GI physiology Flashcards

Question

Pepsinogen is a zymogen. What activates it?

Answer 1

• The acidic environment of the gastric lumen will turn pepsinogen into pepsin

Answer 2

* In the duodenum, where the enteropeptidase is on the enterocyte surface * Trypsin can cleave other trypsinogens

Answer 3

* Plant starch = amylopectin | * Polymer of glucos containing both alpha-1,4 and alpha-1,6 linkages

Answer 4

* Sucrose and lactose | * Both disaccharides

Answer 5

* Amylase in saliva can only break down alpha-1,4 linkages to produce maltose, maltotriose and alpha-limit dextrin * It can never produce free glucose

Answer 6

* Amylose is a straight chain of polymers | * Amylopectin contains branched chains

Answer 7

* Sucrase-isomaltase * This is in the mucosa and represents the last stage of small intestinal digestion of branch points of starch to glucose

Answer 8

* MGA - mucosal maltase-glucoamylase * Final step in small intestinal digestion of linear forms of starch to glucose * Thus SI and MGA complement each other's functions

Answer 9

* Responsible for trasporting glucose through the apical membrane of enterocytes * Sodium dependent glucose tansporter * Transports glucos and galactose along with Na from the intestinal lumen into cytosol

Answer 10

* Sodium independent fructose transporter | * GLUT2 is on the basolateral side and transports all three monosaccharides from cytosol to the blood

Answer 11

* Sodium independent fructose transporter * Also apical like SGLT1 * Facilitative glucose transporter example * Transports fructos from lumen into cytosol * GLUT2 is on the basolateral side and transports all three monosaccharides from cytosol to the blood

Answer 12

• Genetic absence of Na/glucose co-transporter SGLT1 • In the intestinal brush border • Glucose galactose malabsorption in humans • Reduced intestinal Na and fluid absorption osmotically • Also fluid secretion secondary to the osmotic effects of non-absorbed monosaccharide ○ Can treat with replacing dietary glucose with fructose because GLUT5 still works ○ This can be potentially fatal neonatal glucose-galactose malabsorption

Answer 13

``` • Chew up internal peptide bonds (hydrolyze) • Pepsin ○ Aromatic amino acids • Trypsin ○ Arginine and lysine (R,K) • Chymotrypsin ○ Aromatic amino acids • Elastase ○ Neutral aliphatic amino acids ```

Answer 14

• Hydrolyze one amino acid at a time from the C terminus towards the N terminus • Carboxypeptidases A • Carboxypeptidases B • Aminopeptidase ○ Clips off one amino acid from N terminus • Dipeptidyl aminopeptidase ○ Clips off two amino acids from N terminus • Dipeptidase ○ From dipeptides to amino acids

Answer 15

• The main way that humans absorb and digest proteins is through pancreatic proteases secreted into the small intestine from inactive precursors ○ Trypsinogen ○ Chymotripsinogen ○ Pro-elastase ○ Pro-carboxypeptidase A ○ Pro-carboxypeptidase B • Steps of small intestine protein digestion: ○ Activation of trypsinogen to trypsin by brush border protease enterokinase ○ Activation of all other precursors by trypsin ○ Trypsin, chymotrypsin, elastase, carboxypeptidase A and B all hydrolyze protein to amino acids and di, tri, and oligopeptides ○ Brush border proteases hydrolyze oligopeptides to amino acids ○ Pancreatic proteases digest themselves and others

Answer 16

* Occurs by sodium dependent co-transport * Unlike carbohydrate absorption (only monomers) most protein absorption comes through di and tripeptides * These get fully chewed to amino acids by cytoplasmic peptidases * Amino acids exit the basolateral membrane of the enterocyte by facilitated diffusion and enter blood capillaries

Answer 17

• Cysteinuria ○ Lacking Na-dependend amino acid transporter in GI and in kidneys ○ Lose cystein, lysine, arginine and ornithine amino acids in feces and urine • Hartnup disease ○ Defect or absence of neutral amino acid transporter • Chronic pancreatitis ○ Trypsin is gone, so all the others appear to be missing as well as trypsin activates all the others

Answer 18

• Fat droplets are emulsified by bile salts and lecithin to form particles on the order of 1um in diameter • This process increases the surface area for subsequend digestion by lipase and colipase ○ Colipase = protein that helps to anchor lipase to the surface of the droplets • The products of lipase digestion are 2'-monoglycerides and fatty acids which are solubilized in bile-salt micelles • Micelles are cylindrical structures about 10nm across, with hydrophilic groups oriented toward the aqueous phase and hydrophobic groups associating in the interior • Micelles are required to transport the products of fat digestion through the unstirred water layer near the surface of enterocytes • The abundance of hydrophilic glycoproteins protruding from the brush border membrane, as well as mucus are responsible for the unstirred layer • Lipids move in and out of the micelles, and when they strike the cell surface they are able to diffuse passively through the membrane and into the cells • Inside the enterocytes, triglycerides are re-synthesized from monoglycerides and fatty acids • They are then packaged into lipoprotein particles called chylomicrons • In addition to containing triglycerides, chylomicrons contain phospholipids, cholesterol and apolipoproteins • In the golgi they are incorporated into secrtory vesicles • Vesicle migrate to the basolateral membrane and the chylomicrons are released into the interstitial space by exocytosis • They then enter the lacteals because they are too large for capillaries • Fat soluble vitamins are absobed by precisely the same route as fat digestion products and cholesterol

Answer 19

• The most abundant storage form of fat in animals and plants = triglyceride ○ Thus the name of dietary lipid • Harder to absorb and digest than proteins or sugars because of their water insolubility

Answer 20

• Since triglycerides cannot be efficiently absorbed, they are enzymatically digested • 2-monoglyceride and two free fatty acids ○ These CAN be absorbed

Answer 21

• Fat droplets are emulsified by bile salts and lecithin to form particles on the order of 1um in diameter • This process increases the surface area for subsequend digestion by lipase and colipase ○ Colipase = protein that helps to anchor lipase to the surface of the droplets • The products of lipase digestion are 2'-monoglycerides and fatty acids which are solubilized in bile-salt micelles • Micelles are cylindrical structures about 10nm across, with hydrophilic groups oriented toward the aqueous phase and hydrophobic groups associating in the interior • Micelles are required to transport the products of fat digestion through the unstirred water layer near the surface of enterocytes • The abundance of hydrophilic glycoproteins protruding from the brush border membrane, as well as mucus are responsible for the unstirred layer • Lipids move in and out of the micelles, and when they strike the cell surface they are able to diffuse passively through the membrane and into the cells

Answer 22

* Lipids move in and out of the micelles, and when they strike the cell surface they are able to diffuse passively through the membrane and into the cells * Inside the enterocytes, triglycerides are re-synthesized from monoglycerides and fatty acids * They are then packaged into lipoprotein particles called chylomicrons * In addition to containing triglycerides, chylomicrons contain phospholipids, cholesterol and apolipoproteins * In the golgi they are incorporated into secrtory vesicles * Vesicle migrate to the basolateral membrane and the chylomicrons are released into the interstitial space by exocytosis * They then enter the lacteals because they are too large for capillaries * Fat soluble vitamins are absobed by precisely the same route as fat digestion products and cholesterol

Answer 23

* absorbed the same manner as triglycerides or dietary fat * • Fat soluble vitamins are absobed by precisely the same route as fat digestion products and cholesterol * vitamins A, D, E, K

Answer 24

• Excessive loss of fat in the stool • Hallmark of a diverse group of digestive disorders that cause fat malabsorption • Also causes fat-soluble vitamin malabsorption ○ Liver disease - bile salts are gone ○ Pancreatic insufficiency - lipase is gone and enzymatic degradation of fat is gone ○ Weight loss medication - these inhibit lipase activity and increase passage of fat from GI tract

Answer 25

• 1 - water readily moves across the intestinal epithelium ○ Thus chyme entering the duodenum is rapidly brouth into isotonic equilibrium with the blood • 2 - water absorption follows the absoption of solutes and is therefore said to be absorbed isotonically

Answer 26

* No, it all happens paracellularly | * Water will follow the osmotic gradient

Answer 27

• Water and ions. Water and ions. NOTHING else. • Na reabsorption from the apical Na channels ○ Epithelial sodium channel or ENaC • Any rise in plasma aldosterone levels causes very similar changes in colonic epithelia cells of the GI tract to those in the distal nephron of the kidneys • The colon will absorb more Na and consequently more water • Another similarity is that more Na absorption means more K secretion across apical membrane • In the colon there is net K secretion when the lumenal concentration drops below 25mM (I assume this is Na concentration)

Answer 28

• B vitamins • Vitamin C • Niacin • Folic acid • Pantothenic acid • Biotin ○ All absorbed by either by co-transport with Na or by passive diffusion ○ Virtually complete in the upper small intestine ○ B12 is the exception which is absorbed in distal ileum with intrinsic factor

Answer 29

• Hypokalemia because of the same mechanism by which extra flow in the kidney can result in hypokalemia • The K secretion is turned on max because there is such rapid flow that the colon is trying always to raise K levels ○ Think of it like extra flow sucks out K

Answer 30

• Mucins ○ Large glycoproteins that lubricate food and facilitate swallowing • Amylase and lingual lipase ○ Begin digestion of starches and fats • NaHCO3 ○ Helps maintain an optimal pH for enzyme activity and also to reduce Ca solubility ○ When pH is basic, less Ca loss from teeth in oral fluids • IgA, lysozyme, lactoferrin ○ Antibacterial agents that prevent overgrowth of bacteria that require iron ○ Lactoferrin chelates iron

Answer 31

• Submaxillary and sublingual glands contain two types of cells ○ Serous and mucous • Serous - fluid, electrolytes and enzymes • Mucous - mucin secretion • From the acinus, saliva passes relatively unchanged through a short intercalated duct • From intercalated duct into striated duct where more epithelial cells alter the inorganic ion composition

Answer 32

* High flow - slightly hypotonic and rich in bicarbonate * Low flow - VERY hypotonic because there is time for the duct cells to modify the fluid and the duct is water impermeable

Answer 33

• Aqueous and enzymatic components make up pancreatic secretions

Answer 34

• Autodigestion. Chain reaction of zymogens being activated and digestion of the pancreas itself. Bummer

Answer 35

* CCK - cholecystokinin * 33 amino acid peptide where the 8 C-terminal residues are REQUIRED for activity * CCK is the most important stimulus for acinar cell secretion * IP3 and Ca are probably the most important 2nd messengers for CCK * Ach is also stimulatory for secretions

Answer 36

• Mostly water and bicarbonate, produced by duct cells • Neutralizes acid in duodenum, preventing injury to the duodenal mucosa and bringing the pH to optimum level for enzymatic digestion of chyme to proceed ○ Solvates enzymatic secretions ○ Alkalinity necessary to neutralize gastric acid dumped from stomach into small intestine ○ Presence of acid in small intestine leads to secretion from duodenal endocrine cells of secretin ○ Secretin is most important stimulus for NaHCO3 § Thus the presence of acid in the intestine leads to secretion of a neutralizing solution ○ Secretin and CCK both inhibit gastric acid/fluid production and delay gastric emptying until intestine is ready for more

Answer 37

• Similar to reverse of acid secretion by paretal cells • HCO3 is transported into lumen and H+ is transported into interstitial fluid • Primary active transport here is Na/K ATPase in the baslolateral membrane and H is transported into plasma by secondary active transport using energy of inward Na gradient • Low H+ leads to high OH and thus high HCO3 • HCO3 is transported into the lumen by Cl/HCO3 exchange • CO2 is necessary for the OH to make HCO3, and much of that comes from blood, not cellular metabolism ○ H+ into plasma will react with HCO3 and make CO2 which freely diffuses back into cell • The water will follow the NaHCO3 net movement into the lumen

Answer 38

• Secretin (through cAMP) and ach

Answer 39

• High flow rates - HCO3 or bicarb rich fluid as there is less time to exchange this for Cl • Low flow rates - Cl rich fluid, mostly NaCl ○ Bicarb has been replaced by Cl through exchange

Answer 40

• Hormone - CCK - cholecystokinin • Main stimulant for bicarb-rich fluid is secretin • Vagal Ach stimulates enzyme secretion with little or no effect on the fluid composition ○ The fluid is likely influenced by peptidergic pathways like VIP

Answer 41

• Phases = cephalic, gastric and intestinal ○ Intestinal most important • Cephalic - stimulated by Ach and maybe VIP through vagal mechanisms and brain control • Gastric phase is stimulated by distention and vago-vagal reflexes • Intestinal phase - food in intestine leads to increased CCK and acid in intestine is secretin secretion with vagal reflex also

Answer 42

``` • Water ○ Taste, dissolution of nutrients, aids in swallowing and speech • Bicarbonate ○ Neutralizes refluxed gastric acid • Mucins ○ lubrication • Amylase ○ Starch digestion • Lysozyme, lactoferrin, IgA ○ Innate and acquired immune protection • Epidermal and nerve growth fibers ○ Contributes to mucosal growth and protection ```

Answer 43

* Acinus - creates the material that goes into the fluid | * Striated ductal cells - modify the ionic content of the fluid

Answer 44

``` • Otic ganglion ○ Parotid gland • Submandibular ganglion ○ Submandibular gland • Both of these are under parasympathetic stimulatory control, so Ach will stimulate saliva production ```

Answer 45

• Parasympathetic ○ Both vasodilation and secretory cell stimulation ○ Protein and fluid and ion rich solution • Sympathetic ○ No vasodilaiton, only acinar cell secretion ○ High protein, low fluid solution

Answer 46

* Fluid comes paracellularly into the glands * Without vasodilation, water has no way of getting in there * Saliva is primarily formed by passive filtration

Answer 47

``` • Out = lumen, in = cellular and subtract from lumen • Na, H synporter ○ Both in ○ Na/k exchanger will take sodium through basolateral side and increase intracellular K concentration • Cl, HCO3 antiporter ○ Cl in, bicarb out ○ Cl will go out basolateral side • K/H exhanger ○ Potassium out, acid in ```

Answer 48

* In saliva production * Salivary duct cells have tight junctions that don't allow for paracellular movement of water * They do, however, have ionic transporters that modify composition

Answer 49

* Carbonic anhydrase uses water to make CO2 into H+ and HCO3- * During low flow rates there is NaCl reabsorption with K and HCO3 secretion * Faster flow rates means less ionic changes

Answer 50

• These are all NT or hormones involved in regulating exocrine pancreas secretions • Ach ○ Vagus nerve and ENS ○ Stimulates release of digestive enzymes from acinar cells ○ Mostly cephalic stage • Secretin ○ Released from endocrine cells in the proximal small intestines in response to acid ○ Stimulates the release of bicarb rich solution from pancreatic duct cells • CCK (cholecystokinin) ○ Released from endocrine cells in the proximal small intestines in response to fats and proteins ○ Stimulates release of digestive enzymes from acinar cells

Answer 51

``` • Overall = protein, carbohydrate, lipid adsorption and digestion ○ Matching of nutrient delivery to digestive and absorptive capacity • Gallbladder ○ contraction • Pancreas ○ Acinar secretion • Stomach ○ Reduced emptying • Sphincter of oddi ○ relaxation ```

Answer 52

• CCK-RP ○ Fatty acids hitting the cell stimulate release • Monitor peptide ○ From pancreas, mostly Ach stimulated

Answer 53

• Pancreatic (exocrine) secretion here • VIP and secretin will influence cAMP ○ Downstream is phosph of structural and regulatory proteins • GRP, Ach, CCK will all influence Calcium release ○ This is the more important influence on enzyme release from acinar cells

Answer 54

* Neutralize the stomach acid * CCK will stimulate it's release * It will mess with cAMP in the duct cells and increase bicarb secretion

Answer 55

• Ductal cell secretion here in exocrine pancreas • CFTR is the chloride channel in question • Controlled by cAMP, thus can be regulated by secretin (stimulated release by CCK) • Cl is important to release ductally to promote exchange with intracellular bicarb ○ Cl/HCO3 exchanger

Answer 56

* Alkaline. This makes sense as more flow is supposed to help neutralize stomach chyme * The higher the flow rate, the more Cl and HCO3 exchange * Slower flow, probably can't pump out enough Cl to stimulate exchange

Answer 57

* Salivary duct | * NaHCO3 is from the pancreas

Answer 58

• Net fluid secretion from cells in the intestinal crypts • HOWEVER, a net fluid reabsorption from enterocytes on the villi ○ This is because the villi surface area is way greater than the crypt surface area

Answer 59

``` • In the small intestine • The colon is just for the last little bit, which is why increased motility on the part of the small intestine so easily overwhelms the large intestine and leads to diarrhea • Stomach and duodenum secrete 8L of fluid! ○ Diet - 2 ○ Saliva - 1 ○ Gastric - 2 ○ Pancreatic - 2 ○ Bile - 1 ○ Jejunum - 1 • Absorption ○ Jejunum - 4.5 ○ Ileum - 3.5 ○ Colon 0.9 ```

Answer 60

• Sodium is absorbed all along the intestine with the most of it being in the jejunum ○ 60-80% • Dependent on a gradient established by basolateral Na/K ATPase • Water and sodium absorption are linked • Na/glucose and Na/galactose and Na/amino acid transporters play a big role in sodium out of lumen into cell ○ NaCl cotransport and Na/H+ exchange are also important players

Answer 61

* In the colon Na goes through the ENaC channel | * In the small bowel, Na is absorbed mostly through co-transport of other nutrients like sugars and amino acids

Answer 62

• Chloride is passive in proximal intestines b/c of loose tight junctions • Chloride mostly balances the sodium charge which is moving from lumen to basolateral side • In the colon, though, cl is specifically exchanged for HCO3 (bicarb) ○ Offsets the acidic environment of colonic bacteria

Answer 63

* Intracellular b/c of Na/K ATPase * Most of it is in the cells * K passively leaks out in the colon depending on speed of fluid movement * In diarrhea, watch for hypokalemia

Answer 64

* Calcium and magnesium compete for uptake by enterocytes * Ca and Mg will go into enterocytes based on concentration gradient * The Ca concentration gradient in the cytoplasm is maintained by Ca ATPase that bumps Ca into the blood AND by the internal calcium stores within the cell (ER) that store calcium for vesicular fusion and such * However, magnesium isn't so selectively maintained so lost of magnesium in the fluid makes it an osmotic laxative

Answer 65

* Vitamin D * After it is hydroxylated a couple times it stimulates transcription and translation of Ca-binding proteins and Ca-ATPase proteins

Answer 66

• Regulated absorption in the proximal intestines • Transported across apical membrane as either heme or Fe++ ○ Receptor mediated • Either binds to apoferritin to form ferritin that says in the cell • OR it binds to transferrin, a carrier protein, that leaves the cell and goes into blood

Answer 67

* The colon * Interesting to me b/c I thought large intestine was lots of water reabsorption * Instead, the colon wants to more finely regulate water reabsorption by altering ionic composition, not by letting water just rush back in

GI physiology Flashcards

(91 cards)