PBL 3: Small intestine and exocrine pancreas Flashcards
Where are Brunner’s glands located?
In the wall of the proximal duodenum
Mainly between the pylorus and the heptaoduodenal papilla
What is the function of Brunner’s glands?
Secrete large amounts of alkaline mucus
What stimulates the Brunner’s glands to secrete mucus?
- Irritiating stimuli on the duodenal mucosa
- Vagal stimulation
- Gastrointestinal hormones, e.g. secretin
What does retroperitoneal mean?
Lies posterior to the parietal peritoneum
i.e. lies between the parietal peritoneum and the posterior wall of the abdominal cavity
Which regions in the regions of the abdomen does the pancreas lie in?
Epigastrium and left hypochondrium region
The stomach lies ___ to the pancreas?
Anterior
The first part of the duodenum lies __ to the pancreas?
The second part of the duodenum lies ___ to the pancreas?
- Anterior
- Laterally
What is the tranverse mesocolon?
Peritoneum fold that connects the transverse colon to the posterior wall of the abdomen.
Attaches to the anterior surface of the pancreas
The common bile duct descends __ the head of the pancreas?
Behind
What are the 5 anatomical regions of the pancreas?
- Uncinate process
- Head
- Neck
- Body
- Tail
Name these 5 parts of the pancreas?
What is the blood supply to the pancreas?
- The head of the pancreas is supplied by the superior and inferior pancreaticoduodenal arteries
- The rest of the pancreas is supplied by the splenic artery
Describe the branches from aorta to superior pancreaticoduodenal artery?
Aorta
Coeliac trunk
Gastroduodenal artery
Superior pancreaticoduodenal artery
Describe the branches from aorta to inferior pancreaticoduodenal artery?
Aorta
Superior mesenteric artery
Inferior pacreaticoduodenal artery
Name these vessels supplying the pancreas?
What is the venous drainage for the pancreas?
- Head of the pancreas is drained by the superior mesenteric vein
- Rest of the pancreas is drained by the Splenic vein
- Both drain into the hepatic portal vein
What are the two types of tissues in the pancreas?
Describe their distribution
- Acini and Islets of Langerhans
- The vast bulk of the pancreas is composed of exocrine tissue (acini) with scattered exocrine tissue between (islets of langerhans).
What is the name of the exocrine tissue of the pancreas? What is its function
- Acini
- Secretes digestive juices into the duodenum
What is the name of the endocrine tissue of the pancreas? What is its function
- Islets of Langerhans
- Secretes insulin and glucagon into the blood
What is the name of the cells located in acini?
Pancreatic exocrine acinar cells
Describe how the pancreatic exocrine acinar cells are arranged?
- Arranged in grape-like clusters called acini
Describe the secretion and pathway for the digestive enzymes from acinar cells
- Each cell is packed with membrane-bound secretory granules, which contains digestive enzymes.
- The digestive enzymes are exocytosed into the lumen of the acinus (name of a single acini)
- From the lumen they are drained by intercalated ducts.
Describe the capsule that covers the pancreas and what role it has on the composition of the pancreas?
- Thin connective tissue capsule
- The capsule invaginates into the gland to form septae.
- Septae: serves as scaffolding for large blood vessels.
- These septae divide the pancreas into lobules
What are the 4 types of pancreatic ducts?
What secretions does each receive
- Intercalated ducts
- Receives secretions from acini
- Intralobular ducts
- Receives secretions from intercalated ducts
- Interlobular ducts
- Receives secretions from intralobular ducts
- Pancreatic duct
- Receives secretions from interlobular ducts
- Joins with the common bile duct
What are the 3 parts of the small intestine?
Duodenum
Jejunum
ileum
What are the subdivisions of the duodenum?
- Superior
- Descending
- Inferior
- Ascending
What is the name of the point in which the duodenum joins the jejunum?
Joins at the duodenojejunal flexure
Border between the duodenum and the jejunum
What is the name of the slip of muscle at the duodenojejunal junction?
Suspensory muscle of the duodenum
What is the function of the suspensory muscle of the duodenum?
Allows intestinal contents to pass into the jejunum
Name these different parts of the duodenum?
What is the blood supply to the duodenum?
-
Gastroduodenal artery
- Derived from the common hepatic artery, from coeliac trunk, from aorta
- Supplies the area proximal to the greater duodenal papilla
-
Inferior pancreaticoduodenal artery
- Branched from the superior mesenteric artery from the aorta.
- Supplied the area distal to the greater duodenal papilla.
What is the venous drainage for the duodenum?
- Pancreaticoduodenal veins
- Drains into the hepatic portal vein
Which sections of the duodenum is retroperitoneal?
- Descending, inferior and ascending parts of the duodenum are retroperitoneal.
- Superior part (most proximal part) is intraperitoneal
Jejunum and ileum are ___peritoneal?
Intraperitoneal
The jejunum and ileum are attached to the posterior abdominal wall by?
Mesentery
What is a mesentery?
- Its a double layer of peritoneum
- Vessels travels in this layer e.g. artery, vein, lymphatic vessels and nerves
Where does the jejunum begin?
At the duodenojejunal flexure
Where does the ileum end?
At the ileocaecal junction
The ileum invagination into the caecum to form which structure?
ileocaecal valve
What is the function of the ileocaecal value?
- Function is to prevent reflux of material from the caecum into the ileum.
- Not developed enough to control the movement of material from the ileum to the caecum.
What is the blood supply to the jejunum and ileum?
Superior mesenteric artery
Describe the superior mesenteric artery?
- Arises from the aorta below the coeliac trunk at the level of L1.
- Moves between the layers of mesentery
- Splits into 20 branches
- These branches anastomose to form loops called arcades
- From arcades, long and straight arteries arise called vasa recta
What are vasa recta?
They are straight arteries coming off from arcades in the mesentery of the jejunum and ileum, and heading toward the intestines
What are the 3 features of the small intestine to allow it to have a large surface area?
-
Plicae circulares
- Inner surface of the small intestine has circular mucosal folds
- Function to increase surface area and to aid the mixing of the chyme
-
Villi
- Mucosa forms projections which protrude into the lumen
- Covered with epithelial cells
-
Microvilli
- On the luminal plasma membrane of epithelial cells (projections from the villi)
Describe the plicae circulares?
- Inner surface of the small intestine has circular mucosal folds
- Function to increase surface area and to aid the mixing of the chyme
- Contains the mucosa and submucosa layer.
The brush border is made up of what?
- It is made up of the microvillus border of the intestinal epithelial cells
What does the superior mesenteric artery provide blood supply to?
Provides blood to the entire small intestine and extends branches up until the middle third of the transverse colon.
What does the inferior mesenteric artery provide blood supply to?
Provides blood to the left third of the transverse colon and the sigmoid colon.
What kind of epithelium covers the small intestine?
Simple columnar epithelium
Name these different parts of the plicae circulares and villi?
Name the cells in the small intestine?
- Epithelial
-
Goblet
- Secretes mucus
-
Enterocytes
- Aka absorptive cells
- Absorbs nutrients
-
Enteroendocrine
- Located at the base of a villi
- Secretes hormones, secretin, CCK or GIP
-
Paneth
- Located at the base of the crypts
- Secretes lysozymes
-
Stem cells
- Constant turnover of epithelial cells
What is GIP in the small intestine?
- Gastric inhibitory polypeptide
- Secreted from enteroendocrine cells in the small intestine
- Weak inhibitor of gastric acid secretion
- Its main role is to stimulate insulin secretion
What is VIP hormone in the small intestine?
- Vasoactive Intestinal polypeptide
- A peptide hormone that is vasoactive in the intestine
- Regulates smooth muscle activity, epithelial cell secretion, and blood flow in the gastrointestinal tract
Gall Bladder relaxation and closure of Sphincter of Oddi is mediated by?
Vasoactive intestinal polypeptides (VIP)
What is the name of the capillary network at the center of each villi in the small intestine?
Lacteal
Name these different parts of the villi?
Describe the Crypts of Lieberkuhn?
- Invaginations of epithelium around the villi
- Lined with younger epithelial cells that are involved primarily in secretion.
- Towards the base rare stem cells, which replenish the epithelial cells in the crypts and on the villi.
Describe the villi?
- Projections into the lumen covered predominantly by absorptive enterocytes and goblet cells
- These cells have a short lifespan (few days) before they are shredded into the lumen
The exocrine enzymes of the pancreas are secreted by which cells?
Acinar cells
Bicarbonate is secreted by which cells in the pancreas?
Epithelial cells lining the ducts
What does fat gets digested to?
Fatty acids and monoglycerides
What does proteins gets digested to?
Peptides eventually amino acids
What does nucleic acid gets digested to?
Nucleotides
What are the 4 proteolytic enzymes secreted by the pancreas?
- Trypsin
- Chymotrypsin
- Carboxypolypeptidase
- Elastase
Describe the secretion of proteolytic enzymes and how they become active?
- Secreted in their inactive forms (zymogens) in the pancreas.
- Enterokinase, a proteolytic enzyme secreted when chyme comes in contact with the mucosa, splits off a peptide from trypsinogen, forming the active trypsin.
- Once trypsin is active, it activates the other zymogens by splitting off peptide fragment, forming active chymotrypsin, carboxypolypeptidase and elastase
Name these different products of carbohydrate digestion?
Name the 3 pancreatic fat digestion enzymes? and their function
- Pancreatic lipase
- Hydrolyses fats into fatty acids and monoglycerides
- Cholesterol esterase
- Hydrolyses cholesterol esters
- Phospholipase
- Splits fatty acids from phospholipids
What is the pancreatic carbohydrate digestion enzyme?
- Pancreatic amylase
- Hydrolyses starch, glycogen and other carbohydrates (Except cellulose) into disaccharides (e.g. sucrose, lactose and maltose)
Amylase breaks down __ into ___?
Starch
maltose
What are the 2 types of control of pancreatic exocrine secretion?
- Vagus nerve
- Innervates the pancreas
- Low levels in anticipation of a meal
- Enteric endocrine system
- Stimulates the secretion of 3 hormones: CCK, Secretin and gastrin
Which stimulates the hormonal control of the pancreatic exocrine?
Enteric endocrine system
Describe Cholecystokinin (CCK) and its role in pancreatic exocrine secretion?
- Synthesised and secreted by enteroendocrine cells located in the duodenum.
- Stimulated by the presence of partially digested proteins and fats in the small intestine.
- CCK transported via the blood, and binds to receptor on pancreatic acinar cells.
- Stimulates the pancreatic acinar cells to produce the pancreatic digestive enzymes.
Describe Secretin and its role in pancreatic exocrine secretion?
- Secretin is secreted in response to gastric acid in the duodenum.
- Produced by S cells in the duodenum and jejunum
- Stimulates the duct cells to secrete water and bicarbonate
Describe Gastrin and its role in pancreatic exocrine secretion?
- Produced by G cells located in the pyloric antrum and duodenum.
- Released in response to distention
- Stimulates the acinar cells to produce pancreatic digestive enzymes
What type of carbohydrates are in our diet?
- 2/3rd of the carbohydrates in our diet is starch (polysaccharide)
- The remainder consists of sucrose (disaccharide) and lactose (disaccharide).
o Cellulose and other complex polysaccharides are found in vegetables but they are not digested and absorbed. They are partially metabolised by intestinal microflora.
What parts of the ailmentary system does carbohydrate digestion occur?
- Starts in the mouth
- Release of salivary amylase by the salvia gland
- Only digests a small amount of the total starch digested
- As the bolus is received by the stomach, the amylase gets inactivated by the gastric acid.
- 2nd part is in the duodenum
- 95% of starch digestion is completed in the duodenum by pancreatic amylase
What does amylase breaks down?
Starch into maltose
Since amylases only breaks down maltose what happens to the other ingested carbohydrates?
- Lactose and surcose are disaccharides, therefore are broken down by enzymes in the brush border of the small intestine
- Surcrase breaks down sucrose into glucose and fructose
- Lactase breaks down lactose into glucose and galactose.
- Maltose gets broken down as well into monosacchrides
- Maltase breaks down maltose into 2 glucose molcules
Describe the break down of maltose?
Describe the break down of lactose?
Describe the break down of sucrose?
What are the 3 monosaccharides?
Fructose
Glucose
Galactose
What are the 3 disaccharides?
Maltose
Lactose
Sucrose
Nutrients are absorbed from the intestinal lumen against a concentration gradient by the process of secondary active transport.
With which ion is glucose co-transported into the enterocyte?
Sodium
Describe the mechanism in which fructose gets absorbed into the intestinal epithelium?
Enters the epithelial cells by facilitated diffusion via glucose transporter (GLUT).
Describe the mechanism in which glucose and galactose gets absorbed into the intestinal epithelium?
Enter the epithelial cells by secondary active transport coupled to sodium via sodium-glucose cotransporter (SGLT)
Describe the mechanism in which fructose, galactose and glucose gets transported from the epithelial cells to the interstitial fluid?
By facilitated diffusion via glucose transporter (GLUT).
The monosaccharides (now in the interstitial fluid) diffuse into the blood through capillary pores.
About 95% of bile acids recirculate via enterohepatic circulation.
Reabsorption of bile acids mainly occur in which part of the enteric system?
Ileum
Nutrients are absorbed from the intestinal lumen against a concentration gradient by the process of secondary active transport.
With which ions are the dipeptides and tripeptides co-transported into the enterocyte?
Hydrogen ions
Where does the digestion of proteins first begin?
- Begins in the stomach
- Pepsinogen is converted into pepsin in the presence of gastric acid.
- Pepsinogen is released from chief cells
- Pepsin digests proteins into peptide fragments
After the stomach, where and how is the rest of the protein digested?
- Proteins are further broken down in the small intestine by pancreatic exocrine secretions.
- trypsin, chymotrypsin, elastase, carboxypolypeptidase.
- Enterokinase, proteolytic enzyme secreted by intestinal mucosa when chyme comes in contact with the mucosa, splits off a peptide from trypsinogen, forming the active trypsin.
- Once trypsin is active, it activates the other zymogens by splitting off peptide fragment, forming active chymotrypsin and elastase.
- Chymotrypsin, trypsin, carboxypolypeptidase and elastase causes the digestion of proteins.
What is the means in which small peptides and amino acids absorbed into the epithelial cells?
via cotransport with H+
If the digestion of proteins from pancreatic enzymes isnt enough and the peptides are too big to be transports with H+.
What happens to them and how are they absorbed?
- They will be further digested by aminopeptidases, enzyme located in the apical membrane of the epithelial cells in the SI
- These amino acids are absorbed into the epithelial cell by secondary active transport coupled to sodium.
What happens to the peptides that enter the epithelial cells?
They are further hydrolysed into amino acids by peptidases
By what mechanism does amino acids leave the epithelial cell and enter the interstitial fluid?
via facilitated diffusion by amino acid transporters (many different types specific for different amino acids (details not required)).
What is the most abundant fat molecule?
Triglycerides
What lipids are present in our diet?
- Majority is triglycerides
- Other lipids present e.g. cholesterol and phospholipids
Where does the digestion of lipids first begin?
- A small number of triglycerides are digested in the stomach due to the lingual lipase
- Lingual lipase is secreted by lingual glands in the mouth.
- Less than 10%
- Majority of the fat digestion occurs in the small intestine.
Describe the emulsification process of fats?
- The lipids in the ingested foods are insoluble in water
- In the stomach, the lipids aggregate to lipid droplets.
- Pancreatic lipase is a water-soluble enzyme; therefore, its action can take place only at the surface of a lipid droplet.
- Because of the small surface area to volume ratio of the fat droplets the rate of digestion of pancreatic lipase would be very slow.
- To increase the rate of digestion, the large lipid droplets are divided into smaller droplets to increase their surface area.
- More surface area for lipase action.
-
Bile salts and lecithin, a phospholipid, are secreted into the duodenum from bile.
- Function is to emulsify the fat.
- Therefore, makes the fat globule readily fragmented by agitation with the water in the small intestine.
- With each fragmentation, the total surface area of the fat greatly increases.
Describe the digestion of emulsified fat?
- Pancreatic lipase digests these emulsified fats into fatty acids and 2-monoglycerides
- The enterocytes of the small intestine contain additional lipase (enteric lipase) but usually not required.
What are the two things that stimulate the fat into emulisified fat for the digestion?
Bile (containing bile salts and phospholipids)
Agitation
Why is micelle formation important in the absorption of fats?
- Without micelles, lipids would sit on the surface of chyme and never come in contact with the absorptive surfaces of the epithelial cells. Micelles can easily squeeze between microvilli and get very near the luminal cell surface.
Describe the formation of micelle?
- Micelles are lipid aggregates with hydrophilic head (pointing outwards) and hydrophobic tails (pointing inwards).
- Made from bile salts with a core of fatty acids, monoglycerides, phospholipids, cholesterol and fat-soluble vitamins.
- Micelles are constantly being broken down and reformed
- Each time releasing the contents of the micelle, which are absorbed through the epithelial cells by simple diffusion.
- Micelles are in equilibrium with the small concentration of fat-digestion products that are free in soluble that are being absorbed.
What mechanism causes the fatty acids and monoglycerides to enter the epithelium?
Simple diffusion (can pass freely)
What happens to the fatty acids and monoglycerides once they enter the epithelial cells?
- They become resynthesized into triglycerides
- Occurs in the smooth endoplasmic reticulum, where the enzymes for triglyceride synthesis are located.
- As a result, decreases the fatty acid and monoglyceride concentration in the epithelial cell, thereby maintaining a diffusion gradient.
- Reformed triglycerides aggregate to form small droplets (which can also contain other lipids such as phospholipids, cholesterol, and fat-soluble vitamins).
Describe how small droplets are transported from the epithelial cell into the rest of the body?
- The vesicles of small droplets pinch off the smooth endoplasmic reticulum, get processed by the Golgi apparatus, and eventually fuse with the plasma membrane, releasing the small droplet (which is now an extracellular fat droplet known as a chylomicron)
- The chylomicron released passes into lacteals (lymphatic vessels)
What are the 4 fat-soluble vitamins?
Vitamins A, D, E and K
Name these different parts of the digestion and absorption process of the fat?
Define chronic pancreatitis?
- Chronic irreversible inflammation of the pancreas that leads to the destruction of both endocrine and exocrine function of the pancreas.
- Characterised by fibrosis
What is the most common cause of chronic pancreatitis?
- Often due to acute bouts of acute pancreatitis
- Alcohol misuse
- Hereditary
- Trauma
What are the common signs and symptoms of chronic pancreatitis?
- Repeated episodes of epigastric pain that radiate towards the back
- Can be felt in the back because the pancreas is retroperitoneal
- A trigger the causes the pain is unknown but thought to be worse after a meal
- Weight loss
- Loss of appetite
- Diabetes mellitus
- Steatorrhea
- Severe cases where at least 90% of acinar cells have been destroyed.
What are the imaging used to investigate chronic pancreatitis?
Abdominal ultrasound
Compouted tomography
Endoscope ultrasound
What changes would occur in pancreatic damage and function tests for chronic pancreatitis only?
- Reduction in faecal elastase
- While for acute pancreatitis it would be normal
What are the treatments available for chronic pancreatitis?
- Damage to the pancreas is permanent
- Lifestyle changes
- Dietary changes (low fat diet)
- Reduction in alcohol consumption
- Pancreatic enzyme supplements
- Steroids (to relieve the inflammation)
- Pain relief
- Pancreas resection (remove specific parts of the pancreas that are inflamed and causing the severe pain)
- Total pancreatectomy (removal of the entire pancreas)
