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Physio > Digestion And Absorption > Flashcards

Digestion And Absorption Flashcards

1
Q

Gross anatomy of small intestine

A

. Kekring’s mucosal folds
. Villi on folds
. Microvilli on apical surface of enterocytes forming brush border

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2
Q

Salivary and pancreatic amylase

A

. Digest complex carbs at internal alpha 1,4 links to either maltotriose, alpha-limit dextrins, and maltose

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3
Q

Brush border enzymes

A 
. Glycosidases is the general category 
. Lactase: digests lactose to glucose and galactose 
. Maltase 
. Sucrase 
. Alpha-dextrinase
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4
Q

Final monosaccharides formed by action of brush border enzyme

A

. Glucose
. Fructose
. Galactose

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5
Q

Main transporters for absorption of glucose, galactose, and fructose in the small intestine

A

. GLUT-5: apical side, transport fructose into enterocyte from lumen, Na-independent
. SGLT-1: apical side, ransport of glucose/galactose into enterocyte from lumen, Na-dependent (Na-K ATPase drives gradient)
. GLUT-2: basolat. Membrane, transports glucose, galactose, and fructose out of enterocyte to interstitial space, Na-independent

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6
Q

Pepsin

A

. In lumen of stomach

. Digests some dietary protein (10-20% total) to oligopeptides

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7
Q

Function of brush border enzymes and aminopeptidases

A

. Apical membrane of enterocyte

. Hydrolyzes most, but not all, of remaining oligopeptides, Tori-peptides, and di-peptides to AAs

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8
Q

Cytoplasmic peptidases

A

. Inside enterocyte
. Digests tripeptides and dipeptides that have been absorbed intact
. Complete hydrolysis to AAs

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9
Q

Absorption of tri and di-peptides into enterocyte

A

. Pept-1: H-dependent, linked to apical Na/H exchanger
. Specific hydrolysis transport proteins for di-peptides on brush border that hydrolyze them and transport AA formed
. Intact absorption (pinocytosis) by M cells (can also be entry route for bacteria)

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10
Q

Absorption of AAs into enterocyte

A

. Na-dependent transporters (need Na-K ATPase to make gradient)
. Na-independent transporters
. Transporters specific for AA structure: neutral/acidic/basic

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11
Q

Classifications of dietary fat

A 
. Triglycerides 
. Phospholipids
. LCFFA
. Cholesterol
. Medium chain FFA
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12
Q

Triglycerides

A 
. Dietary fat mostly this 
. 3C glycerol sterilized w/ 3 FAs
. Not H2O-soluble 
. Do not form micelles w/ bile salts 
. Must be hydrolyzed to 2-monoglyceride (2-MG) and long-chain FFAs then 2-MG forms micelles
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13
Q

Phospholipids

A

. Digested by phospholipase A2 to release

. Lysophospholipids and LCFFA form micelles w/ bile salts

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14
Q

Long chain free fatty acids (LCFFA)

A

. Over 12C in chain

. Form micelles w/ bile salts

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15
Q

Cholesterol

A

. Sterol w/ same physical and chemical characteristics as fat
. Form micelles w/ bile salts

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16
Q

Medium chain FFA

A

. 8-12 FFA attached

. Do NOT form micelles

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17
Q

2 main problems in fat digestion

A

. Dietary fats are not water soluble but form large droplets in the aqueous lumen
. Enzymes for fat digestion are not lipid-soluble

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18
Q

Enzymatic cleave of fats must occur at ____

A

. Oil-water interface
. Fat digestion and absorption requires coordination of gastric emptying, all sections, and physiochemical events of emulsification and micelle formation

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19
Q

Steps in triglyceride digestion

A

. Emulsification
. Gastric lipase
. Binding of colipase
. Lipase

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20
Q

Emulsification

A

. Reduces large globules of fat into small spheres inc. SA for enzyme activity
. Physical (mastication, peristaltic contractions in stomach, phasic contractions in upper SI)
. Chemical: bile salts and phospholipids in lumen of SI

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21
Q

Gastric lipase

A

. In lumen of stomach
. Digests about 15% of dietary TG to DAG and FFAs
. Important in digestion for infants bc pancreatic lipase in low in them
. In adults it cannot fully compensate for insufficient pancreatic lipase

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22
Q

Binding of colipase

A

. LCFFAs stimulate pancreatic acinar secretion through CCK
. Trypsin activates pro-colipase to form active cofactor colipase
. Anchors lipase to oil/water interface so lipase can efficiently digest dietary triglyceride
. Dietary triglyceride digestion is severely impaired in absence of colipase

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23
Q

Lipase

A

. Found in salivary, gastric, and pancreatic secretions
. Most important is pancreatic lipase
. Secreted in active form from pancreatic acinar cells
. Bile salts inhibit activity (don’t allow access to oil/water interface)
. Colipase displaces bile salts to lipase can access it
. When it adheres to interface, it generates LCFFAs and 2-MG from dietary TG which can when be incorporated into micelles

24
Q

Absorption of lipid contents of micelle

A

. Micelles diffuse to apical membrane of enterocyte
. Micelles break down, they are not absorbed intact
. Contents diffuse through lipid membrane of enterocyte and enter the cytoplasm

25
Q

Absorption of medium-chain FFA

A

. Do not require micellar solubilization for absorption
. Absorbed by simple diffusion
. Go directly into capillaries by going in between enterocytes

26
Q

Process of chylomicron formation

A

. LCFFA and 2-MG or DAGare converted back into TG and phospholipids are synthesized in smooth ER
. Cholesterol enters cholesterol pool in the cell and can be esterified to cholesterol esters
. In smooth ER lipids are packaged for export
. TG, cholesterol, cholesterol esters, and phospholipids form droplets w/ protein coat apo-B (chylomicrons) making it water-soluble
. In Golgi chylomicrons are packaged then exocytosed across basolateral membrane of enterocyte
. Chylomicrons diffuse onto lacteal of villus and pass into larger lymph channels and ultimately enter venous system

27
Q

Fat soluble vitamin absorption

A

. Incorporated into micelles for absorption

28
Q

Vitamin B12 absorption

A

. In gastric lumen, pepsin digests protein and B12 is attached to proteins
. Free vit. B12 binds to salivary R protein in gastric lumen
. When chyme enters duodenal lumen, pancreatic proteases degrade salivary R protein
. Vi. B12 binds to intrinsic factor secreted by parietal cells because it is resistant to H and pancreatic proteases
. IF forms dimers and binds to receptors in brush border of ileum allowing for absorption of vit. B12
. Absorbed into plasma where it attached to binding protein

29
Q

Most ingested Ca is in ____form

A

. Nonionic form as salts
. insoluble at neutral pH
. Gastric acid solubilizes some salts and permits absorption in the SI

30
Q

Dissolved Ca

A

. Absorbed through tight junctions by solvent drag
. Absorbed through Ca channels
. Absorbed primarily in the duodenum

31
Q

Calcium absorption

A

. Enters enterocytes down its electrochemical gradient through an epithelial Ca channel (TRPV5/6)
. Inside enterocytes, Ca binds to binding protein calbindin
. At basolateral membrane Ca-ATPase and Na/Ca exchangers extrude Ca against an electrochemical gradient

32
Q

Vitamin D in relation to Ca absorption

A

. Inc. number of apical m membrane Ca channels
Inc. number of cystolic CaBP (calbindin protein)
. Inc, activity of basolateral Ca-ATPase

33
Q

Main mechanisms of Na absorption in small intestine

A

. Co-transport w/ glucose/galactose or AA

. In exchange w/ H

34
Q

Na absorption in colon

A

. ENaC channels
. Inc. by aldosterone
. Inc. number of channels and insertioni of ENaC into apical Membrane of enterocyte

35
Q

Mechanisms for iron absorption by duodenal enterocytes

A

. Divalent metal transporter 1 (DMT 1): ferric iron (Fe3+) is reduced to ferrous iron by ferrireductase DCYTB in apical membrane
. Heme gets transported into enterocyte and inside cell released Fe2+ via heme oxygenase

36
Q

Inside cell, ferrous Fe2+ is ___

A

. Stored as ferritin

. Exported across basolateral membrane by ferroportin 1

37
Q

Iron in circulation

A

. Hephaestin is a ferroxidase in basal membrane that oxidizes Fe 2+ to 3+
. Ferric iron binds to apotransferrin in plasma
. Hepcidin synthesized by liver can bind to ferroportin and block iron export under conditions of Fe overload

38
Q

Digestion and absorption of carbs in colon

A

. Digested by colonic bacteria
. Produce short chain FFA (under 8C) and gases
. Short chain FFA are absorbed by simple diffusion and. Promote Na and H2O absorption
. Gases eliminated as farts

39
Q

Digestion and absorption of protein in colon

A

. Digested by colonic bacteria into ammonia and cadavarine

. Gases leave as farts

40
Q

Digestion and absorption of triglycerides by colon

A

. Not digested by colonic bacteria or absorbed in colon

. Excreted intact in feces (steatorrhea)

41
Q

Digestion and absorption of bile salts in colon

A

. Salts Not absorbed in terminal ileum are converted into secondary bile salts by colonic bacteria
. Secondary bile salts are lipid-soluble and are absorbed by simple diffusion

42
Q

How much fluid is absorbed by intestine every day

A

. 2L of ingested fluid
. 7L of secreted fluid
. Less than 2% lost in poo

43
Q

Functions of water in small intestine

A

. Maintains osmolarity of luminal contents equal to plasma
. Maintains fluidity of luminal contents
. Fluid luminal contents distend visceral smooth m. To activate mechanoreceptive IPANS and inc. frequency of phasic contractions
. Brings digestive enzymes into contact w/ food particles
. Allows diffusion of digested nutrients to apical surface of enterocytes so absorption can occur

44
Q

Water flux occurs in response to ____

A

. Osmotic gradients produced by luminal accumulation of nutrients and electrolytes (Na) and absorption of nutrients and electrolytes into extracellular space
. Hydrostatic pressures generated by presence of water in extracellular space or intestinal lumen

45
Q

If net water flux is from blood to lumen it is called ____

A

. Secretion

46
Q

If net water flux is from lumen to blood it is called ___

A

Absorption

47
Q

Water absorption from intestinal lumen to blood

A

. H2O from lumen enters extracellular space by following osmotic gradient established by absorbed solutes and-or electrolytes
. Entrance of water into extracellular space inc. hydrostatic pressure w/in space
. Inc. hydrostatic pressure causes bulk flow of H2O and solutes into fenestrated capillaries

48
Q

Passive diffusion of water is much greater in SI than colon because of ___

A

. Leakier barriers btw adjacent enterocytes allowing for easier paracellular transport of water
. Much larger SA available for H2O absorption

49
Q

Reasons for osmotic diarrheas

A

. Intraluminal maldigestion (pancreatic exocrine insufficiency, reduced bile salt conc. Lactose intolerance)
. Mucosal malabsorption (mucosal diseases, short bowel syndrome)

50
Q

Pancreatic exocrine insufficiency

A

. Inadequate secretion of lipolytic enzymes and colipase and/or proteolytic enzymes
. Over 90% pancreas must be damaged for maldigestion of fat and protein
. Presence of undigested fat and protein in intestinal lumen creates osmotic driving force for water flux into lumen promoting diarrhea
. Treatment: freeze-dried pancreatic extracts

51
Q

How dec. luminal bile salt conc. Promotes osmotic diarrhea

A

. Impaired formation of micelles dec. digestion of dietary triglycerides and absorption of lipid digestion products
. Results in water flux into lumen
. Triglycerides and lipid digestion products enter colon creating osmotic driving force for retention of water in lumen and secretion of water into lumen

52
Q

How lactose intolerance promotes osmotic diarrhea after lactose ingestion

A

. Lactose not digested by brush border enzyme lactase and is not absorbed
. In colon it is digested by bacteria to form gas (H2 and methane) and lactic acid
. Undigested lactose and lactic acid exert osmotic force promoting water flux into lumen causing diarrhea

53
Q

Mucosal diseases

A

. Promote osmotic diarrhea
. Disruption in integrity of mucosa and/or weakening of barrier prevents adequate absorption of nutrients by enterocytes (celiac, IBD)

54
Q

Short bowel syndrome

A

. Inadequate absorptive SA prevents adequate absorption of nutrients

55
Q

Secretory diarrhea

A

. Initiating event is excessive electrolyte and H2O secretion into intestinal lumen
. Result from altered epithelial transport, apoptosis/necrosis of enterocytes, disruption/dysfunction of intestinal mucosal barrier
. Transit time dec.
. Nutrient absorption dec.
. Osmotic gradients favor H2O retention and additional water secretion into lumen

56
Q

Main causes of secretory diarrhea

A

. Secretory Agents assoc. w/ adenylate cyclase (entero-toxins, methylxanthines/caffeine)
. Agents not assoc. w/ adenylate cyclase (bacterial endotoxins: shigella, staph, clostridium)
. Mucosal injury, altered cell permeability/ tight junction issue (IBD, salmonella, E. Coli)
. Tumors (gastrinoma, carcinoid syndrome, pancreatic cholera syndrome (VIP)

Physio (62 decks)

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