GI Secretion Fuctions, Digestion And Absorption - Lec15

Question 1

What is the general composition of saliva? How does it change according to flow rate?

Answer 1

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.

Question 2

Salivary gland secretion occurs in 2 stages, what is characteristic of the first?

Answer 2

Occurs in acini
Secretion contains ptyalin (α-amylase)
Composition is isotonic, ionic conc sim to plasma

Question 3

What is characteristic of the second stage of salivary secretions?

Answer 3

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

Question 4

What factors increase saliva production?

Answer 4

Food in mouth - parasympathetic response

Smells

Conditioned reflexes

Nausea

Question 5

Factors the decrease saliva production

Answer 5

Inhibition of parasympathetic system:

• sleep
• dehydration
• fear
• anticholinergic drugs

Question 6

How is salivary secretion regulated?

Answer 6

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

Question 7

What is the mechanism of parietal secretion of HCL into the gastric lumen? What about H+ ions?

Answer 7

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

Question 8

What’s the direct pathway of an increase in hydrogen ion secretion by parasympathetic stimulation?

Answer 8

By CN X innveration using acetylcholine and muscarinic M3 receptor

Question 9

What’s the indirect pathway of an increase in hydrogen ion secretion by parasympathetic stimulation?

Answer 9

CN X innervation of G cells which stimulates gastrin secretion. Gastrin stimulates H+ secretion. Neurotransmitter is GRP (gastrin releasing peptide)

Question 10

What effect does gastrin and histamine have of gastric secretions?

Answer 10

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

Question 11

What is the effect of somatestatin and prostaglandins on gastric secretions and how?

Answer 11

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

Question 12

What are the major enzymes secreted by the pancreas and their, basic, function?

Answer 12

Digestive enzymes for proteins
-trypsin, chymotrysin and carboxypolypeptidase

Digestive enzymes for carbohydrates
-pancreatic amylase

Digestive enzyme for fat
-pancreatic lipase, cholesterol esterase and phopholipase

Question 13

How does the pancreas make the bicarbonate ion?

Answer 13

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-

Question 14

How does acetylcholine, cholecystokinin and secretin regulate pancreatic secretions?

Answer 14

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

Question 15

What factors stimulate the release of bile from the gall bladder?

Answer 15

Presence of fatty food in duodenum

Question 16

How is bile formed?

Answer 16

• 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

Question 17

How are gallstones formed?

Answer 17

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.

Question 18

What is the structure of the crypt of Lieberkuhn?

Answer 18

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)

Question 19

What to Paneth cells secrete?

Answer 19

which secrete antimicrobial proteins to keep bacteria at bay

Question 20

Define and compare condensation reactions and hydrolysis. What is the equation for the processes?

Answer 20

Condensation - removal of hydrogen and hydroxyl ions to combine monomeres into polymers

Hydrolysis - reverse - incorporating water molecules to breakdown polymers into monomers

Question 21

Digestion of carbohydrates

Answer 21

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

Question 22

Digestion of fats

Answer 22

Fat is emulsified by bile and agitation

emulsified fat is converted to fatty acids and 2-monoglycerides by pancreatic lipase

Question 23

Digestion of proteins

Answer 23

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.

Question 24

How is glucose transported? What about other monosaccarides?

Answer 24

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

Question 25

How is glucose absorbed?

Answer 25

secondary active transport

Question 26

How are peptides and amino acids absorbed?

Answer 26

sodiium-amino acid co-transport into enterocyte

faciliatate transport from enterocyte to blood
transportor types- neutral, acidic, basic and imino

hydrogen dipeptides and tripeptides can be co-transported as well

Question 27

How are fats absorbed?

Answer 27

CCK - slows gastric emptying

bile acids - emulsify lipids

pancreatic lipases hydrolyze lipids to fatty acids, monoglycerides, cholesterol and lysolecithin

(pancreatic lipase, cholesterol ester hydrolase and phospholipase A2)

forms micelles -> micelles fuse with cell and empty contents into enterocytes.