10. Secretions of the pancreas and small intestine

Question 1

Pancreas:
Innervation?
Enzyme storage?

Answer 1

Innervation:
PS= From vagus stimulates secretion
Symp= Inhibits secretion

Pancreatic enzymes are stored in condensed zymogen granules until release

Question 2

What is the process of formation and release of pancreatic secretions?
Composition of pancreatic fluid?
Modification of ductal cells?

Answer 2

Enzymes released from acinar cells, passes centroacinar cells then follow out via ductal cells as an isotonic pancreatic secretion.

Pancreatic fluid is an isotonic fluid containing Na+, K+Cl- and HCO-3

Ductal cells concentrate HCO-3 in fluid

Question 3

Enzymatic component of pancreatic secretions?

Answer 3

Pancreatic amylase and lipase are secreted as active enzymes

Pancreatic proteases are secreted in an inactive form and activated in duodenum (enterokinase)

Question 4

Regulation of pancreatic secretion, 3 phases?

Answer 4

Cephalic phase initiated by taste, smell and conditioning mediated by the vagus nerve ( mainly enzymatic secretion)

Gastric phase initiated by distension of the stomach and mediated by the vagus nerve ( mainly enzymatic secretion)

Intestinal phase accounts for 80% of pancreatic secretion and both enzymatic and aqueous secretions are stimulated

Question 5

How are acinar cells regulated?

Answer 5

Duodenal I cells secrete CCK in response to the presence of amino acids, small peptides and fatty acids in the intestinal lumen.
Vagal release of ACh potentiates CCK action

Question 6

How is ductal cell secretion regulated?

Answer 6

Ductal cells secrete Na+, K+Cl- and HCO-3

1. Arrival of acidic chyme in the duodenum triggers secretin release
2. S cells of duodenum secrete SECRETIN, stimulating HCO-3 release from ductal cells

ACh and CCK potentiate secretin action.

Question 7

Bile secretion:
Function?
Produced/stored?
Composition?

Answer 7

Bile is essential for the digestion and absorption of lipids (water insoluble)

Produced and secreted by the liver and stored in the gall bladder

Mixture of bile salts, bile pigments and cholesterol

Question 8

Function of bile salts?

Answer 8

Bile salts emulsify lipids to prepare them for digestion and solubilise the products of digestion into ‘packets’ called micelles

Question 9

3 functions of the gallbladder

Answer 9

1. Reservoir for bile
   Stores the bile which is continuously produced by the hepatocytes and flows to the gallbladder through the bile ducts
2. Concentration of bile
   Epithelial cells lining the gallbladder absorb ions and water isosmotically
3. Ejection of bile
   Begin 30 minutes after a meal. The major stimulus for ejection is the release of cholecystokinin from the I cells in the duodenum and jejunum.

Question 10

The IMA ends as the ____ ___ artery supplying the rectum and anal canal

Answer 10

The IMA ends as the superior rectal artery supplying the rectum and anal canal

Question 11

What are the different layers of the GIT? (Inner to outer)

Answer 11

Lumen
Epithelium
Muscularis mucosae
Submucosa
Inner circular muscle
Auerbach's myenteric plexus of ganglia and nerves
Outer longitudinal muscle

Question 12

What is hirchsprung disease?

Answer 12

Lack of normal development of the colonic innervation leads to a constricted, aganglonic segment proximally

Question 13

Rectum is ___peritoneal

Answer 13

Rectum is retroperitoneal

Question 14

What is cholecystokinin?

When/why/where is it secreted

Answer 14

A 33 amino acids peptide hormone related to gastrin

Secreted from the I cells of the duodenal and jejunal mucosa in response to the presence of monoglycerides, fatty acids, small peptides and amino acids

Question 15

5 hormonal action of cholecystokinin

Answer 15

1. Contraction of the gall bladder and relaxation of the
   sphincter of Oddi to eject bile (emulsification and solubilisation of dietary fat)
2. Secretion of pancreatic enzymes (both lipases and proteases)
3. Secretion of pancreatic HCO3
4. Growth of exocrine pancreas and gall bladder
5. Inhibition of gastric emptying

Question 16

Control of the biliary system:

1. ______ synthesise and secrete the constituents of bile (bile salts, cholesterol, phospholipids, bile pigments, ions and water)
2. Bile flows from liver in bile ____ and is stored and _____ in the gall bladder
3. Chyme in the small intestine triggers ___release:
   - stimulates gall bladder _____
   - stimulates relaxation of the Sphincter of ____
4. When lipid absorption is complete bile salts are _____ to the liver by the enterohepatic ___ circulation
5. Bile salts are extracted from the portal blood by _____ (20% loss per day)

Answer 16

1. Hepatocytes synthesise and secrete the constituents of bile
   (bile salts, cholesterol, phospholipids, bile pigments, ions and water)
2. Bile flows from liver in bile ducts and is stored and concentrated in the gall bladder
3. Chyme in the small intestine triggers CCK release stimulates gall bladder contraction
   stimulates relaxation of the Sphincter of Oddi
4. When lipid absorption is complete bile salts are recirculated to the liver by the enterohepatic portal circulation
5. Bile salts are extracted from the portal blood by hepatocytes (20% loss per day)

Question 17

What is an emulsion?

Answer 17

Lipid coated in bile salts

Question 18

What are mixed micelles?

Structure?

Answer 18

Products of lipid digestion (cholesterol, monoglycerides, lysolecithin and free fatty acids) and solubilised in mixed micelles.
Structure:
-The core contains the products of lipid digestion
-Surface coating of bile salts which are amphipathic

Question 19

What are the two pathways of absorption?

Answer 19

Cellular and paracellular

Question 20

Structure of the intestinal mucosa:

• Arrangement of SI surface?
• Structure of villi?
• Apical surface of epithelial cells covered by?

Answer 20

Surface of the small intestine isarranged in circular folds of Keckring

Villi project from the folds.
Surface of the villi are covered with epithelial cells (enterocytes) with mucus secreting cells (goblet cells)

Apical surface of epithelial cells covered by microvilli – brush border

Question 21

Carbohydrate:
Products?
Site of digestion?
Mechanism of digestion?
Limitation?
Begins in?

Answer 21

Products:

• Glucose
• Galactose
• Fructose

Site: SI

Mechanism:
Na+-dependant cotransport Na+-dependant cotransport
Facilitated diffusion\

Limitation: Only monosaccharides can be absorbed

Begins with salivary -amylase (minor role)

Question 22

Absorption of carbohydrate, 2 forms. Secondary active transport and facilitated diffusion. Explain

Answer 22

Secondary active transport
– SGLT 1 (sodium dependent glucose transporter 1) located on the apical membrane transports glucose and galactose

Facilitated diffusion
– GLUT 5 (glucose transporter 5) transports fructose across the apical membrane

Question 23

Protein digestion:
Begins in?
Completed in? How?

Answer 23

Begins in the stomach with pepsin

Completed in the small intestine with pancreatic and brush-border proteases
– Endopeptidases hydrolyse the interior peptide bonds of proteins
– Exopeptidases hydrolyse one amino acid at a time from

Question 24

Two proteins int eh SI that digest proteins?

Answer 24

1. ENDOPEPTIDASES hydrolyse the interior peptide bonds of proteins
2. EXOPEPTIDASES hydrolyse one amino acid at a time from

Question 25

Process of activation of protein digesting proteins

Answer 25

Pancreatic proteases secreted as inactive precursors (E.g. Trypsinogen)

Initial step- activation of trypsinogen to the active form, trypsin, by the brush-border enzyme enterokinase
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This trypsin catalyses the conversion of the other inactive precursors to active enzymes