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Block D - Digestion and Metabolism > Physiology -- Intestinal Absorptive and Secretory Processes > Flashcards

Physiology -- Intestinal Absorptive and Secretory Processes Flashcards

1
Q

4 enzymes that break down proteins to small peptides

A
  • Pepsin
  • Trypsin
  • Chymotrypsin
  • Carboxypeptidase
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2
Q

Location of Brunner’s glands

A

Duodenum submucosa

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3
Q

Contents of Brunner’s glands

A
  • No digestive enzymes
  • Lots of HCO3-
  • Mucin
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4
Q

Location of goblet cells

A

Throughout the small intestine

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5
Q

Secretions of goblet cells

A

Mucin

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6
Q

Characterization of intestinal mucosa

A

Crypts and villi

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7
Q

Location of complete digestion and absorption in terms of the small intestine

A

Villi

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8
Q

Location of secretion in terms of small intestine

A

Crypt

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9
Q

Rate of cell shedding at the tip of the villus

A

100 x 106 cells/min

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10
Q

Location of villus enzymatic activity

A

Brush border

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11
Q

Secretions from crypt cells and volume produced

A

No digestive enzymes

Succus entericus (3L per day)

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12
Q

Ionic composition of succus entericus

A

Isotonic:

  • Na+
  • K+
  • Cl-
  • HCO3-
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13
Q

pH of succus entericus secretions

A

~7.5 - 9

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14
Q

What is located in the lumen of the SI crypts?

A

Cotransporter protein transporting Na+ and Cl-

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15
Q

Describe the function of the cotransporter of the crypts

A
  • Na+ brought out of cell by Na+-K+-ATPase
  • Na+ cotransported with Cl- into cell
  • Cl - secreted out of the other end of the cell through Ca++ dependent cAMP
  • Na+ also flows between cells
  • Water follows osmotic gradient
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16
Q

Effect of vibrio cholerae bacterium

A

Enterotoxic effect: bind to receptors on apical membrane –> stimulate adenylate cyclase –> increase cAMP = maximally stimulates intestinal secretion

Net result = overwhelms the absorptive capacity of SI and colon (need rapid rehydration)

Also, cAMP decreases absorption of neutral NaCl in villi

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17
Q

Describe the regulation of succus entericus secretions

A

Poorly understood interactions of ENS, ANS, gut peptides (gastrin, VIP) acting in paracrine/endocrine fashion

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18
Q

Effect of prostaglandins and histamine on succus entericus secretion

A

Increase secretion

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19
Q

Effect on somatostatin on succus entericus secretion

A

Inhibition of secretion

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20
Q

Function of villi

A
  • Synthesize enzymes (retained in brush border)
  • Absorb nutrients and fluids
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21
Q

6 enzyme types synthesized by the intestinal villi

A
  • Enterokinase
  • Amylase
  • Lipase
  • Aminopeptidases
  • Dipeptidases
  • Disaccharases
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22
Q

4 disaccharases synthesized by the intestinal villi

A
  • Sucrase
  • Maltase
  • Isomaltase
  • Lactase
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23
Q

Location of paneth cells

A

Base of crypts

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24
Q

2 substances secreted by paneth cells

A

Lysozymes

Defensins

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25
Q

Function of paneth cell secretions

A

Possible importance in protecting mucosa against bacteria

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26
Q

Contents of SI lamina propria

A

Lymphocytes and plasma cells

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27
Q

Role of SI lamina propria contents

A

Secretion of Ig = important in mucosal immune system

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28
Q

Volume of colonic secretions

A

Small

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29
Q

pH of colonic secretions

A

Alkaline

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30
Q

Composition of colonic secretions

A
  • [HCO3-] = 100 - 150 mEq/L
  • [K+] = 100 - 150 mEq/L
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31
Q

Protein included in colonic secretions

A

Mucin

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32
Q

Characteristic feature of colonic secretions

A

Bacterial activity

NO DIGESTIVE ENZYMES

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33
Q

Approximate daily input and output of solids

A

Input = 500 g

Output = 50 g

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34
Q

Approximate daily water input and output

A

Input = 2000 mL

Output = 200 mL

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35
Q

Approximate composition of solid output

A
  • 30% bacteria
  • 30% undigested fiber
  • 10 - 20% lipids
  • 10 - 20% inorganic matter
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36
Q

Volume of fluid produced by salivary glands

A

1500 mL

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37
Q

Sources of fluids input to the GI

A
  • Oral intake
  • Salivary glands
  • Stomach
  • Bile
  • Pancreas
  • Intestine
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38
Q

Volume of stomach fluids produced daily

A

2500 mL

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39
Q

Volume of bile produced daily

A

500 mL

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40
Q

Volume of pancreatic fluid produced daily

A

1500 mL

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41
Q

Volume of intestinal fluids produced daily

A

1000 mL

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42
Q

Approximate volume of fluid that must be absorbed daily

A

9000 mL

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43
Q

Location of fluid absorption in GI

A

Duodenum

Colon

44
Q

Approximate daily fluid absorbed from duodenum

45
Q

Approximate daily fluid absorbed by colon

46
Q

Describe the composition of proteins released into the lumen of the GIT

A
  • 50g as enzymes
  • 30g as cells
  • Total = 30g
47
Q

Fate of amino acids derived from digested proteins

A

Go to the amino acid pool

48
Q

2 characteristics of sites of exchange

A
  • Very large surface area
  • Intimate contact with blood vessels
49
Q

What part of the GI tract is absolutely essential to life?

A

Intactness of at least part of the small intestine (i.e. colon cannot take over nutrient absorption)

50
Q

Response of small intestine to resection

A
  • Hypertrophy
  • Hyperplasia

i.e. increase + of villus cells and increaase height of villi to increase absorptive capacity by up to 5x

51
Q

4 trophic factors for SI

A
  • Luminal nutrients
  • Hormones (G, CCK, glucagon, etc)
  • Local neural factors
  • Growth factors (epidermal, PGs, insulin-like, etc)
52
Q

What promotes the integrity of the GIT

A

Normal utilization (i.e. regular eating)

53
Q

Consequence of parental feeding (i.e. no stimulation of the gut) on GIT

A

Intestinal mass decreases

54
Q

Dscribe the process of diarrhea

55
Q

Describe the state of blood flow during a meal

A

50 - 100% increase, most of which is shunted to the mucosa to maximize absorption

56
Q

Rate of postprandial blood flow to the intestine

A

1 - 2 L/min

57
Q

Lympho flow to intestine

A

1 - 2 mL/min

58
Q

4 mechanisms for vasodilatoin

A
  • Metabolites
  • Bradykinin
  • NANC neurons (VIP? NO? etc)
  • Hormones
59
Q

Describe the blood flow through the layers of the intestinal walls

A

High density of cells in the mucosa = high blood flow

60
Q

Describe the changes in surface area from duodenum to ileum

A

Gradual decrease in:

  • Diameter
  • Thickness of wall
  • Number of folds
  • Number of villi
  • Size and shape of villli
61
Q

Where is the efficiency of nutrient absorption greatest?

A

Proximal SI

62
Q

Describe the relative surface area of small intestinal sections

A

50% of total SI surface area is the in proximal 25% of the SI

63
Q

Describe the permeability of the SI sections

A
  • More permeable tight junctions
  • Apical membranes are leakier
64
Q

4 reasons why the efficiency of absorption is greatest in the proximal SI

A
  • Greater surface area
  • More permeable
  • Greater concentration gradients
  • Brush border enzymes and transporters are denser in proximal SI
65
Q

Major area of Iron and Ca++ absorption in SI

66
Q

Major area of carbohydrate absorption in SI

A

Whole SI but decreasing distally, notably mid-jejunum

67
Q

Major area of proteins, lipids, salt and water in SI

A

Whole SI but decreasing distally

68
Q

Major area of vitamin B12 absorption in SI

69
Q

Major area of bile acid absorption in SI

A

Slight absorption throughout the whole SI with gradual increase through duodenum to jejunum, but marked increase at ileum

70
Q

5 methods of absorption in SI

A
  • Simple diffusion
  • Facilitated diffusion
  • Active transport
  • Pinocytosis
  • Osmosis
71
Q

Percentage of carbohydrates absorbed

72
Q

Percentage of fat absorbed

73
Q

Percentage of protein absorbed

74
Q

Absorptive capacity for H2O

75
Q

Absorptive capacity for glucose

76
Q

Absorptive capacity for amino acids

77
Q

Absorptive capacity for fat

78
Q

4 factors in absorption

A
  • Adequate form of absorption
  • Adequate surface for absorption
  • Adequate rate of transit
  • Specific co-factors and transporters (for facilitated diffusion and active transport)
79
Q

Describe carbohydrate absorption from its initial state to the SI

80
Q

4 causes of carbohydrate malabsorption

A
  • Sever pancreatic insufficiency
  • Selective deficiency of Brush Border disaccharases (or transporters)
  • Impaired enterocyte function (i.e. celiac disease)
  • Loss of mucosal surface
81
Q

Describe how proteins are absorbed through the small intestine

82
Q

Describe how fats are digested and absorbed through the SI

83
Q

8 things to consider when analyzing malabsorption

A
  • Number of nutrients (single or multiple?)
  • Enzyme deficiencies (single or multiple?)
  • Enzymes NOT activated?
  • Enzymes inactivated?
  • Co-factors absent?
  • Co-factors ineffective?
  • Surface area inadequate?
  • Transporter abnormalities?
  • Too rapid transit?
84
Q

What is water absorption largely secondary to?

A

Na+ absorption

85
Q

2 pathways of water absorption

A
  • Transcellular
  • Paracellular
86
Q

What is critical to Na+ absorption?

A
  • Na+/K= ATPase in the basolateral membrane
  • Favors Na+ entry into the cell and then into intercellular spaces
87
Q

Describe the mechanism for Na+ absorption in the distal colon alone

A

Electrogenic absorption

  • Occurs as a result of the pump actively extruding Na+ from cell to interstitial spaces
    • Lowers intracellular Na+ concentration
    • Makes the interior of the cell electronegative to lumen (favors entry of Na+ into cell)
88
Q

Describe the mechanism for Na+ absorption in the jejunum and ileum

A

Glucose (or other non-electrolyte solute) stimulated absorption; Na+ movement across the brush border by either:

  • Carrier-mediated solute-coupled transport or
  • Passive, secondary to solvent drag denerated by the active transport of glucose
89
Q

Describe the mechanism of Na+ absorption i nthe jejunum and distal colon

A

Neutral Cl- dependent absorption; coupled Na+/Cl- co-transport driven by the basolateral Na+ pump

90
Q

Describe the mechanism of Na+ absorption in the ileum and proximal colon

A

Neutral Na+/Cl- absorption by dual ion exchange

  • (Na+/H+ and Cl-/HCO3-) or countertransport controlled by intracellular pH restricting HCO3- and H+ available for exchange
91
Q

In cholera, what mechanisms of Na+ absorption are non-functional

A
  • A = Electrogenic absorption
  • C = Neutral Cl- dependent absorption
  • D = Neutral Na+/Cl- absorption by dual ion exchange
92
Q

In cholera. which mechanism of Na+ absorption is functional?

A

B = glucose (or other non-electrolyte solute) stimulated absorption

93
Q

Treatment for cholera

A

Oral replacement to reverse dehyrdation

94
Q

Compare the permeability and efficiency of the SI versus the colon in terms of water absorption

A
  • SI is more permeable but less efficient
  • Colon is less permeable but more efficient
95
Q

Describe the normal flow of water through the colon in a healthy individual

96
Q

Describe the flow of water through two variations of small intestinal disease

A

B = high ileocecal flow but compensatory colonic absorption = same stool H2O as normal (no diarrhea)

C = high ileocecal flow, compensatory absorption is maximized = increase H2O in stool (diarrhea)

97
Q

Describe the flow of water through a diseased colon

A

D = normal ileocecal flow, but absorptive capacity of colon is REVERSED so that water actually ENTERS the colon instead of being absorbed FROM it –> high H2O in stool = diarrhea

98
Q

Define diarrhea

A

Loss of fluid and solutes in excess of 500 mL/day

99
Q

5 mechanism-defined causes of diarrhea

A
  • Absorptive defect
  • Non-absorbable osmotic effect
  • Secretory defect
  • Motility defect
  • Excessive intake??
100
Q

How do absorptive defects cause diarrhea?

A

Decrease in absorptive surface

101
Q

How do non-absorbable osmotic effects cause diarrhea?

A

Through non-absorbable osmotic agents

102
Q

How do secetory defects cause diarrhea?

A
  • Mechanism of cholera enterotoxin (can be applied to all of this kind of defect)
  • Decreased electroneural NaCl absorption
103
Q

How do motility defects cause diarrhea

A

Decrease contractile activity and decreased resistance

104
Q

Definition of acute diarrhea

A

Diarrhea for less than 14 days

105
Q

Definition of chronic diarrhea

A

Diarrhea for >30 days

Block D - Digestion and Metabolism (29 decks)

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