Pancreatic Exocrine Secretions and It's Control

Question 1

Outline the flow of acini secretions

Answer 1

Acini form sacs → connect to ductal tree → enter duodenum at main common bile duct

Question 2

What is the route taken by secretions through the ductal tree?

Answer 2

Intercalated ducts→Intralobular ducts→Extralobular ducts →interlobular ducts → common bile duct →duodenum

Question 3

What are the Islets of Langerhans?

Answer 3

Clusters of endocrine cells in the pancreas

Question 4

Where are islets of Langerhans located?

Answer 4

Found throughout the pancreas, mainly in the tail of the pancreas

Question 5

What is the role of islets of Langerhans?

Answer 5

Secrete the following -
Insulin : 65% ꞵ cells
Glucagon : 20% ꭤ cells
Somatostatin : 10% δ cells

Question 6

What are acinar cells?

Answer 6

Berry-like secretory portion consisting of pyramidal epithelial cells that secrete digestive enzymes into the Pancreatic duct

Question 7

Where are microvilli located in acini?

Answer 7

Microvilli extend for the apical surface of epithelial cells into the acini lumen

Question 8

Describe the structure of Intercalated ducts

Answer 8

Squamous epithelium
Begin with acini
Smallest pancreatic duct members
Drain into Intralobular ducts

Question 9

Describe the structural features of Intralobular ducts

Answer 9

Lined with cuboidal or low columnar epithelium

Question 10

Describe the structure of interlobular ducts

Answer 10

Larger columnar epithelium

Goblet cells in a fibrocollagenous connective tissue

Question 11

How does the pancreatic duct accommodate pancreatic secretions?

Answer 11

Pancreatic duct wall composed of smooth muscle; as size of duct increases, the surrounding connective tissue also gets more dense

Question 12

Outline the flow of secretions through the exocrine pancreas

Answer 12

acini→microvilli→Intercalated→Intralobular→interlobular→Pancreatic duct

Question 13

How much fluid is secreted by the exocrine pancreas?

Answer 13

secretes 1.5 L per day

Question 14

What does the pancreatic juice consist of?

Answer 14

Na+ and HCO₃⁻-rich juice, albumin, globulin an digestive enzymes

Question 15

What is the role of acinar cells?

Answer 15

Acinar cells secrete digestive enzymes to hydrolyse carbohydrates, proteins, fats and nucleic acids

Question 16

How is auto-digestion of the pancreas prevented?

Answer 16

Enzymes are secreted in their inactive form to prevent auto-digestion (proenzymes)

Question 17

Where are the inactive enzymes activated?

Answer 17

In the duodenum

Question 18

Describe the concentration of solution secreted by the pancreas

Answer 18

is a HCO₃⁻-rich hypertonic juice

Question 19

What is the purpose of the alkaline pancreatic secretions?

Answer 19

HCO₃⁻-rich juice and secretions from gallbladder neutralise gastric acid (↑pH to 6 or 7)

Question 20

How is the fluid concentration regulated?

Answer 20

Composition of the fluid is modified as it travels through the ducts

Question 21

How is the fluid composition altered?

Answer 21

• Cl- actively exchanged for HCO₃⁻- by epithelial cells
• H+ actively eliminated; more CO₂ and HCO₃⁻ produced
  in blood
• H+ exchanged for K+ and Na+
  –> H+ neutralises HCO₃⁻ (forming H₂CO₃)

Question 22

How does a low secretion rate effect where its secreted from?

Answer 22

Low secretion rate = Most fluid secreted via Intralobular ducts

Question 23

What causes a high secretion rate?

Answer 23

Secretin

most additional fluid produced by Extralobular ducts

Question 24

What are the effects of Secretin on pancreatic juice?

Answer 24

composition and volume affected

reciprocal change in [Cl-] and [HCO₃⁻] after the infusion of Secretin
HCO₃⁻ increases, Cl- decreases

Question 25

What causes the decrease in amylase after Secretin infusion?

Answer 25

The fall in [amylase] probably reflects dilution effect as volume of pancreatic juice increases

Question 26

What are the 3 major types of enzymes secreted by the pancreas?

Answer 26

Proteolytic enzymes - trypsin - chymotrypsin - carboxypeptidases Amylase - starch→maltose Lipase

Question 27

What other enzymes are secreted by the pancreas?

Answer 27

Deoxyribonuclease | ribonuclease

Question 28

How can pancreatic juice be used to diagnose acute pancreatitis?

Answer 28

Level of pancreatic amylase is increased => good diagnostic test

Question 29

What is the role of proteolytic enzymes?

Answer 29

Cause digestion of proteins | => pancreas is at risk of auto digestion

Question 30

Explain the dual effects of Chymotrypsin C (Caldecrin)

Answer 30

At high [Ca2+] - enables activation of trypsinogen | At low [Ca2+] - facilitates trypsin degradation

Question 31

How long do the enzymes last within the duodenum?

Answer 31

Digestive enzymes survive different lengths of time in the small intestine

Question 32

What Percentage of enzymes reach the small intestine?

Answer 32

Amylase ; 75% Trypsin ; 20% Lipase ; 1%

Question 33

What evidence supports the fact there is bidirectional permeability at the basolateral membrane for digestive enzymes?

Answer 33

Chymotrypsinogen and amylase cross the basolateral membrane of the pancreas

Question 34

What controls the Pancreatic secretions?

Answer 34

- Neuroendocrine signals - Vagal (parasympathetic stimulation - Sympathetic stimulation - Secretin and CCK

Question 35

Explain how vagal innervation affects pancreatic secretions

Answer 35

Enhances secretion rate of enzymes and aqueous components of pancreatic juice

Question 36

What is the effect of sympathetic stimulation on the exocrine pancreas?

Answer 36

Inhibits secretion via decreased blood flow

Question 37

Explain the effects of CCK and Secretin

Answer 37

Stimulate the secretion of pancreatic juice

Question 38

What occurs during the cephalic phase of digestion?

Answer 38

Vagal (Ach and VIP) stimulation of gastrin release from antrum → some protein-rich pancreatic juice

Question 39

Explain the events of the Gastric digestion phase

Answer 39

Distension (vagal reflex on fundus or antrum), amino acid- and peptide- (in antrum)-stimulated gastrin secretion → release of enzyme-rich pancreatic juice

Question 40

What effect does a vagotomy have on the Gastric phase?

Answer 40

Vagotomy → 50%↓ in response to chyme

Question 41

What happens in the Intestinal phase?

Answer 41

Chyme in duodenum and jejunum induces secretion of pancreatic juice via CCK and Secretin

Question 42

Outline the effects of CCK

Answer 42

- released from duodenum & jejunum in response to FA - stimulates acinar cells to release pancreatic juice - stimulates secretion of conc. bile for fat absorption - contracts gallbladder - relaxes sphincter of Oddi - Potentiates effects of Secretin (weak acinar cell agonist)

Question 43

What are the effects of Secretin?

Answer 43

- released from duodenum and jejunum - induces pancreatic duct cells to secrete HCO₃⁻ rich pancreatic juice - decreased enzyme content - stimulates bile production

Question 44

How does CF affect the pancreatic ducts?

Answer 44

Abnormal mucus blocks pancreatic ducts affecting pancreatic secretions; carbohydrate, protein and lipid digestion will be affected

Question 45

What is the effect of obstructed pancreatic duct in CF patients?

Answer 45

Obstruction of the pancreatic duct by thick mucus contributes to GI symptoms in patients with cystic fibrosis (CF)

Question 46

What are the major GI symptoms of CF?

Answer 46

- Due to dysfunction of exocrine glands, sweat glands secrete excessive sodium and chloride - Pancreatic ducts become blocked with thickened mucus and this leads to pancreatic insufficiency - Lungs also produce thickened mucus

Question 47

What are the consequences of the GI symptoms in CF patients?

Answer 47

- Severe malabsorption - Steatorrhoea - Recurrent chest infections - Sterility in 98% of males

Question 48

How does CF cause sterility in males?

Answer 48

Sterility in 98% of males due to congenital bilateral aplasia of the vas deferens (CBAVD) – vas deferens fails to develop properly. Recall that normally the vas deferens carries the newly made sperm to the back of the prostate gland where it joins the outlets of the seminal vesicles. The sperm can then be released into the semen during intercourse