The Pancreas and Small Bowel Flashcards
What is the pancreatic embryology?
Use the image below to explain:
The top of the image forms the stomach, the bottom of the image forms the duodenum
The pancreas and other organs (such as the gallbladder and liver) arise from the foregut outgrowth
Afterwards, the proximal duodenum rotates clockwise so the ventral pancreas comes to rest under the dorsal pancreas. The ventral and dorsal pancreatic ducts and buds fuse
Explain using the image below:
Where do the bile duct and pancreatic duct join?
What is the main pancreatic duct made of?
Bile and pancreatic ducts join at the major papilla
Main pancreatic duct switches direction half way - as one half is the dorsal pancreatic duct, and the other half is the ventral pancreatic duct
During endoscopies, look for the major papilla as that is where the main pancreatic duct can most easily be found
Where is the pancreas located and why is it difficult to access?
Fill in the covered labels of the pancreatic anatomy below:
The pancreas is right at the back - retroperitoneal. So to gain access to the pancreas, stomach needs to be moved superiorly, large bowel moved inferiorly
In the diagram, the transverse colon and the stomach has been removed
Pancreas is divided into the head, neck, body and tail
Superior mesenteric vein runs under the neck of the pancreas
The mesenteric vein meets the splenic vein, where it becomes the portal vein and eventually joins onto the IVC
The head of the pancreas is to the right, the tail of the pancreas is to the left (narrower pointier bit)
Which structure follows around the pancreas?
How many sections is it divided into and what are its names?
The duodenum follows around the pancreas
Divided into 4 sections - D1 to D4
What does the pancreas look like on CT scans? Label the pancreas on the images below:
What is MRCP?
What is it used for?
The image below shows a normal biliary tree:
Magnetic resonance cholangiopancreatography - uses MRI to visualise the biliary and pancreatic ducts in a non-invasive manner
Can be used to detect gallstones or look for abnormalities e.g. if the dorsal and ventral ducts did not fuse etc.
What is angiography? Why is it useful?
How is an angiography performed?
Where is the pancreas found in each of these images (angiograms)?
Placing dye into an artery to decipher how the artery is working - see if there is an aneurysm, or any bleeding e.g. in pancreatitis can cause erosion of gastroduodenal artery
Use local anaethesia, access the femoral artery in the groin, place in a wire and tube (catheter), and then the dye. The catheter can be guided all the way up to the aorta - and an image of the arteries can be produced
How is endocrine VS exocrine secretion defined?
What are the main secretions of the pancreas and their actions?
Endocrine = secretion into the bloodstream, affect on distant organ, more widespread effect (ductless glands)
Exocrine = secretion into a duct that has a direct local effect
Insulin = anabolic hormone that lowers BGL, promotes transport of glucose into cells for storage as glycogen, promotes protein synthesis and lipogenesis; glucagon = increases rate of gluconeogenesis and glycogenolysis to increase BGL; and somatostatin = inhibits other pancreatic hormones (i.e. insulin and glucagon)
What are the endocrine and exocrine functions of the pancreas?
Endocrine = 2% of the gland - via the islets of langerhan, secrete insulin, glucagon, somatostatin and pancreatic polypeptide; for regulation of blood glucose, metabolism and growth effects
Exocrine = 98% of the gland - secretes pancreatic juice into the duodenum via sphincter of Oddi; has a digestive function
The pancreas is made up of which 2 structures (that produce the endo and exocrine secretions?
What are acini?
What are islets? Where are they foundmore abundantly in the pancreas?
Acini and islets
A functional unit of the exocrine function = produce enzymes of the pancreatic juice (pro-ezymes secreted into the ducts that then become activated pancretic juice)
A functional unit of the endocrine function = produce the hormones, the idlets are split into the alpha and beta cells of langerhan; more islets in the tail of the pancreas than the head. If part of pancreas taken out during surgery, more likely to become diabetic if the tail was taken out rather than the head
What is the microanatomy of the pancreas? Describe the image below:
Green = islets of langerhans
Within each acinus is a canaculi, which then connects onto an intercalated duct
The small intercalated duct that connects to the main pancreatic duct
The canaculi contain specialised centroacinar cells, whilst the ducts contain regular pancreatic ductal cells
What is the compositon of the islets and what hormones do they produce? Use the image below:
Where are the acini?
Why are the islets highly vascular?
A = alpha-cells, form 15-20% of islet tissue and secrete glucagon
B = beta-cells, form 60-70% of islet tissue and secrete insulin
D = delta-cells, form 5-10% of islet tissue and secrete somatostatin
Acini surround the islets of langerhan - labelled ‘2’
Highly vascular so close access for site of secretion
The acini are the exocrine pancreatic units - what are the 2 cell types they are composed of?
- Secretory acinar cells - large with apical secretion granules
- Duct cells - small and pale
What are the 2 components of pancreatic juice?
- Produced by acinar cells - low volume, high viscosity, enzyme rich
- Produced by duct and centroacinar cells - high volume, low viscosity (watery), bicarbonate (HCO3-) rich
Which cells produce bicarbonate ions?
In what concentration is bicarbonate found in pancreatic juice?
What is the purpose of the bicarbonate ions in the pancreatic juice?
Bicarbonate ions are produced by the duct and centroacinar cells
120mM (mmol/L) - pH = 7.5 - 8.0
Neutralises acidic chyme from the stomach, prevents damage to the duodenum mucosa, raises the pH to the optimum range for the function of the pancreatic enzymes, and washes the low volume enzymes out of the pancreas into the duodenum
What is the graph below showing?
(HINT: how does the pH of the duodenum affect bicarbonate secretion)
Why does the bicarbonate secretion rate not increase below duodenal pH 3?
It shows the effect of duodenal pH on bicarbonate secretion rate
Between pH 3-5 of the duodenum, the bicarbonate secretion rate increases linearly
Below pH 3 of the duodenum, the bicarbonate secretion rate plateaus - this is because bile also contains bicarbonate and brunners glands (found in the duodenum) secete alkaline fluid, so more bicarbonate production is not necessary
What is the duct cell mechanism to producing bicarbonate ions?
- Water and CO2 enter the duct cells from the blood
They react together, catalysed by carbonic anhydrase to form H+ and HCO3-
There is a higher concentration of Na+ and H2O in the blood than in the lumen of the pancreatic duct, so they diffuse down the concentration gradient between the duct cells (paracellular pathway)
- The bicarbonate ion moves out to the lumen, and is exchanged for a Cl- ion, as there is a higher Cl- concentration in the lumen than in the cell so it travels down the concentration gradient (providing the energy for that exchange)
H+ leaves the cell via the H+/Na+ antiportar, and Na+ moves in down the concentration gradient from the blood into the duct cell
- The Na+ gradient between the blood and duct cell is maintained by a Na+/K+ exchange pump that uses ATP to pump Na+ back out from the duct cell into the blood, and K+ is pumped in
- In order for the K+ and Cl- to leave the duct cell, K+ exits via a K+ channel into the blood, and Cl- leaves via a Cl- channel into the lumen
What is cystic fibrosis caused by?
A non-functioning Cl- channel
Pancreas gets clogged up
Treatment requires patients to take pancreatic enzymes
What is the reaction between H2O and CO2 and what are its products?
Where are the two common instances in the body where htis reaction takes place and why?
H2O + CO2 –> H2CO3 –> H+ + HCO3-
- Gastric juice - must be acidic so this reaction occurs in the gastric parietal cells, so the H+ in secreted into the gastric juice whilst the biarbonate is secreted into the blood. Therefore, gastric venous blood is alkaline
- Pancreatic juice - must be alkaline so this reaction occurs in the pancreatic duct cells, so the bicarbonate is secreted into the pancreatic juice whilst the H+ is secreted into the blood. Therefore, pancreatic venous blood is acidic
What and where are the enzymes in the pancreatic juice manufacture, what are their functions?
What is pancreatitis?
What = lipases (fat digestion), proteases (protein digestion), amylase (carbohydrate digestion)
Where = manufactures in the acinar cells of the pancreas, once made are stored in the zymogen (containing pro-enzymes - inactive enzymes that may be metabolised to active enzymes) granules
Pancreatitis = autodigestion, pro-enzymes are actived before they reach the duodenum
Why are proteases inactive in the pancreas?
What is the protective mechanism incase the proteases are accidentally activated too early?
What may overload these protetive mechanisms?
To protect the acini and duct cells of the pancreas from autodigestion
Pancreas has a trypsin inhibitor to prevent trypsin activation
Therefore, the enzymes (esp. proteases) are only activated once they reach the duodenum
Overload = from e.g. blockage of main pancreatic duct, so when the pancreatic juices are trapped in the duct for a long period of time, it may result in activation of the enzymes leading to autodigestion (acute pancreatitis)
What controls pancreatic juice enzyme secretion from the acini?
Acini are innervated by the vagus nerve, ACh released upon vagal stimulation causing enzyme secretion; also controlled by cholecystokinin (CCK) release from the duodenal I cells that feeds back to the pancreas i.e. an increase in CCK results in an increase of pancreatic enzymes secretion
CCK release from the duodenum is controlled by fatty acids and amino acids (which increase CCK release), and trypsin (which decreases CCK release)
How is bicarbonate secretion controlled in the centroacini and ducts?
The initial acinar fluid release is controlled by CCK and it produces an isotonic solution of Na+, K+, Cl- and HCO3-, all resembling plasma concentrations
When the acinar fluid enters the duct, secretin stimulates the secretion of H2O and HCO3- from the extralobular cells lining the ducts, Cl- exits the ducts whilst HCO3- enters the ducts, making the fluid more alkaline
What is the classic negative feedback loop to control HCO3- secretions?
When the pH in duodenum decreases, S cells in the duodenum secrete more secretin
So then the secretin feeds back to the pancreatic ductal HCO3- secretion
HCO3- secretion has a negative feedback on the pH of the duodenum (i.e. increases the pH of the duodenum)
How effectively do secretin and CCK increase bicarbonate secretion? Use the graph below to help:
How does the vagus nerve work with secretin?
Individually, secretin causes a very slight increase in the bicarbonate secretion, and CCK does not cause a significant increase. CCK and secretin together, however, cause a significant increase in bicarbonate secretion
Vagus nerve acts similarly to CCK, when alone has little effect, but when paired with secretin produces similar effects (significant increase in bicarbonate secretion)
What is the mesentry and what is its function?
Fill in the covered labels on the diagram below:
Describe which part of the bowel each artery is keeping alive?
What would happen if there was thrombus in the superior mesentric artery?
Suspends the small and large bowels from the posterior abdominal wall - anchors them in place whilst allowing for some movement
Also provides a passage (conduit) for the blood, lymphatic and nerve supply
Superior mesentric artery = has many tributaries
Middle colic artery = keeps alive the transverse colon, hapatic flexure and splenic flexure alive
Right colic artery = keeps live the right colon
Ileocolic artery = keeps alive the caecum and part of the ileum
Jejunal and ileum arteries = numerous arteries keep the remainder of the small bowel alive
Thrombus in the superior mesenteric artery - blocked off blood supply = whole of the small bowel dies except the descending colon
What are the layers of the small bowel?
What are plicae circulares?
How do the plicae circulares differ between the ileum and jejunum?
Serosa = outermost layer; tough, protective layer
Muscularis = longitudinal muscle layer; then circular muscle layer
Submucosa
Muscosa
Plicae circulares (valves of Kerckring) = only found in the small bowel, formed from the small bowel being squished causing rumpled mucosa
Jejunum = many plicae circulares that are deeper and more condensed, which is why it feels thicker; ileum = fewer plicae circulares
Where are villi found?
What is the name given to the dips between the villi?
Where are microvilli found?
What is the blood and lymph supply to the villi like?
On the plicae circulares
Crypts
On the villi
Rich blood and lymph supply for absorption of digested nutrients and drainage of fluid
What is the structure of the villi? Use the diagram below to explain:
The villi are motile structures
Their epithelium is simple, one cell thick
The main cell type is the enterocyte
Goblet cells mainly scattered around the villi
Paneth and stem cells are found in the crypts
What are enterocytes?
Most abundant cell type in the small bowel
Tall columnar cells with microvilli at the top, and a basal nucleus
They are specialised for absorption and transport of substances
Short life span, 1-6 days, usually = 36hrs
Crypts, villi and microvilli increase surface area by 500 fold
What is the structure of microvilli? What are they covered in?
What is the purpose of microvilli?
Found on top of enterocytes - covered ina substance called glycocalyx
Glycocalyx is rich in carbohydrates and is known as the unstirred layer as it traps a layer of water an mucus to protect the enterocyte from being digested by the enzymes in the lumen. It also regulates the of absorption from the intestinal lumen
What are stem cells and where are they found?
What is their function?
How do they carry out their function?
Undifferentiated cells found in the crypts
Stem cells = pluripotent, can differentiate into any of those cell types, essential to replace / replenish the surface epithelium
Continually divide by mitosis, and slowly migrate to the top of the villus, replacing older cells that die by apoptosis (dead cells are digested and reabsorbed)
Although the duodenum, jejunum and ileum are continuous with no major separations, what factors differ them from each other?
Duodenum = contains specialised glands called Brunner’s glands (found at the base of crypts producing alkaline secretions)
Ileum = thinner walls than the jejunum, mesentery holding it to the posterior abdominal wall lies to the right of the aorta and lower abodomen. Also contains greater number of arterial arcades than the jejunum with a greater number of shorter branches
Jejunum = thicker walls due to greater number and thicker plicae circulares that are packed closer together, mesentery holding it to the posterior abdominal wall lies to the left of the aorta and upper abdomen. Also contains fewer arterial arcades than the ileum with long branches
