35) Pancreatic exocrine secretions and its control Flashcards
1
Q
What are duct cells?
A
- The cells which line the lumen of the pancreas/ pancreatic duct and secrete HCO3- in response to particular hormones secreted by the duodenal cells
- It allows the acidic chyme to be neutralised
2
Q
What are islet cells?
A
- Cells found in the pancreas that are responsible for secreting hormones such as insulin
3
Q
What are acinar cells?
A
- Cells in the pancreas that synthesise and secrete digestive enzymes in the lumen of the pancreatic duct
- These enzymes are in their inactive form and breakdown carbohydrates, fats, proteins and nucleic acids
- They are found in circular structures with adjacent acini cells (called acinus)
- They form sacs which connect to the ductal tree and empty into the duodenum via the pancreatic duct
- The acini form the exocrine portion of the pancreas
4
Q
Describe the flow of material within the ductile tree.
A
- Intercalated ducts empty their contents into the intralobular ducts
- The intralobular ducts empty their contents into the extralobular ducts
- The extralobular ducts empty their contents into the interlobular ducts
- Finally the interlobular ducts empty their contents into the pancreatic duct
- The pancreatic duct is connected to the bile duct which eventually connects to the duodenum
5
Q
What is secretin’s role in the pancreas?
A
- Secretin is a hormone secreted by the duodenal cells and stimulates the duct cells of the pancreas
- This causes the duct cells to secrete HCO3- which in turn causes levels of Cl- to decrease
6
Q
How are the actions of the pancreatic cells controlled?
A
- Material within the duodenum that senses what the ductile cells and the acini cells need to do
- If there is acidic chyme secretin is secreted by the duodenal cells which travels in the blood to the pancreas and stimulates the ductile cells to secrete HCO3- (bicarbonate)
- If there are lipids within the duodenum CCK is released duodenal cells which promotes the synthesis and secretion of digestive enzymes
7
Q
What is the function of the microvilli found in the ductile system?
A
- They act as little tubes that collect the secretions that make their way into the duodenum
- They are found on the pancreatic duct epithelial cells
8
Q
What substances are found in the pancreatic juice?
A
- Enzymes are secreted in their inactive form and are activated in the duodenum
- They are secreted in their inactive form to prevent the auto-digestion of the pancreas
- In the pancreatic juice we also find cations (e.g. Na+, K+, Ca2+ and Mg2+) and anions (e.g, HCO3-, Cl-, SO42-, HPO42-)
9
Q
What are the alkaline secretions of the pancreas?
A
- HCO3- rich juice is secreted from the pancreas (all gallbladder) to help neutralise gastric acid
- Composition of the juice changes as it moves through the duct
- Epithelial cells actively exchange Cl- (into the duct cells) and HCO3- (into the lumen)
- H+ is actively eliminated by Na+/H+ exchanger into the blood from the duct cell
- H+ exchanged for K+ which is driven by the Na+/K+ ATPase which transports Na+ back into the blood and brings K+ into the duct cells
- H+ neutralises HCO3- to form H2CO3
- CO2 and HCO3- is produced in the blood
- This CO2 diffuses into the cell and reacts with H2O to form H2CO3
(The [HCO3-] in pancreatic juice is proportional to [HCO3-] on blood)
10
Q
What are the major types of enzymes of the pancreas?
A
- Proteolytic enzymes (trypsin, chymotrypsin and carboxypeptidase)
- Amylase
- Lipase
- Ribonucleases
- Deoxyribonucleases
11
Q
How are pancreatic secretions controlled?
A
- There is neuroendocrine signalling which means there are hormonal and nervous factors which control pancreatic secretions
- Vagal (parasympathetic) innervation enhances the rate of secretion of enzymes and pancreatic juice
- Sympathetic innervation inhibits this secretion
- Secretin and CCK stimulate secretion of pancreatic fluid (HCO3- and enzyme rich secretions)
12
Q
What are the phases of control of the pancreatic secretions?
A
- Cephalic: Vagal (ACh and VIP) stimulation of gastrin release from the antrum causing some enzyme rich juice to be released
- Gastric: Distension causes a vaso-vagal reflex in the fundus or the antrum. The presence of amino acid and peptides stimulates gastrin release causing the release of pancreatic juices
- Intestinal (duodenal): Secretin and CCK take part in enteropancreatic reflexes take place. Acidic chyme in the duodenum and jejunum induces the secretion of pancreatic juices
13
Q
How do pancreatic secretions occur during the cephalic phase?
A
- First we see, smell or taste food which sends an impulse to the brain.
- This impulse stimulates the vagal fibres which causes ACh to be released and interact with M2 receptors found on the acini cells within the acinus
- This results in the secretion of water and bicarbonate.
- These secretions make their way towards the duodenum via the pancreatic duct.
14
Q
How do pancreatic secretions occur in the gastric phase?
A
- Food in the stomach causes distension which sends impulses to the brain
- These impulses stimulate vagal fibres which triggers G-cells to secrete gastrin
- Gastrin will interact with its receptors on the acini cells within the acinus which stimulates enzyme release.
- These secretions make their way towards the duodenum via the pancreatic duct.
- Impulses in the vagal fibre also stimulates the release of ACh which will interact with acinus cells and on pancreatic ductal cells which causes enzymes and bicarbonate secretions which make their way into the duodenum also
15
Q
How do pancreatic secretions occur in the intestinal phase?
A
- If the food within the duodenum contains protein will be an increased distension of the duodenum.
- This will cause a vago-vagal reflex which will cause ACh to be secreted
- These ACh will interact with acinus cells (to secrete water and enzymes) and will interact with pancreatic ductal cells to cause bicarbonate secretion
- If there is acidic chyme within the duodenum it will stimulate the S-cells which secrete secretin
- Secretin will act on the pancreatic ductal cells to secrete bicarbonate rich pancreatic secretions into the duodenum via the common bile duct
- Food containing lipids will stimulate I cells which secrete CCK
- CCK will act on the CCK receptors on the acinus cells only and so will cause enzyme rich secretions
