GI VII: Integrated Response to a Meal - Sm. Intest. Phase

Question 1

What are the mucosal folds on the inside of the submucosa called, and what is their purpose?

Answer 1

plica circularis - increase SA of small intestine to allow more time for digestion and absorption

Question 2

What is the basis for digestion/absorption in the small intestine?

Answer 2

• large SA of mucosal cells
• intense motility
• secretions in lumen (from gallbladder and pancreas)

Question 3

Is emptying of gastric contents into the duodenum fast or slow?

Answer 3

slow - takes about 3 hours

Question 4

Solids must be reduced to particles of what size to enter the duodenum?

Answer 4

less than or equal to 1 mm^3 (via retropulsion)

Question 5

Are duodenal segmental contractions inhibited or stimulated during gastric emptying?

Answer 5

Inhibited! Propulsive activity must be momentarily stopped in order to allow food to enter from the stomach.

Question 6

As a meal enters the duodenum, it initiates _______ of gastric emptying.

Answer 6

feedback inhibition

Question 7

2 major factors contribute to inhibition/slowing of gastric emptying…

Answer 7

1) presence of fat in duodenum

2) presence of H+ in duodenum

Question 8

What are the 2 main components of pancreatic secretions?

Answer 8

1) aqueous component (high in bicarb.)

2) enzymatic component

Question 9

When is CCK released, and what does it do?

Answer 9

It is secreted when fatty acids are present in the duodenum, and it slows gastric emptying / pyloric contraction to ensure there is adequate time for fat to be digested/absorbed.

Question 10

The effect of H+ in the duodenum is mediated by what?

Answer 10

reflexes in the enteric nervous system; H+ receptors in duodenal mucosa detect low pH and send info to gastric smooth muscle via interneurons in myenteric plexus; reflex ensures gastric contents are delivered slowly to duodenum to allow ample time for neutralization of acid by pancreatic bicarb

Question 11

What is the largest contributor to enzymatic digestion of a meal?

Answer 11

pancreatic secretions

Question 12

The pancreas consists 100% of which type of acinar cell?

Answer 12

serous acini (source of pancreatic juices)

Question 13

acinar cells are under the control of _____, while ductal cells are under the control of ______.

Answer 13

CCK; secretin

Question 14

In the endocrine pancreas, the islets of Langerhans secrete what?

Answer 14

insulin, glucagon, somatostatin, and pancreatic polypeptide

Question 15

What is the tonicity of pancreatic secretions?

Answer 15

isotonic

Question 16

The acinar cells secrete the _______ component of the exocrine pancreas, while the centroacinar and ductal cells secrete the ________ component.

Answer 16

enzymatic; aqueous

Question 17

When luminal pH is below ___, the ______ are triggered to release _______.

Answer 17

4.5; S cells; secretin

Question 18

What is the mechanism of action of secretin?

Answer 18

It increases cAMP in duct cells, which opens CFTR Cl- channels and allows outflow of Cl- into ductal lumen. This drives the antiporter that exchanges Cl- ions for bicarbonate. The net result is secretion of bicarb into the lumen, and absorption of H+ into the blood.

Question 19

When considering the mechanism of action of secretin, how is H+ transported into the blood?

Answer 19

It is transported by the Na+/H+ exchanger, NHE-1.

Question 20

Why is pancreatic function defective in cystic fibrosis?

Answer 20

CFTR is mutated, so bicarbonate secretory process is defective. This results in a decrease in pancreatic ductal secretion, as well as a high concentration and precipitation of acinar enzymes in the duct, ultimately destroying the gland.

Question 21

CCK release from I cells in small intestinal epithelium is triggered by which 3 things?

Answer 21

1) direct interaction of FAs or AAs with I cells
2) binding of FAs or AAs to sensor paracrine cells that release CCK-RP
3) release of monitor peptide by pancreatic acinar cells

Question 22

What is the secretin stimulation test?

Answer 22

It is a test designed to assess the digestive function of the pancreas. Secretin normally causes the pancreas to release a fluid containing digestive enzymes. In cases of chronic pancreatitis, cystic fibrosis, or pancreatic cancer, there may be a lack of digestive enzymes in the fluid that comes from the pancreas.

Question 23

What are the 2 ways in which CCK stimulates pancreatic acinar secretion?

Answer 23

1) as an endocrine factor that binds to acinar cell CCK-1 receptor
2) it stimulates neural reflexes that activate vagovagal reflexes leading to release of ACh, VIP, and GRP
(some substances act through cAMP pathway, others act through Ca2+ pathway to stimulate fusion of granules w/ apical membrane and discharge of pancreatic enzymes)

Question 24

Where are pancreatic proteases converted to their active forms, and what is this process initiated by?

Answer 24

in the lumen of duodenum, initiated by trypsin

Question 25

What is the enzyme responsible for converting trypsinogen to trypsin?

Answer 25

enterokinase

Question 26

What do pancreatic enzymes contain that prevent premature activation of pancreatic proteases?

Answer 26

trypsin inhibitors; however, some people have genetic mutations where trypsin is resistant to degradation, leading to pancreatitis

Question 27

What is the major constituent of bile?

Answer 27

bile acids (which are detergents and form micelles)

Question 28

When chyme reaches the terminal ileum and lipid absorption is complete, conjugated bile acids are reabsorbed by a symporter known as ________.

Answer 28

Na+ dependent bile transporter (asbt)

Question 29

Regarding peristaltic contraction, what is needed for constriction vs. relaxation?

Answer 29

ACh and Substance P for orad contraction; VIP and NO for caudad relaxation

Question 30

Where do segmentation contractions predominantly occur?

Answer 30

in the small and large intestine

Question 31

Describe the Migrating Motor Complex (MMC).

Answer 31

Periodic contractions in duodenum and jejunum, mediated by the hormone motilin. They occur at 90 minute intervals during fasting to clear out any remaining gastric/intestinal contents into the colon in preparation for the next meal.

Question 32

What are the structures in the small intestine that increase absorption area?

Answer 32

• simple columnar epithelium with brush border
• villi
• plica circularis in jejunum

Question 33

Where is the site of stem cells in stomach vs. small intestine?

Answer 33

• stomach: sit at mucus neck cells

- sm. intestine: sit at bottom of crypt

Question 34

What is the effect of H+ presence (low pH) in the duodenum?

Answer 34

It causes inhibition/slowing of gastric emptying, mediated via reflexes in the enteric nervous system. H+ receptors in duodenal mucosa detect low pH of chyme and relay to gastric smooth muscle via interneurons. This reflex ensures gastric contents are delivered slowly to duodenum so ample time is available for neutralization of acid by pancreatic bicarb.