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630: Medical Physiology > Gastrointestinal Physiology (Part 2) > Flashcards

Gastrointestinal Physiology (Part 2) Flashcards

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USMLE® Step 1 flashcards
1
Q

What are the ultimate functions of the GI tract?

A

digestion and absorption

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2
Q

Define digestion

A

The chemical breakdown of ingested foods into absorbable molecules

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3
Q

Define absorption

A

The movement of nutrients, water, and electrolytes from the lumen of the intestine into the blood

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4
Q

What are the 2 paths for absorption?

A
  • cellular path

- paracellular path

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5
Q

Describe the cellular path for absorption

A

The substance must cross the luminal membrane, enter the intestinal epithelial cell, and then be extruded from the cell across the basolateral membrane into blood

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6
Q

Describe the paracellular path for absorption

A

Substances move across the tight junctions between intestinal epithelial cells, through the lateral intercellular spaces, and into the blood.

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7
Q

The surface of the small intestine is arranged in longitudinal folds called what?

A

Folds of Kerckring

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8
Q

What increases the surface area of the small intestine?

A

Villi and microvilli

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9
Q

Where are villi the longest and the shortest? Why?

A

Longest in the duodenum, where most digestion and absorption occurs.
Shortest in the terminal ileum.

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10
Q

What covers the surface of the villi?

A

epithelial cells

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11
Q

The apical surface of the epithelial cells is further expanded by what?

A

microvilli

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12
Q

What is the microvillar surface called?

A

the brush border

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13
Q

What are the 3 types of ingested carbohydrates?

A
  • polysaccharides
  • disaccharides
  • monosaccharides
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14
Q

What type of carbohydrate is the only one that the intestinal epithelial cells can absorb? What does this mean?

A

Monosaccharides

Therefore, to be absorbed, all ingested carbohydrates must be digested to monosaccharides.

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15
Q

What are the 3 monosaccharides?

A
  • glucose
  • galactose
  • fructose
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16
Q

Describe the process of starch digestion

A

1) alpha-amylase digests the interior 1,4-glycosidic bonds in starch
2) this results in the formation of 3 disaccharides (alpha dextrins, maltose, and maltotriose)
3) these disaccharides are further digested to monosaccharides by the intestinal brush-border enzymes
4) the end result is glucose

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17
Q

What are the 3 disaccharides in food?

A
  • trehalose
  • lactose
  • sucrose
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18
Q

Do trehalose, lactose, and sucrose require the amylase digestive step?

A

No, since they are already in the disaccharide form

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19
Q

What is trehlaose digested into?

A

2 molecules of glucose

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20
Q

What is lactose digested into?

A

glucose and galactose

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21
Q

What is sucrose digested into?

A

glucose and fructose

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22
Q

How is glucose absorbed by intestinal epithelial cells?

A

via a Na+ dependent cotransporter

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23
Q

How is galactose absorbed by intestinal epithelial cells?

A

via a Na+ dependent cotransporter

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24
Q

How is fructose absorbed by intestinal epithelial cells?

A

via facilitated diffusion

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25
Q

What is an example of failure to digest a carbohydrate to an absorbable form?

A

lactose intolerance

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26
Q

Describe the basis behind lactose intolerance

A

The brush-border lactase is deficient or lacking, which means lactose is not digested to glucose and galactose

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27
Q

Proteins must be digested to one of 3 absorbable forms, what are they?

A
  • amino acids
  • dipeptides
  • tripeptides
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28
Q

Where does protein digestion begin? By way of what?

A

In the stomach by way of pepin

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29
Q

Protein digestion is completed where? By way of what?

A

In the small intestine with pancreatic and brush-border proteases

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30
Q

What are the 2 classes of proteases?

A
  • endopeptides

- exopeptides

31
Q

What do endopeptides hydrolyze?

A

The interior peptide bonds of proteins

32
Q

What are the 4 endopeptides of the GI tract?

A
  • pepsin
  • trypsin
  • chymotrypsin
  • elastase
33
Q

What do exopeptides hydrolyze?

A

One amino acid at a time from the C-terminal ends of proteins and peptides

34
Q

What are the 2 exopeptides of the GI tract?

A
  • carboxypeptidase A

- carboxypeptidase B

35
Q

What is the first step in intestinal protein digestion?

A

The activation of trypsinogen to its active form, trypsin, by the brush-border enzyme enterokinase

36
Q

What does trypsin do next?

A

It catalyzes the conversion of all of the other inactive precursors to their active enzymes

37
Q

These active enzymes function to do what?

A

To break down dietary protein to amino acids, dipeptides, or tripeptides

38
Q

So, in summary what are the 3 products of protein digestion?

A
  • amino acids
  • dipeptides
  • tripeptides
39
Q

How are amino acids transported from the lumen into the cell?

A

via Na+-amino acid cotransporters

40
Q

How are amino acids transported from the cell into the blood?

A

via facilitated diffusion

41
Q

Most ingested protein is absorbed by intestinal cells in what forms?

A

dipeptide and tripeptide

42
Q

When do disorders of protein digestion or absorption occur?

A

When there is a deficiency of pancreatic enzymes or when there is a defect in the transporters of the intestinal epithelial cells

43
Q

What are 2 disorders of the exocrine pancreas in which there is a deficiency of all enzymes?

A
  • chronic pancreatitis

- cystic fibrosis

44
Q

What disease is caused by a defect in or absence of an Na+ amino acid cotransporter

A

Cystinuria

45
Q

What are the 3 dietary lipids?

A
  • triglycerides
  • cholesterol
  • phospholipids
46
Q

What are the 5 final products of lipid digestion?

A
  • monoglycerides
  • fatty acids
  • cholesterol
  • lysolecithin
  • glycerol
47
Q

What is the only end product of lipid digestion that is not hydrophobic?

A

glycerol

48
Q

Describe the mechanism of absorption of the 4 hydrophobic lipids in the small intestine

A

1) they are solubilized in micelles
2) the micelles diffuse to the brush-border and the lipids diffuse down their concentration gradients into the cell
3) the products of lipid digestion are reesterified with free fatty acids
4) These reesterified lipids are packaged with apoproteins in chylomicrons
5) the chylomicrons are packed in secretory vesicles on the Golgi apparatus
6) these vesicles migrate to the basolateral membrane and there is exocytosis of the chylomicrons

49
Q

An abnormality at any step of lipid digestion or absorption will result in what?

A

steatorrhea (fat excreted in feces)

50
Q

What are 6 situations in which steatorrhea will occur?

A
  • Pancreatic insufficiency
  • if the acidic chyme delivered to the duodenum is not adequately neutralized by HCO3-
  • Deficiency of bile salts
  • Bacterial overgrowth
  • Decreased number of intestinal cells for absorption
  • Failure to synthesize apoproteins
51
Q

What are the 2 categories of vitamins?

A

fat or water soluble

52
Q

What are the 4 fat soluble vitamins?

A
  • A
  • D
  • E
  • K
53
Q

How are fat soluble vitamins absorbed?

A

Via the same mechanism as dietary lipids

54
Q

What are the 9 water soluble vitamins?

A
  • B1
  • B2
  • B6
  • B12
  • C
  • biotin
  • folic acid
  • nicotinic acid
  • pantothenic acid
55
Q

How are water soluble vitamins absorbed?

A

via an Na+ dependent cotransport in the small intestine

56
Q

What does the absorption of B12 require?

A

intrinsic factor

57
Q

What is a consequence of gastroctomy?

A

A loss of the source of intrinsic factor

58
Q

Pateints who are vitamin B12 deficient may develop what?

A

pernicious anemia

59
Q

How is pernicious anemia treated?

A

via vitamin B12 injections (not orally)

60
Q

What are the functions of the liver?

A
  • processing of absorbed substances
  • synthesis and secretion of bile acids
  • bilirubin production and excretion;
  • participation in metabolism of key nutrients (including carbohydrates, proteins, and lipids)
  • detoxification and excretion of waste products
61
Q

Where does the majority of the liver’s blood supply come from?

A

The GI tract

- spleen
- stomach
- small and large intestines
- pancreas
62
Q

What do bile acids aid in?

A

The digestion and absorption of dietary lipids

63
Q

How is bilirubin formed?

A

Hemoglobin is degraded by the reticuloendothelial system to form biliverdin which is converted to bilirubin

64
Q

Bilirubin is then bound to albumin in the circulation and carried where?

A

to the liver

65
Q

Once the bilirubin is in the liver what happens to it?

A

It is conjugated with glucoronic acid via the enzyme UDP glucuronyl transferase

66
Q

The liver participates in the metabolism of what 3 things?

A
  • carbohydrates
  • proteins
  • lipids
67
Q

In carbohydrate metabolism, what 3 things does the liver do?

A
  • gluconeogenesis
  • stores glucose as glycogen
  • releases stored glucose into the bloodstream
68
Q

What 3 things does the liver do in protein metabolism?

A
  • synthesizes the non-essential amino acids and modifies amino acids
  • synthesizes almost all plasma proteins
  • converts ammonia to urea
69
Q

In lipid metabolism what 2 things does the liver participate in?

A
  • fatty acid oxidation

- synthesis of lipoproteins, cholesterol and phospholipids

70
Q

Toxic substances that are absorbed from the GI tract are presented to the liver via the portal circulation, and the liver modifies them in so-called ________ metabolism

A

first pass

71
Q

Bacteria absorbed from the colon are phagocytized by what cells and thus are never entered into systemic circulation?

A

hepatic Kuffer cells

72
Q

What else modifies toxins so that they do not enter systemic circulation?

A

liver enzymes

73
Q

Liver enzymes render toxins water-soluble via __ phases

A

2

630: Medical Physiology (24 decks)

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