GI: Intestinal Phase

Question 1

List some functions of the intestinal phase.

Answer 1

1. secretion of biliary and pancreatic juices
2. digestion and absorption
3. large surface area
4. control of the pylorus to regulate gastric emptying
5. inhibits gastric secretion

Question 2

What motility things need to occur to allow for gastric emptying?

Answer 2

1. increased pressure in the proximal stomach
2. increased contractions of the antrum
3. relaxed pyloric sphincter
4. relaxed duodenum

Question 3

Describe feedback inhibition of gastric emptying.

Answer 3

1. vagal afferents detect the presence of chyme and nutrients in the duodenum
2. vagal efferents slow down gastric contractions and close up the pyloric sphincter

Question 4

Describe the effect of fat in the duodenum on gastric emptying.

Answer 4

1. fatty acids stimulate the secretion of CCK
2. CCK inhibits gastric emptying and closes the pylorus to allow for proper digestion and absorption
3. CCK allows pancreatic and biliary secretion release to help in fat digestion and absorption

Question 5

Describe the effect of duodenal acidity on gastric emptying.

Answer 5

1. proton receptors on the duodenal mucosa sense low pH of duodenal contents
2. signal is sent through the myenteric plexus to the gastric smooth muscle
3. slows down gastric contractions to allow time for pancreatic HCO3 secretions to neutralize acid in the duodenum

Question 6

What factors contribute to the inhibition of gastric emptying?

Answer 6

• fat in the duodenum
• acid in the duodenum
• chyme in the duodenum

Question 7

What is the largest contribution to enzymatic digestion?

Answer 7

exocrine pancreatic secretions (1L/day)

Question 8

List the components of exocrine pancreatic secretions.

Answer 8

HCO3, aqueous solution

enzymes for macromolecular digestion

Question 9

What is the purpose of HCO3 in the pancreatic secretions?

Answer 9

• neutralizes incoming acidic chyme
• allows for correct pH for pancreatic enzymes
• reduces risk for mucosal damage from the pepsins entering via chyme

Question 10

What glands make up the endocrine pancreas? What do these glands secrete?

Answer 10

• islet of Langerhans’

- insulin, glucagon, somatostatin, pancreatic polypeptides

Question 11

Describe the structure of endocrine pancreatic glands.

Answer 11

1. acinus - blind end that secretes enzymes; made of acinar cells
2. ductules - modify the secretions, adding HCO3 aqueous

Question 12

What stimulates pancreatic gland secretion (specifically, each part of the gland)?

Answer 12

1. acinus is stimulated by CCK

2. ductules are stimulated by secretin

Question 13

Describe the cells that line the pancreatic glands.

Answer 13

1. acinus = acinar cells

2. ductules = centroacinar cells, ductal cells

Question 14

Describe the effect of decreased pH on the pancreatic secretions.

Answer 14

• activate S cells=> secretin
• stimulates ductule cells to release HCO3
• negative feedback once lumen pH reaches normal levels to stop secretin secretion

Question 15

How does secretin work?

Answer 15

1. increases cAMP in ductal cells
2. opens CFTR Cl- channels
3. ductal cells release Cl- into the ductal lumen
4. stimulates Cl-/HCO3- antiporter => secretion of HCO3-, reuptake of Cl-

Question 16

Where does HCO3 in pancreatic secretions come from?

Answer 16

1. Na-HCO3 co-transporter Type 1 (NBC-1) brings Na and HCO3 into the ductal cells (basolateral membrane)
2. Carbonic Anhydrase

Question 17

Why does pancreatic venous blood get acidified?

Answer 17

Because H+ are produced from carbonic anhydrase and are transported to the blood via Na/H exchangers on the basolateral membrane

Question 18

How does Cystic Fibrosis affect pancreatic secretions?

Answer 18

• mutated CFTR Cl- channel prevents secretion of Cl-
• secondary prevention of HCO3- secretion
• highly acidic contents in the glands leads to destruction of exocrine glands
• impaired pancreatic function

Question 19

List the triggers of CCK release.

Answer 19

1. fatty acids and amino acids bind to I cells
2. fatty acids and amino acids bind to sensor paracrine cells that release CCK-Releasing Peptide
3. Monitor Peptide released by acinar cells binds to I cells

Question 20

Where is CCK released from?

Answer 20

intestinal I cells

Question 21

What are the effects of CCK?

Answer 21

1. binds to acinar cell CCK-1 receptor => increase intracellular Ca2+ => stimulates secretion of enzymes
2. neural: stimulates vagovagal reflex => stimulates secretion of ACh, GRP, and VIP from ENS

Question 22

Describe the mechanism of acinar enzymatic secretion.

Answer 22

1. CCK, ACh, and GRP => increase intracellular calcium
2. VIP and secretin => increase cAMP
3. phosphorylation cascade
4. zymogen granules fuse with acinar membrane
5. exocytosis
6. ductal secretions wash away enzymes out of the gland

Question 23

How do pancreatic proteases get activated?

Answer 23

enterokinase activates trypsinogen to trypsin

trypsin then activates all the other proteases

Question 24

What causes pancreatitis?

Answer 24

premature activation of proteases or trypsin is resistant to degradation

Question 25

How does distension and hypertonicity effect pancreatic function?

Answer 25

via enteropancreatic reflexes | increases secretion

Question 26

In general, what is the pathway of bile?

Answer 26

hepatocytes => bile canaliculi => bile duct => R/L hepatic duct => common bile duct => gallbladder => sphincter of Oddi

Question 27

List the components of bile.

Answer 27

- bile acids - phospholipids - electrolytes - proteins - cholesterol - bile pigments (bilirubin)

Question 28

Describe enterohepatic circulation of bile acids.

Answer 28

liver => duodenum 1. if unconjugated, can get passively reabsorbed 2. if conjugated, gets actively reabsorbed in the terminal ileum via ASBT (Na-dependent Bile acid Transporter) => portal vein

Question 29

What is the purpose of motility in the small intestines?

Answer 29

- more digestion - increased biochemical digestion by exposing to more enzymes - propels the bolus to the epithelium for absorption

Question 30

Describe the migrating motor complex.

Answer 30

- motilin hormone causes periodic contractions every 90 minutes - clears the small intestine of its contents by opening pylorus and ileocecal sphincters - motilin decreases after a meal (to allow for proper gastric emptying), increased during fasting

Question 31

Define Brunner's glands.

Answer 31

- located in duodenum | - secrete mucus and HCO3

Question 32

Define Lieberkuhn glands.

Answer 32

- secrete peptidases and amylases

Question 33

Define Paneth cells.

Answer 33

- located at the bottom of villi - secrete antimicrobial peptides and enzymes - ribonucleases, esterases