GI: Pancreas, Gallbladder & Liver Flashcards
Describe the key properties of chyme leaving the stomach
- Hypertonic
- Low pH
- Partially digested
List the secretions of the exocrine pancreas
Acini
- Amylases
- Lipases
- Proteases (trypsin, chymotrypsin, Elastase, Carboxypeptidase
Duct cells
- Aqueous compenent
- Bicarbonate
What stimulates pancreatic and biliary secretions?
Secretin
CCK (enzymes)
Autonomic (PNS stimulates, SNS inhibits)
Describe the mechanism of secretion of alkaline component?
- Stimulated by secretin
- Release of aqueous bicarbonate component of pancreatic secretions by duct cells to neutralise chyme. Also released as part of bile
Describe how the microscopic structure of the liver relates to its functions
- Hexgonal arrangement
- Triad of structures at each corner (portal vein, hepatic artery, bile duct)
- Substance brought liver start at periphery and work towards middle
- Central vein in the middle
- Blood enter lobule via hepatic artery and portal vein
- Blood flows toward central vein via sinusoids (low pressure)
- Bile flows out alone canaliculi then bile duct into duodenum
Why are pancreatic proteases release in inactive forms?
- Inactive protease enzymes (zymogen) produced within acinar cells and are concentrated and stored in zymogen granules. Prevent autodigestion of pancreas
- The enzymes are converted to active forms where they need to be used
Describe the digestive functions of the liver and the components of bile
Liver secretes bile into duodenum to emulsify fat so they can be readily digested by lipases secreted by pancreas
Bile consists of
- Bile acids and bile pigments
- Alkaline solution
The liver acinus is divided in zone 1, 2, 3 starting from the periphery towards the centre. Where is toxic and ischaemic damage likely to have a greater effect?
Toxic
-Zone 1. Last region to receive blood supply
Ischaemic
-Zone 3. Last region to receive blood supply
Describe the function of the gall bladder and the relationship to the formation of gallstones
- Bile is stored in the gallbladder
- Concentrates bile which can lead to gallstones
Describe the secretion of bile acids and the entero-hepatic circulation of bile acids
-CCK stimulates bile release by causing gallbladder contraction
What are bile salts?
Two primary acids
- Cholic acid
- Chenodeoxycolic acid
Bile salts are bile acids conjugated with amino acids
Why are bile acids conjugated?
- Not always soluble at duodenal pHs
- Bile salts have amphipathic structure so can fact at oil/water interface
Describe the mechanisms of digestion of fats
- Lipids tend to form large globules by time reached duodenum which results in small surface area for enzymes to act
- Bile salts emulsify fat into smaller units to increase surface area and allow lipases to act
- Bile salts then create micelles with product of lipid breakdown
- Micelle transports digested lipids to luminal membrane of enterocyte
- Lipids diffuse into intestinal epithelial cells
What is the enter-hepatic circulation of bile acids?
- Bile salts remain in gut
- Reabsorbed in terminal ileum
- Returned to liver in portal blood
Liver recycle bile acids
How are digested fat transported to the blood circulation?
- Inside the enterocyte, lipid molecules are built back up again into triglycerides, phospholipids and cholesterol
- Lipids packed with apoproteins within enterocyte into chylomicrons
- Chylomicrons exocytosed from basolateral membrane of enterocyte ad enter lymph capillaries into thoracic duct
What is steatorrhoea?
- Certain pathologies may cause bile acids or pancreatic lipases to not be secreted in adequate amounts
- Undigested fat appears in faeces. It is pale, floating and foul smelling
How does chyme becomes isotonic as it leaves the duodenum?
- Stomach is impermeable to water
- Duodenum is relatively permeable to water
- Hypertonic chyme draws movement of water from ECF/circulation into duodenum
What are the key points of carbohydrate digestion?
- Carbohydrates are chains of sugars (Polysaccharides, Disaccharides, Monosaccharides)
- Only monosaccharides can be absorbed (Fructose, Galactose, Glucose)
- Final breakdown occurs in brush border by brush border hydrolases
- Glucose can only enter with Na+
- Fructose can enter from lumen through GLUT-5
What are the key points of carbohydrate digestion?
- Carbohydrates are chains of sugars (Polysaccharides, Disaccharides, Monosaccharides)
- Only monosaccharides can be absorbed
- Final breakdown occurs in brush border by brush border hydrolases
- Glucose can only enter with Na+
- Fructose can enter from lumen through GLUT-5
How is starch digested?
- Amylase
- Break in the middle results in maltose. Maltase can digest maltose
- Break at the end results in glucose
- Isomaltase can disrupt branching chains to form glucose from alpha dextrin
How are monosaccharides absorbed?
- Na+/K+ ATPase on basolateral membrane maintains a low intracellular Na+
- SGLT1 bind to Na+. Allows glucose to bind which allows Na+ and glucose to move into cell
- GLUT2 transports glucose out of enterocyte as the basolateral membrane. Diffuses down gradient into capillary blood
What are the principles of oral rehydration?
- Uptake of Na+ generate osmotic gradient and water follows
- Glucose uptake stimulate Na+ uptake
- Mixture of glucose and salt will stimulate maximum water uptake
What are the principles of protein digestion in stomach?
- Only amino acids, dipeptides and tripeptides absorbed
- Pepsinogen released from chief cells which gets converted to pepsin by HCl
- Pepsin acts on protein to form oligopeptides /amino acids which move to the small intestine
What are the principles of protein digestion in small intestine?
- Pancreas release proteases as zymogens
- Trypsinogen conver to trypsin by enteropeptidase. Trypsin then activates other proteases
- Endopeptidases (Trypsin, Chymotrypsin, Elastase) produce shorter polypeptides
- Exopeptidases (carboxypeptidase A & B) produce dipeptides and amino acids
How are protein product absorbed?
- Amino acids are transported into cell by Na+-amino acid co-transporters (neutral, acidic, basic, imino)
- Dipeptides/tripeptides moved by H+ co-transporter called peptide transporter 1 into the cell where they are converted to amino acids by systolic peptidases
Describe the basis of electrolyte and water uptake in the GI tract?
- Na+ moved by active transport of the cell on basolateral membrane
- Na+ diffuses into epithelial cells
- Osmotic gradient from all bsoritpn leads to uptake of water. The fluid absorbed is isoo-smotic
What are the similarities and differences in electrolyte/water uptake in small intestine vs the large intestine?
-Both have Na+-K+ ATPase on basolateral membrane
Apical membrane
- Na+ is co-transported in the small intestine
- Na+ channels in the large intestine which is induced by aldosterone (ENaC)
Describe uptake of calcium in the intestine?
When calcium intake is low
- Active transcellular absorption so it enters cell via facilitated diffusion
- Ca+ ATPase removes Ca+ from basolateral membrane
- Process requires Vitamin D and is stimulated by parathyroid hormone.
When calcium intake is normal/high
-Passive paracellular absorption
Describe the uptake of iron in the GI tract?
- Mostly in haem/Fe2+
- Gastric acid is important in the process
- Iron absorbed across apical membrane. This is via co-trasnport with H+
- If iron levels are low, iron binds to transferrin to be transported to stores
- If iron levels are high, iron contained in ferritin complexes and trapped in cells. Lost when enterocyte is replaced
How are water soluble vitamins absorbed?
Absorbed by Na+ co-transport (Vitamin C/B)
-Vitamin B12 absorbed in terminal ileum bound to intrinsic factor which is secreted by gastric parietal cells. Removal of terminal ileum and gastritis can cause B12 deficiency
