Overview
This lecture covers the exocrine functions of the pancreas and the biliary system in digestion: the two components of pancreatic secretion (an alkaline, bicarbonate-rich fluid and digestive enzymes) and how each is formed and regulated; the formation, storage and release of bile, including the enterohepatic circulation of bile salts; and the neurohormonal control shared by both systems via Secretin and Cholecystokinin (CCK). It closes with related clinical conditions: Cystic fibrosis, chronic pancreatitis, gallstones and biliary obstruction with jaundice.
Digestive secretions and fluid balance
- Each region of the alimentary canal secretes characteristic substances: salivary glands - mucus, alpha-amylase; stomach - HCl, pepsinogens, mucus; pancreas - HCO3-, hydrolytic enzymes; liver and gallbladder - HCO3-, bile salts, bile pigments; small intestine - hydrolytic enzymes from desquamated cells; large intestine - mucus.
- Approximate daily fluid volumes: oral intake 1.5 L; saliva 1.5 L; gastric juice 3.0 L; bile 1.0 L; pancreatic juice 1.0 L; intestinal secretion 2.0 L. Of the total entering the lumen, about 9.0 L is absorbed in the small intestine and 0.9 L in the colon, leaving only about 0.1 L excreted in faeces. Sodium follows a parallel pattern (mmol/24 h) at each step.
- Bile and pancreatic juice each contribute about 1 L/day to total GI fluid volume.
Biliary tree and pancreatic duct anatomy
- Bile ducts from the liver join to form the common hepatic duct.
- The common hepatic duct joins the duct from the gallbladder to form the common bile duct.
- The common bile duct joins the pancreatic duct at the Sphincter of Oddi, which opens into the duodenum.
Pancreatic exocrine structure
- The pancreas secretes about 1.0 L/day of exocrine fluid via the pathway: acini (glands) -> ducts -> common bile duct -> sphincter of Oddi -> duodenum.
- The pancreas also has an endocrine function (insulin and glucagon from the islets), but exocrine secretion is this lecture’s focus.
- Exocrine (acinar) cells secrete enzymes; duct cells secrete bicarbonate; endocrine cells of the islets are interspersed among the acini.
- Pancreatic islets are distinguished from surrounding exocrine tissue by a continuous connective tissue capsule and extensive vascularity. Glucagon-secreting alpha cells stain red; insulin-secreting beta cells stain blue.
An unlabelled histology image of densely packed glandular (acinar-type) tissue was shown with no caption or reference identifying it on the slide.
The two pancreatic secretions
Pancreatic exocrine secretion has two major components: an alkaline fluid rich in HCO3-, and digestive enzymes.
Alkaline fluid
- Isosmotic with plasma.
- HCO3- rich; concentration increases with flow rate.
- Neutralises acid chyme arriving from the stomach, raising pH to the optimum for pancreatic enzyme activity (pH 6.7-9).
- Along the GI tract generally, pH is about 1 in the stomach, rises steeply through the duodenum and upper jejunum, and plateaus at about pH 7.5 from the middle jejunum through the ileum and colon.
- Produced by duct cells as an isosmotic, HCO3—rich solution; final composition is variable because of Cl-/HCO3- exchange.
Duct cell mechanism of HCO3- secretion
- Inside the duct cell, CO2 + H2O combine, via carbonic anhydrase, to form HCO3- and H+.
- At the luminal membrane, a Cl-/HCO3- exchanger secretes HCO3- into the lumen in exchange for Cl- entering the cell.
- CFTR, a Cl- channel at the luminal membrane, recycles Cl- back into the lumen, sustaining the exchanger.
- Water and Na+ follow into the lumen, presumably paracellularly.
- At the basolateral (interstitial) membrane, a Na+/H+ exchanger extrudes the H+ generated in step 1 in exchange for Na+ entering the cell.
- Na+/K+-ATPase pumps Na+ out and K+ into the cell (using ATP/ADP); a K+ channel allows K+ to leak back out.
Digestive enzymes
- Amylolytic: pancreatic amylase.
- Nucleolytic: ribonuclease, deoxyribonuclease.
- Lipolytic: lipase, phospholipase.
- Proteolytic: trypsin, chymotrypsin, carboxypeptidase.
- Proteolytic enzymes are synthesised in acinar cells and secreted by exocytosis as inactive precursors, preventing the pancreas from digesting itself.
- Activation cascade: membrane-bound enterokinase on the intestinal epithelial cell converts trypsinogen to trypsin; trypsin then activates the other inactive proenzymes to their active forms.
Regulation of pancreatic secretion
- Secretion increases during a meal, with cephalic, gastric and intestinal phases; the intestinal phase is dominant.
- Two intestinal-phase stimuli: (1) acid in the duodenum triggers secretin release; (2) food (fatty acids, amino acids, sugars) in the duodenum triggers CCK release.
Secretin pathway (acid -> bicarbonate secretion)
- Increased duodenal acidity (acid chyme from the stomach) stimulates S cells of the duodenum to release secretin.
- Increased plasma secretin acts on the pancreas to increase bicarbonate secretion from the ducts.
- Increased bicarbonate flow into the small intestine neutralises intestinal acid.
- The resulting fall in acidity feeds back negatively to reduce further secretin secretion.
CCK pathway (fat/protein -> enzyme secretion)
- Arrival of fatty acids, amino acids and sugars in the duodenum stimulates I cells of the duodenum to release CCK.
- Increased plasma CCK acts on pancreatic acinar cells to increase enzyme secretion.
- Increased enzyme flow into the small intestine increases digestion of fats and protein.
Biliary secretion: formation and storage
- Bile is formed by hepatocytes lining the liver sinusoids and excreted into bile canaliculi, which modify its composition.
- In the liver sinusoid, blood flows from branches of the hepatic artery and portal vein past the liver cells toward the central vein; bile flows through canaliculi and bile ducts in the opposite direction to blood flow.
- Bile is produced continuously, then concentrated and stored in the gallbladder, and ejected after a meal by gallbladder contraction, passing via the bile duct and sphincter of Oddi into the duodenum.
- Bile volume is approximately 1.0 L/day.
Bile composition
- Bile pigments (bilirubin).
- Cholesterol and phospholipids.
- Metabolites and excretory products, including drugs.
- HCO3-, contributing to neutralisation of acid.
- Bile salts, which are synthesised and recycled and are needed for fat digestion and absorption.
Enterohepatic circulation
- Many substances secreted from the liver in bile are reabsorbed in the GI tract and recycled rather than lost; most of what the gut absorbs was the body’s own material to begin with.
- Bile salts flow from the liver via the common bile duct (with the gallbladder as a side reservoir) into the duodenum, pass along the small intestine, and are reabsorbed in the ileum back into the hepatic portal vein to the liver.
- Only about 5% of bile salts are lost in faeces per cycle; the liver replaces this with about 5% new synthesis.
- Some drug metabolites are also recycled via Enterohepatic circulation, which helps maintain plasma drug concentrations.
- Reabsorption often requires (de)conjugation of metabolites, which is carried out by gut bacteria; if antibiotics reduce these bacteria, the sequence is: less (de)conjugation -> less reabsorption -> plasma drug levels drop -> drug effect reduced.
Regulation of biliary secretion
- Bile salt recycling via enterohepatic circulation.
- Secretin stimulates HCO3- secretion, as for the pancreas.
- CCK contracts the gallbladder and relaxes the sphincter of Oddi.
CCK pathway for bile delivery
- Fatty acids in the duodenum stimulate increased CCK secretion.
- Increased plasma CCK causes gallbladder contraction and sphincter of Oddi relaxation.
- Both actions increase bile flow into the common bile duct and then into the duodenum.
Clinical correlations
Cystic fibrosis
- A CFTR defect reduces HCO3-/water secretion in pancreatic ducts, producing viscous mucus that plugs the ducts, restricting enzyme release and causing malabsorption and pancreatic damage.
Chronic pancreatitis (alcohol-related)
- Inflammation impairs pancreatic secretion, causing malabsorption and steatorrhoea; ongoing inflammation causes tissue damage and pain, and can also affect endocrine function.
Gallstones
- A saturated (cholesterol-rich) bile solution precipitates stones. Duct blockage causes pain (biliary colic) and inflammation (cholecystitis). Managed by cholecystectomy or lithotripsy.
Two clinical images (a plain abdominal X-ray showing calcified opacities, and an abdominal ultrasound showing echogenic foci within the gallbladder) carried no on-slide caption or label; the imaging findings are inferred from the preceding "Gall stones" slide context, not stated on the slide itself.
Biliary obstruction and jaundice
Biliary obstruction, caused by tumour, stone or inflammation (e.g. hepatitis), leads to accumulation of bilirubin and produces jaundice.
Self-test
- List the digestive secretions produced by each region of the alimentary canal covered in this lecture (salivary glands, stomach, pancreas, liver/gallbladder, small intestine, large intestine).
- Approximately how much fluid do bile and pancreatic juice each contribute to total GI fluid volume per day?
- Trace the pathway of bile and pancreatic secretions from their point of production to the duodenum, naming the structures involved.
- What are the two major components of pancreatic exocrine secretion?
- Describe the composition and function of the pancreatic alkaline fluid, including the pH range it optimises pancreatic enzymes for.
- Describe the steps by which pancreatic duct cells secrete a bicarbonate-rich fluid, including the role of CFTR.
- List the four categories of pancreatic digestive enzymes with an example of each.
- Why are pancreatic proteolytic enzymes secreted as inactive precursors, and describe the steps of their activation cascade.
- Describe the intestinal-phase pathway by which duodenal acid stimulates pancreatic bicarbonate secretion, including the cells involved and the negative feedback step.
- Describe the intestinal-phase pathway by which fatty acids and amino acids in the duodenum stimulate pancreatic enzyme secretion.
- Describe how bile is formed and how its composition is modified before reaching the duodenum.
- List the major components of bile.
- Describe the enterohepatic circulation of bile salts, including the proportion recycled versus lost.
- Predict the effect of antibiotic use on plasma levels of an enterohepatically recycled drug, and explain the mechanism.
- Describe the two actions of CCK on the biliary system that increase bile delivery to the duodenum.
- Explain how a CFTR defect in cystic fibrosis leads to pancreatic malabsorption.
- A patient presents with right-upper-quadrant pain after fatty meals and yellowing of the skin. Explain the mechanism linking gallstone disease to jaundice.
- Distinguish the roles of secretin and CCK in regulating pancreatic and biliary secretion.
Answers
Reveal answers
- Salivary glands: mucus, alpha-amylase. Stomach: HCl, pepsinogens, mucus. Pancreas: HCO3-, hydrolytic enzymes. Liver/gallbladder: HCO3-, bile salts, bile pigments. Small intestine: hydrolytic enzymes from desquamated cells. Large intestine: mucus.
- About 1.0 L/day each.
- Bile: liver bile ducts -> common hepatic duct -> joins gallbladder duct -> common bile duct -> joins pancreatic duct at the sphincter of Oddi -> duodenum. Pancreatic secretion: acini -> ducts -> common bile duct -> sphincter of Oddi -> duodenum.
- An alkaline (HCO3—rich) fluid and digestive enzymes.
- Isosmotic with plasma, with HCO3- concentration rising with flow rate; it neutralises acid chyme from the stomach, raising pH to 6.7-9, the range needed for optimal pancreatic enzyme activity.
- Carbonic anhydrase converts CO2 + H2O to HCO3- + H+ inside the duct cell; a luminal Cl-/HCO3- exchanger secretes HCO3- into the lumen for Cl- in; CFTR (a Cl- channel) recycles Cl- back to the lumen to sustain the exchanger, with water and Na+ following paracellularly; basolaterally a Na+/H+ exchanger extrudes the H+ for Na+ in, and Na+/K+-ATPase (with a K+ channel) maintains the ionic gradients.
- Amylolytic - pancreatic amylase; nucleolytic - ribonuclease/deoxyribonuclease; lipolytic - lipase/phospholipase; proteolytic - trypsin, chymotrypsin, carboxypeptidase.
- They are secreted as inactive precursors to prevent the pancreas digesting itself; membrane-bound enterokinase on the intestinal epithelial cell converts trypsinogen to trypsin, and trypsin then activates the other inactive proenzymes to active enzymes.
- Increased duodenal acidity stimulates S cells to secrete secretin; increased plasma secretin acts on the pancreas to increase bicarbonate secretion, which flows into the small intestine and neutralises the acid; the resulting fall in acidity is the negative feedback signal that reduces further secretin secretion.
- Fatty acids, amino acids and sugars in the duodenum stimulate I cells to secrete CCK; increased plasma CCK acts on pancreatic acinar cells to increase enzyme secretion, increasing enzyme flow into the small intestine and digestion of fat and protein.
- Bile is formed by hepatocytes lining the liver sinusoids and secreted into bile canaliculi, which modify its composition as bile flows (opposite to blood flow) toward the bile ducts; it is produced continuously, then concentrated and stored in the gallbladder until a meal triggers ejection.
- Bile pigments (bilirubin), cholesterol, phospholipids, metabolites/excretory products (e.g. drugs), HCO3-, and bile salts.
- Bile salts pass from the liver via the bile duct/gallbladder to the duodenum, travel along the small intestine, and are reabsorbed in the ileum back to the liver via the hepatic portal vein; about 5% is lost in faeces per cycle and replaced by about 5% new synthesis.
- Plasma drug levels fall and drug effect is reduced: gut bacteria normally (de)conjugate drug metabolites to allow reabsorption; antibiotics reduce these bacteria, so less (de)conjugation occurs, less drug is reabsorbed, and plasma concentration falls.
- CCK contracts the gallbladder, expelling stored bile, and relaxes the sphincter of Oddi, allowing bile to flow into the duodenum.
- The CFTR defect reduces HCO3-/water secretion from pancreatic ducts, producing viscous mucus that plugs the ducts; this restricts enzyme release into the duodenum, causing malabsorption and progressive pancreatic damage.
- Gallstones can obstruct the biliary ducts; obstruction prevents normal biliary excretion of bilirubin, which accumulates in the blood and produces jaundice; obstruction also causes pain (biliary colic) and can lead to inflammation (cholecystitis).
- Secretin is released in response to duodenal acid and acts mainly on pancreatic duct cells to increase HCO3- secretion that neutralises the acid; CCK is released in response to fatty acids/amino acids/sugars in the duodenum and acts on pancreatic acinar cells to increase enzyme secretion, and on the biliary system to contract the gallbladder and relax the sphincter of Oddi, together delivering the enzymes and bile needed for fat and protein digestion.