (fat droplets stabilised by bile salts and phospholipids)
- digestion (gastric and pancreatic lipases/ co-lipases/ triglycerides to 2 fatty acids and monoglyceride)
- diffusion into epithelial cells
- re-synthesis of triglycerides
- release as chylomicrons
micelles
5nm polymolecular aggregates
monoglyceride/ fatty acids/ bile salts
cholesterol and fat-soluble vitamins
