case 3 Flashcards
pancreas
large compound gland, both exo and endocrine. Its retroperitoneal. Main pancreatic duct with drains at the ampulla of Vater. Some have accesory duct of santorini, they drain exocrine hormones from acinar into duodenum. Islets of Langerhans contain the endocrine cells which drain to blood stream. Head supplied by S and I pancreaticoduodenal arteries, nech body and tail by splenic artery. Head drained by superior mesenteric and portal veins. the common bile duct meets the main pancreatic duct at ampulla of Vater. sphincter of Oddi. It has large stores of digestive carb and protein enzymes but not for lipids.
exocrine function of the pancreas
digestive enzymes secreted by pancreatic acini. sodium bicarbonate solution secreted by small ductules (duct cells) leading from acini. combined enzymes and NaHCO3-pancreatic juice flows through pancreatic duct joins bile duct empties duodenum through papilla of Vater surrounded by sphincter of Oddi. Pancreatic juice most when chyme present in duodenum.
endocrine function of the pancreas
secretes insulin and glucagon. Insulin by beta cells, glucagon alpha cells of islet of Langerhan. directly into blood. secretory unit composed acinus and intercalated duct, which merge to form interlobular ducts, then main pancreatic duct.
protein digestive enzymes
three protein digestive enzymes secreted by the pancreas: trypsin: most abundant, splits whole and partially digested proteins into peptides, dont cause release of amino acids.
Chymotrypsin: splits whole and partially digested proteins into peptides but no amino acids.
Carboxypolypeptidase: splits peptides into amino acids so completes digestion
trypsin
the most abundant of the protein digestive enzymes to be secreted. It splits whole and partially digested proteins into peptides of various sizes but do not cause release of individual amino acids.
chymotrypsin
It splits whole and partially digested proteins into peptides of various sizes but do not cause release of individual amino acids.
carboxypolypeptidase
this splits peptides into individual amino acids, thus completing the digestion of some proteins all the way to the amino acid state.
• When first synthesized in the pancreatic cells, the proteolytic digestive enzymes are in the inactive forms ( termed ‘zymogens’) which are inactive enzymatically:
- Trypsinogen
o Activated by the enzyme enterokinase (secreted by the intestinal mucosa when chyme comes into contact with the mucosa).
o It can also be autocatalytically activated by trypsin that has already been formed from previously secreted trypsinogen. - Chymotrypsinogen
o Activated by trypsin to form chymotrypsin. - Procarboxypolypeptidase
o Activated by trypsin to form carboxypolypeptidase.
• They become activated only after they are secreted into the intestinal tract.
enterokinase
secreted intestinal mucosa when chyme comes into contact with mucosa, activates trypsinogen.
activation of chymotrypsinogen
trypsin to form chymotrypsin
procarboxypolypeptidase
activated trypsin to form carboxypolypeptidase, only after secreted into intestinal tract.
carbohydrates digestive enzymes
pancreatic amylase, hydrolyses starch, glycogen and other carbs except cellulose to form disaccharides and few trisaccharides.
fat digestive enzymes
pancreatic lipase: hydrolyses neutral fat into fatty acids and monoglycerides. Cholesterol esterase: causes hydrolysis of cholesterol esters. Phospholipase: splits fatty acids from phospholipids.
Trypsin Inhibitor
secretion prevents digestion of the pancreas itself. Its important proteolytic enzymes arent activated until theyve left the pancreas as trypsin would digest the pancreas itself. Cells that secrete enzymes also secrete trypsin inhibitor. formed in the cytoplasm of glandular cells prevents activation of trypsin both inside the secretory cell and in acini and ducts. This also prevents activation of the other enzymes that trypsin activates. When a duct is blocked large amounts of juice is pooled, so trypsin inhibitor is overwhelmed so the pancreas is digested giving rise to acute pancreatitis.
secretion of sodium carbonate solution
CO2 diffuses into ductal cells from blood, with carbonic anhydrase it combines with water to form carbonic acid, then dissociates into bicarbonate ions and H ions. some HCO3 enter cells across basolateral mem, via Na/HCO3 transporter. Then actively transported into lumen via Cl-HCO3 exchanger. H ions exchanged for Na ions through blood barrier by active transport. this supplies Na to provide nutrality. This causes osmotic pressure so water moves into P ducts.
stimuli causing pancreatic secretion
Ach (m3 receptors) released from parasymp vagus nerve. CCK secreted duodenal and jejunal mucosa in responce fats and amino acids. Secretin secreted duodenal mucosa in responce highly acidic food.
what do Ach and CCK stimulate the acinar cells to secrete
pancreatic digestive enzymes but small amounts water and electrolytes. without the water most enzymes remain stored in acini and ducts. Secretin stim ductal epithelial cells to secrete water solution of Na bicarbonate.
• The pancreatic acinar cell has at least two pathways for stimulating the insertion of zymogen granules and thus releasing digestive enzymes.
- ACh and CCK both activate Gαq, which stimulates PLC, which ultimately leads to the activation of PKC and the release of Ca2+.
- Elevated [Ca2+]i also activates calmodulin (CaM), which can activate protein kinases (PK) and phosphatases (PP).
- Finally, VIP and secretin both activate Gαs, which stimulates adenylyl cyclase (AC), leading to the production of cAMP and the activation of PKA.
- The duct cells have receptors for secretin, GRP, all of which stimulate HCO3- secretion.
- The duct cells have receptors for substance P which inhibits HCO3- secretion.
the presence of food in the stomach stimulates pancreatic secretions from acinar cells how?
distention of the stomach activates vagovagal reflex. protein digestion products stimulate g cells in the antrum to release gastrin which is a poor agonist of CCKA receptor on acinar cells.
s cells
secrete secretin stimulated by acidity of chyme which stim HCO3 production
I cells
stimulated by protein and lipid breakdown which secretes CCK which stim enzyme production. also causes gallbladder contractions
secretin
27 amino acid polypeptide. inactive form prosecretin in S cells in duo and jejunal mucosa.
what 4 enzymes are in enterocytes in the lining of villi of the small intestine
Lactase: lactose into galactose and glucose. Sucrase: sucrose into fructose and glucose. Maltase: split maltose into multiple molecules of glucose. alpha dextinase: splits small glucose polymers into multiple mol of glu.
which two peptidase enzymes are important in the SI
aminopolypeptidase and dipeptidase. split polypeptides into tripeptides and dipeptides and amino acids.
emulsification of fats
physical breakdown of fat globules into very small sizes so that the water-soluble digestive enzymes can act on the globule surfaces. first agitation in the stomach to mix fat with other. emulsification occurs in duodenum with bile which contains salts and phospholipid lecithin. The polar parts (the points where ionization occurs in water) of bile salts and lecithin molecules are soluble in water, whereas most of the remaining portions of their molecules are soluble in fat.
o Therefore, the fat-soluble portions of these liver secretions dissolve in the surface layer of the fat globules, with the polar portions projecting.
polar projections
- The polar projections, in turn, are soluble in the surrounding watery fluids, which greatly decreases the interfacial tension of the fat and makes it soluble as well.
o When the interfacial tension of a globule of non-miscible fluid is low, this non-miscible fluid, on agitation, can be broken up into many very minute particles far more easily than it can when the interfacial tension is great.
o Consequently, a major function of the bile salts and lecithin, especially the lecithin, in the bile is to make the fat globules readily fragmentable by agitation with the water in the small bowel.
lipase enzymes
water-soluble compounds and can attack the fat globules only on their surfaces. The main enzyme to further break down fat globules is the pancreatic lipase enzyme.
o The triglycerides of the diet are split by pancreatic lipase into free fatty acids and 2-monoglycerides.
formation of micelles
hydrolysis of tri result in many monogly and fatty acids in the vicinity of digesting fats which blocks further digestion. Bile salts prevent this as form micelle around fat globule to be digested, develop because of hydrophilic and hydrophobic bile salts.• Micelles also help transport the monoglycerides and free fatty acids to the brush borders of the intestinal epithelial cells.
• There the monoglycerides and free fatty acids are absorbed into the blood, but the bile salts themselves are released back into the chyme to be used again
methods of absorption in the small intestine
simple diffusion-mainly by lipids.
Endocytosis-vit b12 and intrinsic factors
Carrier mediated-amino acids sugars and some lipids
sites of absorption
mouth oesophagus stomach-limited diffusion
Duo and jejunum-major sire of nutrient and ion absorb
Ileum-vit b12 and bile salts
colon-Na and H20 and short chain fatty acids
rectum-limited diffusion
why is the small intestine the main site of absorption
- Large absorptive surface area.
- Rich blood supply of blood vessels and lacteals (to drain lipids) found in the mucosal lining.
- Expansion of nutrient specific transport proteins.
o These belong to a family of transport proteins called “Solute Carrier (SLC) Transport Proteins”.
o Although, these transport proteins are found on the plasma membrane of the intestinal epithelial cells, they too can be found on the organelles inside the cell itself, thus leading to undefined side effects.
surface area of small intestine comprises of
- Folds of Kerckring
- Villi (+ crypts of Lieberkuhn)
- Microvilli
functional differences throughout the small intestine regarding absorption
segmental heterogenity: diff parts are involved in absorption of diff components of diet.
Crypt-villus/surface heterogenity: Absorptive function is located in villous cells in the small intestine, whereas secretory processes reside in the crypt cells.
Cellular heterogenity: specific transport mechanisms are restricted to certain cells.
3 monosaccharides absorbed in the small intestine
glucose most abundant monosaccharide absorbed (80%). This is because glucose is the final digestion product of carbohydrates, galactose, fructose. All absorbed by active transport process.
absorption of glucose
dependant on Na absorpt. Cotransport with active transport of Na. first Na AT into blood depleting in cell, so dec causes absorpt of Na from lumen into apical mem by facilitated diffusion, 2NA-glucose. Then gluvose moved out cell again into blood by facilitated diffusion.