Small Intestine Phase

Question 1

Why is the small intestine so important for digestion?

Answer 1

The small intestine is the most significant site for digestion and absorption of nutrients in the GI system.

Question 2

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

Answer 2

Intense motility aids in the break down of food, and a large surface area of mucosal cells allows for massive absorption of nutrients.

Question 3

What is the negative feedback that happens when food enters the duodenum?

Answer 3

As a meal enters the duodenum, it initiates feedback inhibition of gastric emptying - this allows for more digestion time in both compartments

Question 4

Vagal regulation of gastric emptying relies on what sensory information?

Answer 4

the pH and hyperosmotic content of chyme stimulates the vagal response to the gastric emptying

Question 5

What allows gastric emptying to start again?

Answer 5

Chyme moves further down the small intestine and the feedback inhibition is reduced

Question 6

What two major factors contribute to the inhibition/slowing of the gastric emptying.

Answer 6

1) The presence of fat in the duodenum

2) The presence of low pH in the duodenum

Question 7

What is the main stimulator of CCK release?

Answer 7

fatty acids in the duodenum

Question 8

What is the main effects of CCK?

Answer 8

Release of bile from the gallbladder and digestive enzymes from the pancreas

Question 9

What cells secrete CCK?

Answer 9

I-cells

Question 10

What are the two main component of pancreatic secretions into the duodenum?

Answer 10

HCO3- and digestive enzymes

Question 11

What important hormone for digestion is released by the endocrine pancreas?

Answer 11

Somatostatin (inhibitor of the G-cells of the stomach)

Question 12

Where do the enzymatic components of the pancreas come from?

Answer 12

The acinar cells

Question 13

Where does the aqueous component (HCO3-) of the pancreatic secretions come from?

Answer 13

The centroacinar cells

Question 14

Describe the role of secretin in the regulation of pH in the duodenum.

Answer 14

At low pH, S-cells (of the duodenum) are stimulated to secrete secretin. Secretin then stimulates the secretion of HCO3- from the centroacinar cells of the pancreas.

An increase in pH inhibits further release of secretin

Question 15

How does secretin cause the release of bicarbonate from duct cells?

Answer 15

Secretin, released from S-cells of the duodenum, binds to duct cells of the pancreas and stimulates cAMP production. This opens the CFTR Cl- channels and Cl- flows into the lumen of the pancreatic ducts.

This influx of Cl- drives the HCO3-/Cl- exchanger, and Cl- from the lumen is exchanged for HCO3-.

Question 16

Other than secretion of HCO3- into the pancreatic ducts, what is the other net effect of secretin release?

Answer 16

Acidification of the blood. H+ is exchanged for Na+ on the basolateral membrane- this leads to net H+ absorption by the blood.

Question 17

What is the problem with a defective CFTR channel, as in cystic fibrosis?

Answer 17

the bicarbonate secretory process is defective, as Cl- cannot leak out into the duct, and therefore Cl- cannot be exchanged for HCO3-.

This results in a decrease in a high concentration of precipitation of the acinar enzymes in the duct and destruction of the gland.

Question 18

What cell types/where is CCK released?

Answer 18

CCK is released from I cells located in the small intestinal epithelium

Question 19

What triggers the release of CCK?

Answer 19

Direct interaction with fatty acids, amino acids, CCK-releasing peptide (CCK-RP), or monitor peptide

Question 20

What causes the release of CCK-RP?

Answer 20

Fatty acids or amino acids

Question 21

Where is monitor peptide released from?

Answer 21

pancreatic acinar cells

Question 22

How does CCK stimulate pancreatic acinar secretion?

Answer 22

1) binds to the CCK1 receptor on the acinar cells
2) Stimulates neural reflexes that leads to the release of ACh, GRP and VIP by pancreatic enteric neurons

Both of these processes leads to the phosphorylation of structural and regulatory proteins that allows for the fusion of granules containing digestive enzymes with the apical membrane of the acinar cells and secretion into the pancreatic ducts.

Question 23

What is enterokinase?

Answer 23

Enterokinase (a brush border enzyme of the small intestine) cleaves trypsinogen –> trypsin (the active form)

Question 24

Where are the pancreatic digestive enzymes converted by trypsin into their active form?

Answer 24

The duodenum.

Question 25

What is trypsin?

Answer 25

Trypsin is the active form of trypsinogen.

Trypsin is the main enzyme responsible for cleaving pancreatic proenzymes into their active form.

Question 26

Throughout the entire digestion process, when are pancreatic secretions first stimulated/increased?

Answer 26

During the cephalic phase via vagal and enteric nerves

Question 27

What 4 factors increase pancreatic secretions during the intestinal phase?

Answer 27

Secretin release (from low pH)
CCK secretion (from peptides, AAs, fatty acids)
Enteropancreatic reflexes (from distention, hypertonicity)
Vagovagal reflexes (from CCK sensory enteric neurons)

Question 28

What is the major constituent of bile?

Answer 28

The majority of bile is made up of bile acids

Question 29

What is the purpose of bile acids?

Answer 29

Bile acids are detergents and form micelles around fat to prevent them from globbing together

Question 30

How is bile recycled?

Answer 30

Enterohepatic circulation: the majority of bile acids are recycled from the intestine back to the liver via the enterohepatic circulation.

Question 31

How are conjugated bile acids reabsorbed?

Answer 31

Conjugated bile acids are reabsorbed by a symporter known as the apical sodium dependent bile acid transporter (ASBT)

Question 32

Where do segmental contractions occur?

Answer 32

Small and large intestine

Question 33

Where does do peristaltic contractions occur?

Answer 33

pharynx, esophagus, gastric antrum, small and large intestine

Question 34

What two neurotransmitters are involved in orad contraction?

Answer 34

Ach and substance P

Question 35

What two neurotransmitters are involved in caudad relaxation?

Answer 35

VIP and NO

Question 36

What is the role of the hormone motilin?

Answer 36

Motilin is responsible for initiating periodic contractions that clear any remaining gastric or intestinal contents out into the colon. The movement that occurs is called the Migrating Motor Complex

Question 37

What is secreted from Brunner’s glands (small intestinal glands)

Answer 37

mucus and bicarbonate

Question 38

What is secreted from Lieberkuhn glands of the small intestine?

Answer 38

peptidases and enzymes that digest carbohydrates

Question 39

What is secreted from the Paneth cells of the small intestine?

Answer 39

antimicrobial peptides and enzymes