GI Secretion Fuctions, Digestion And Absorption - Lec15 Flashcards
What is the general composition of saliva? How does it change according to flow rate?
High potassium ion and bicarbonate concentrations
Low sodium and chloride ion concentrations
Hypotonicity
Presense of α-amylase, lingual lipase and kllikrein
Low flow rate - lowest osmolarity, lowest sodium and bicarbonate conc, highest potassium ion conc - normal secretion
Highest flow rate - compositon close to plasma - max secretions
The faster the flow rate the less adjusting the gland can do to the substances to be secreted.
Salivary gland secretion occurs in 2 stages, what is characteristic of the first?
Occurs in acini
Secretion contains ptyalin (α-amylase)
Composition is isotonic, ionic conc sim to plasma
What is characteristic of the second stage of salivary secretions?
Occurs in salivary ducts
Active reabsorption of sodium ions
Active secretion of potassium ions
Antive/passive secretion of bicarbonate ions
Passive reabsorption of chloride ions due to -70mv ducts
What factors increase saliva production?
Food in mouth - parasympathetic response
Smells
Conditioned reflexes
Nausea
Factors the decrease saliva production
Inhibition of parasympathetic system:
- sleep
- dehydration
- fear
- anticholinergic drugs
How is salivary secretion regulated?
Regulated mainly by the parasympathetic system, but the sympathetic system also has some control, resulting in increased saliva production
Parasympathetic receptors: muscarinic cholinergic recepters. Second messengers are IP3 and Ca++ increase
Sympathetic system receptor: β-adrenergic receptors with second messenger cAMP
What is the mechanism of parietal secretion of HCL into the gastric lumen? What about H+ ions?
Water and carbon dioxide are taken up from interstitial fluid by parietal cells. Water is dissassociated and with CO2 forms bicarbonate ion and hydrogen ion.
The bicarbonate ion is traded for Cl- from the interstitial fluid.
H+ is pumped out into canaliculi lumen by exchange with K
Cl- gets to the lumen by a cl- channel.
Parasympathetic stimulation increase H+ secretion
What’s the direct pathway of an increase in hydrogen ion secretion by parasympathetic stimulation?
By CN X innveration using acetylcholine and muscarinic M3 receptor
What’s the indirect pathway of an increase in hydrogen ion secretion by parasympathetic stimulation?
CN X innervation of G cells which stimulates gastrin secretion. Gastrin stimulates H+ secretion. Neurotransmitter is GRP (gastrin releasing peptide)
What effect does gastrin and histamine have of gastric secretions?
Gastrin is released in response to presence of protein in pylorus, and vagal stimulation. Gastrin causes enterochromaffin-like cells to stimulate H+ secretion.
Histamine stimulates H+ secretion by activating H2 receptors on parietal cells. (Receptor coupled to AC by Gs protein) second messsenger is cAMP
What is the effect of somatestatin and prostaglandins on gastric secretions and how?
Somatostatin inhibits gastric H2 secretion by both the direct and indirect pathways
Direct- by binding receptors on parietal cells
-antagonistic to histamine funciton
Indirect - inhibits release of histamine and gastrin
Prostaglandins - inhibits gastric H2 secretion by activation Gi protein and inhibits Adenyl Cyclase
What are the major enzymes secreted by the pancreas and their, basic, function?
Digestive enzymes for proteins
-trypsin, chymotrysin and carboxypolypeptidase
Digestive enzymes for carbohydrates
-pancreatic amylase
Digestive enzyme for fat
-pancreatic lipase, cholesterol esterase and phopholipase
How does the pancreas make the bicarbonate ion?
Pacreatic duct cell picks up CO2 and H2O from the interstitial fluid/blood and forms HCO3- and H+ using carbonic anhydrase. Bicarbonate ion is pumped in to the lumen by exchange for Cl-
How does acetylcholine, cholecystokinin and secretin regulate pancreatic secretions?
acetylcholine - (parasympathetic + enteric NS) released in response to H+, small peptides, AA, and fatty acids in duodenum.
cholecystokinin - stimulated by food in upper intestine - small peptides, amino acids and fatty acids. secreted by duodenal and upper jejunal mucosal I cell. causes increase in pancreatic secretion and potentiates effect of secretin and stimulates bicarbonate secretion. Second messengers - I3 and intracellular Ca2+
Secretin - stimulatted by acidic foods in upper intestine, secreted by duodenal and upper jejunal 5 mucosal cells and causes release of large amounts of sodium bicarbonate by ductal cells. 2nd messenger - cAMP
What factors stimulate the release of bile from the gall bladder?
Presence of fatty food in duodenum
How is bile formed?
- Cholic acid and chenodeoxycholic acid are synthesized from cholesterol
- Bile salt synthesis - by liver, amphipathic with hydrophilic and hydrophobic.
- conjugated with glycine or taurine, then addition of electrolytes
Secreted continuously by hepatocytes in the liver and stored in the gall bladder where it is concentrated by active transport of sodium and absorption of Cl-, water and other diffusible constituents.
Function - emulsification, form micelles for absorption across intestinal mucosa
How are gallstones formed?
Too much absorption of water from bile
Too much absorption of bile acids fro mbile
Too much cholesterol in ble
Inflammation of epithelium
So… basically the bile gets too concentrated in the gallbladder and crytalizes.
What is the structure of the crypt of Lieberkuhn?
Pits between the intestinal villi in the small intestine.
Made up of globlet cells (secrete mucous), enterocytes (which are absorptive, paneth cells and enteroendocrine cells (which secrete peptide hormones controlling several GI system functions)
What to Paneth cells secrete?
which secrete antimicrobial proteins to keep bacteria at bay
Define and compare condensation reactions and hydrolysis. What is the equation for the processes?
Condensation - removal of hydrogen and hydroxyl ions to combine monomeres into polymers
Hydrolysis - reverse - incorporating water molecules to breakdown polymers into monomers
Digestion of carbohydrates
Starch broken into maltose and glucose polymers
further conversion/breakdown into glucose monomers
Lactose gets converted into galactose, galactose into glucose
sucrose breaks into fructose (and glucose), fructose gets converted into glucose with a galatose intermediate
Digestion of fats
Fat is emulsified by bile and agitation
emulsified fat is converted to fatty acids and 2-monoglycerides by pancreatic lipase
Digestion of proteins
proteins broke down by pepsin into [proteoses, peptones, polypeptides] pancreatic enzymes (trypsin, chymotrypsin, carboxypolypeptidase and proelastase) into polypeptides and Amino Acids. further breakdown into all amino acids by peptidases.
How is glucose transported? What about other monosaccarides?
glucose and galactose:
secondary active transport by sodium-glucose co transport in luminal membrane
fructose diffuses in by concentration gradient
lactose requires lactase on brush border -> glucose and galactose
