Pancreas and Small Bowel Flashcards
What part of the pancreas is sometimes absent as an adult
Accessory pancreatic duct and minor papilla
What is pancreas divisum
Where the ducts are separated
Higher occurance of pancreatitis
What are endocrine glands
Secretion into the blood stream to have an effect on distant target organ - ductless glands
What are exocrine glands
Secretion into a duct to have direct local effect
What are the main endocrine secretions of the pancreas
Insulin: anabolic hormone
Glucagon
Somatostatin - inhibitor
What is the percent of endocrine and exocrine glands of pancreas
Endocrine - 2%
Exocrine - 98%
What are the two pancreatic cells
Acini - grap like clusts of secretory units, secrete pro-enzymes into ducts
Islets - derived from branching duct system, lose contact with ducts and become islet, differentiate into alpha and beta cells into blood
Composition of islets
Alpha cells - 15-20% of islet tissue and secrete glucagon
Beta cells - 60-70 of islet tissue and secrete insulin
Gamma cells - 5-10% of islet tissue and secrete somatostatin
Acini
Islets are highly vascular
What is the composition of acini
Secretory acinar cells
Duct cells
What are the two components of pancreatic juice
Acinar cells - decrease volume, viscous, enzyme-rich
Duct and centroacinar cells - increase volume, watery, HCO3 rich
What is the fucture of bicarbonate secretion
Neutralises acid chyme - preevnts damage to duodenal mucosa, raises pH to optimum range for pancreatic enzymes to work
Washes low volume enzyme secretion out of pancreas
Why does HCO3 secretion stop when pH still acid
Bile also contains HCO3
Brunners glands secrete alkaline fluid
Duodenal ph<3 - not much more increase in HCO3 secretion
How does HCO3 secretion work
- Catalysed by carbonic anhydrase - separation of H and HCO3
Na moves down gradiant via paracellular tight junctions water follows
- Cl/HCO3 exchange at lumen (AE) , Na/H exchange at basolateral membrane into bloodstream (sodium-hydorgen exchanger (antiporter) typer 1 [NHE-1]
Exchange driven by electrochemical gradients
- Na gradient maintained by Na/K pump
- K returns to blood via K channel, Cl returns the blood via Cl channel (cystic fibrosis transmembrane conductance regulator (CFTR)
Composition of H and HCO3 in juice
Stomach
H+ - gastric juice
HCO3 - blood
Gastric venous blood is alkaline
Pancreas
HCO3 - juice
H+ - blood
Pancereatic venous blood is acidic
What are zymogens
Pro enzymes which enzymes for digestions are released and stored in
How are enzymes activated in the intestine
Pancreas contains a trypsin inhibitor to prevent trypsin activation
Enzymes only activated in duodenum
Blockage of MPD may lead to autodigestion
Activated in duodenum by enterokinase which converts trypsinogen to trypsin
Lipase secreted in active form but requires colipase and presence of bile salts
What are the side effects of Orlistat
Increase facal FAT
Pancreatic secretion decreases
Steatorrhoea - fatty stool
What are the phases of pancreatic juice secretion
Cepahlic phase - sight/smell/taste of food, enzyme rich component only, low volume
Gastric phase - stimulation of pancreatic secretion originating from food arriving in stomach
Intestinal phase - 70-80% of pancreatic secretion - hormonally mediate, both components of pancreatic juice stimulated - enzymes and HCO3 juice
How is the pancreatic juice enzyme secretion controlled by
Vagus nerve - cholinergic, vagal stimulation of enzyme secretion
Cholecystokinin (CKK) (Ca/PLC)
How is the pancreatic juice bicarbonate secretion controlled by
Duct and centroacinar cells
Secretin
What are the stimulaatory effect snad inhibitory effects of CCK from duodenal 1 cells
Stimulatory - Fatty acids and amino acids in lumen
Inhibitory - trypsin
What does secretin do
Stimulate secretion of H20 and HCO3 from cells lining extralobular ducts
Secretion-stimulated secretion is richer in HCO3 in constrant to acinar secretion because of Cl/HCO3 exchange
What is the negative feedback loop of bicarbonate
S cells increase scretin
Secretin increas HCO3
Cause pH to decrease
Which will increase production of S cells
What is the coupled stimulation reactions of CCK
CCK and secretin markedly increase HCO3
Vagus nerve has similar effect to CCK
When does pancreatic enzymes release peak
30 mins
What is the function of the small bowel
Absorb nutrients, salt and water
What are the dimension of small bowel
25cm -duodenum
Jejunum -2.5m
Ileum - 3.75m
3.5cm diameter
What is the function of mesentery
Suspends small and large bowel from posterior abdominal wall achoring them in place and allowing some movement
Provides conduit for blood vessels, nerves and lymphatic vessels
Facts about villi
Only in small intestine
Motile
Rich blood and lymph supply
1 cell thick
Dominated by enterocytes - columnar absorptive cells
What are the different cell types in small bowel
Enterocytes
Scattered goblet cells
Enteroendocrine cells
Paneth cells
Stem cells
Facts about enterocytes
Specialised for absorption and transport of substances
Short lifespan of 1-6 days
500flod of surface area of small bowel to folds, villi and microvilli
Facts about microvilli
Make up the brush border
Surface of microbilli covered with glcocalyx
What is the glycocalyx
Rich carbohydrate layer on apical membrane
Serves as protection from digestional lumen yet allows for absorption
Traps a layer of water and mucous known as unstirred laeyr
Regulates rate of absoprtion from intestinal lumen
Facts about goblet cells
2nd most abbundatn epithelial cell
Mucous - large glycoprotein that gacilities passage of amterial
Increase abundance of goblet cells along entire length of bowel
Facts about enteroendocrine cells
Columnar epithelial cells
Lower parts of crypts
Hormone secreting - influences gut motility
Facts about paneth cells
Bases of crypts
Large, acidophilic granules
Antibacterial enzyme lyosozyme - protect stem cells
Glycoproteins and zinc to aid number of enzymes
Engulf bacteria and protozoa
Regulating intestinal flora
Facts about stem cell sin GI tract
Undifferentiated cells
Eptihelial stem are essnetial in GI tract to continually replenish surface epithelium
Divide by mitosis
Migrate to tip of villus
Old cells digested and reabsorbed
Why do enterocytes have a rapid turnover
Enterocytes are first line defense agaisnt GI pathogens
Effects of agents will be diminished
Lesions will be short live
If escaltor like transit diminised, severe intestinal dysfunction will occur - radiation
How is the duodenum distinguish
Presence of brunner’s glands
Submucosal coiled tubular mucous glands secreting alkalin fluid
Open into base of crypts
Difference between jejunum and ileum
Jejunum - wider, redder, thicker, numerous plicae circulares
Arterial arcades more sufficient in ileum
Functions of small bowel motility
Mix ingested food with digestive secretions and enzymes
Facilitate contact between contents of intestine and intestinal mucosa
Propel intestinal contents along alimentary tract
Types of motility in small bowel
Segmentation - mixes content, contraction of circular muscles, more frequent contractions in duodenum, net effect is movement to colon
Peristalsis (propelling) - sequential contraction of adjacent rings, propels towards colon, travels only 10m
Migrating Motor Complez - smooth muscle contractions sweeping through gut, begin in stomach to colon, next wave starts in duodenum, prevents migration of colonic bacteria into ileum
Digestion in the duodenum
Occurs in an alkaline enviornment
Enter through main pancreatic duct and common bile duct
Eptihelium also produces its own digestive enzymes
How does digestion of carbs work
Salivary alpha amylase which is destroyed in stomach
Pancreatic alpha amylase - needs cl for optimum activity and slightly alkaline pH, actsm mainly in lumen, digestion of amylase products and simple carbs happen at the brush border
How are the carbs absorbed
Glucose adn galactose - secondary active transport - SGLT-1 on apical membrane
Fructose - facilitated diffusion - GLUT-5 on apical membrane
GLUT-2 facilitats exits at basolateral membrane
How are proteins digested
Protein digestion begins in lumen of stomach by pepsin
Pancreatic proteases secreted as precursors - lumen of small bowel (trypsinogen)
Trypsin activated by enterkinase
Trypsin makes singel amino acids and oligopeptides
How do enterocytes absorb proteins
Via action fo H+/oligopeptide cotransporter PepT1
Small peptides are digested to AA by peptidases in cytoplasm
How is the digestion of lipids happening
Secretion of bile salts and pancreatic lipases
Emulsification - increase surface area
Enzymatic hydrolysis of ester linkages - colipase complexes with lipase, prevents bile salts displacing lipase from fat droplet
Solubilisation of lipolytic products in bile salt micelles
How does the absorption of lipids happen
FA and MG leave micelles and enter enterocytes
FA and MG resyntehsized in triglycerides by monoglyceride acylation (major) and phosphatidic acid pathway (minor)
Chylomicrons - lipoprotein particles synthesised as an emulsion(80-90% TG) phospholipids, protein, in golgi
Chylomicrons secreted across basment membrane by exocytosis
Lymph transports them away from bwoel
What is the ileocaecal valve
Prevents balck flow of bacteria into ileum
Relaxation and contraction controls passage of amteral into colon
