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M104 T2 > M104 T1 L7 > Flashcards

M104 T1 L7 Flashcards

1
Q

What quantity of water and minerals is lost in faeces?

A

~200 mL

minimal loss

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2
Q

What processes regulate secretion and absorption?

A

the osmolarity of the lumen contents
enteric and autonomic signals
endocrine hormones
immunogenic signals

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3
Q

How does the osmolarity of the lumen contents regulate secretion and absorption?

A

in the absence of food, electrolytes are primarily responsible for creation of an osmotic pressure gradient in gut lumen
this pressure gradient will either draw water and electrolytes into the gut or allow absorption

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4
Q

What processes occur across GI epithelium daily?

A

bidirectional secretion and absorption

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5
Q

Where are secretions most common?

A

in the upper GI tract

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6
Q

What volume of secretions are released in the upper GI tract?

A

~7L

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7
Q

What substances are part of the secretions released in the upper GI tract?

A

saliva, bile

pancreatic, intestinal and gastric juice (PIG)

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8
Q

What is the role of the secretions released in the upper GI tract?

A

they facilitate movement along the GI tract
they mix with digestive enzymes
they allow chemical reactions to occur
they allow nutrient absorption to occur

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9
Q

What percentage and quantity of ingested and secreted water and electrolytes are absorbed?

A

98%

~8.8 L

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10
Q

What substances are predominantly absorbed in the jejunum of the SI?

A

ingested and secreted water and electrolytes

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11
Q

How is the SI adapted for absorption?

A

the presence of enterocyte actin microfilaments
has a rapid response to chyme
epithelium in the SI is more permeable than in the LI
increased SA via folds, villi and microvilli
the villi have lymphatics and blood vessels

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12
Q

How does the SI respond to a hypertonic environment?

A

allows the movement of water into the lumen by osmosis to form isotonic chyme

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13
Q

What is the role of enterocyte actin microfilaments in the SI?

A

they rhythmically contract to move microvilli for maximum exposure to lumen contents

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14
Q

How does the SI respond to an acidic environment?

A

rapidly increase of HCO3- rich secretions into the lumen to neutralise the acid

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15
Q

How is the LI adapted for absorption?

A

the surface is covered with crypts / intestinal glands
contains the bacterial microbiome, which has a role in protein digestion and vitamin synthesis
the epithelium is less permeable than in the SI
Has additional absorptive capacity for water and NaCl

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16
Q

How does the additional absorptive capacity for water and NaCl in the LI work?

A

if the SI can’t absorb the water and electrolytes presented to it, the LI has a capacity to increase absorption of up to 4-6 litres.
it occurs in exchange for K+ loss

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17
Q

What are the different types of active transport?

A

1o; runs on ATP hydrolysis

2o; primary pump

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18
Q

How does primary active transport work?

A

a pump hydrolyses ATP

this provides E to move ions against their electrochemical gradient

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19
Q

How does secondary active transport work?

A

uses the gradient created by the primary active transport pump, which allows a substance to move against its electrochemical gradient

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20
Q

What is an example of a pump used in primary active transport?

A

Na+-K+ ATPase pump

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21
Q

What are two examples of types of secondary active transport mechanisms?

A

Symport: two molecules in same direction
Antiport: two molecules in opposite direction

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22
Q

What is the movement of the two molecules during symport mechanisms?

A

one moves down a conc grad created by the primary pump

the other moves against this conc grad

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23
Q

What is the movement of the two molecules during antiport mechanisms?

A

one moves down a conc grad

the other moves in the same direction but against its conc gradient using the E provided by the primary pump

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24
Q

What is the role of the sodium potassium ATPase pump?

A

to keep Na+ in the cell to remain low

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25
Q

How does sodium potassium ATPase keep Na+ levels in the cell low?

A

3 Na+ exported and 2 K+ imported against electrochemical gradient using E from ATP hydrolysis by ATPase
Keeps Na+ cytosol concentration low

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26
Q

What affects bidirectional fluid flux under the principles of enterocyte transport?

A

the tonicity of chyme entering duodenum

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27
Q

What is the polarity of enterocytes like?

A

they are polarised with an apical and basolateral membrane

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28
Q

How is the free flow of gut lumen contents restricted?

A

by tight junctions between the entry sites

these tight junctions tend to be more permeable in the proximal SI where the majority of absorption occurs

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29
Q

By what two routes can the transfer of products occur?

A

transcellular routes

paracellular routes

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30
Q

How are products transfered via transcellular routes?

A

occurs through the enterocyte itself

may be against conc grad and require ATP

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31
Q

How are products transfered via paracellular routes?

A

occurs between cells

doesn’t require E

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32
Q

What are the three different types of transport proteins involved in transcellular electrolyte transport?

A

pumps - against concentrations
channels
carriers

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33
Q

What are the three different types of electrolyte transport?

A

Passive
Solvent drag
Active

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34
Q

How does passive electrolyte transport work?

A

down electrochemical gradient through ion channels or carriers or permeable tight junctions

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35
Q

How does solvent drag electrolyte transport work?

A

water follows Na+ gradient via osmosis, taking other ions

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36
Q

How does active electrolyte transport work?

A

requires ATP
the Na+/K+ ATPase pump depletes cellular Na + and draws Na + across apical membrane from gut lumen via channel or cotransporter

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37
Q

What is an example of solvent drag electrolyte transport?

A

in the upper intestine where tight junctions more permeable

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38
Q

What happens when chyme enters the duodenum from the stomach?

A

rapid osmotic equilibration of the chyme occurs
forms isotonic chyme
into hypertonic chyme
from hypotonic chyme

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39
Q

What happens to hypertonic and hypotonic chyme after undergoing rapid osmotic equilibration?

A

hyper - water is secreted into it

hypo - water is absorbed from it

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40
Q

What volume of chyme enters the LI?

A

~2 L

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41
Q

What volume of chyme is excreted from the LI?

A

100-200 ml

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42
Q

What is the role of tight junctions in the LI?

A

they prevent back-diffusion of ions into the lumen

this allows for a more complete NaCl absorption, so there is limited loss in faeces

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43
Q

What substances are secreted in the LI?

A

HCO3- and K+

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44
Q

Why is sodium absorption important?

A

bc it drives the absorption of ions, organics and water

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45
Q

By what process are Na+ ions absorbed?

A

basolateral active transport into the interstitial space occurs

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46
Q

How are Na+ ions absorbed?

A

Na+ is pumped out of the cell into the blood vessels
so there is a low Na+ conc inside the cell
this allows Na+ movement from the gut lumen into the entry site
where it is pumped out and then taken up by the capillary network

47
Q

What are the mechanisms by which Na+ is taken up from the gut lumen?

A

passive diffusion

apical membrane carrier proteins

48
Q

What are the four different pathways for passive diffusion?

A

Organic substrate Na/glucose co-transporter
Na/amac co-transporter
Na/H antiport
Na/Cl symport (ileum)

49
Q

How is Na+ and Cl- absorption linked?

A

Na+ absorption with nutrients provides an electrochemical gradient for Cl- absorption to occur

50
Q

What is special about Na+ absorption with nutrients?

A

it is electrogenic

51
Q

What is the effect of Na+ absorption with nutrients being electrogenic?

A

leads to net negative charge in the lumen and net positive charge in the paracellular spaces)

52
Q

What is the dominant mechanism for Cl- absorption in the SI that occurs in the proximal ileum?

A

Cotransport with Na+

53
Q

What is the dominant mechanism for Cl- absorption in the SI that occurs in the distal SI?

A

Counter-transport in exchange for HCO3-

54
Q

Why is the mechanism for Cl- absorption in the SI most dominant in the distal SI?

A

bc the conc of Na+ ions in the lumen decreases bc the majority of Na+ ions have already been absorbed
so they won’t interfere with the bicarbonate and chloride counter transport

55
Q

What happens during the counter transport of bicarbonate and Cl- ions?

A

they’re synthesised in the cell via carbonic anhydrase
secreted into the gut lumen in exchange for Cl-
Cl- < cell < basolateral membrane < interstitial space

56
Q

What are the routes by which chloride ions are absorbed in the SI?

A

down an electrochemical gradient
cotransport with na+
Counter-transport in exchange for HCO3-

57
Q

What is the process of Na+ absorption in the LI driven by?

A

Na+/K+ ATPase in the basolateral membrane

58
Q

How do the Na+ ions in the lumen enter the cells?

A

Na+ channels (facilitated diffusion)
Na+/H+ transporter
No glucose/amino acid carrier activity

59
Q

Why is HCO3- secreted during Cl- absorption in the SI?

A

Cl- absorption is associated with acid production by bacteria
so HCO3- acts as a buffer

60
Q

What is the effect of tight junctions during chlorine absorption in the SI?

A

to ensure no ion backflow into lumen

61
Q

What is the effect of Na+ and Cl- movement during chlorine absorption in the SI?

A

creates osmotic gradient for transcellular water movement

62
Q

What happens when K+ ions become concentrated in lumen as water is absorbed from gut?

A

it may be absorbed down a conc grad or secreted when lumen concentration low (net secretion)

63
Q

What substances regulate absorption? (GAS)

A

glucocorticoids, aldosterone, somatostatin

64
Q

What is aldosterone release triggered by?

A

dehydration

released from the adrenal cortex

65
Q

What is the effect of aldosterone?

A

it acts on the LI to upregulate Na absorption from the gut lumen

66
Q

How does aldosterone cause the LI to upregulate Na absorption?

A

it increases in Na+ channel expression in the apical membrane
it stimulates the Na+/K+ ATPase pump

67
Q

What is the effect of the stimulation of Na+ channels and the Na+/K+ ATPase pump on Na absorption?

A

there is increased uptake and absorption of Na+ ions from the LI
this occurs at the expense of K+, which are then secreted in in a greater amount into into the gut

68
Q

What is the effect of glucocorticoid and somatostatin action?

A

they increase water and NaCl absorption by upregulation of Na+/K+ ATPase pump

69
Q

How are enterocytes in crypts and villi different?

A

they express different combinations of transport proteins - indicates a differing role

70
Q

What is the role of enterocytes on the SI villi?

A

they are absorptive and dominate nutrient transport

71
Q

What are the different locations of enterocytes?

A

on the SI villi

in the crypts

72
Q

What are the roles of enterocytes in the crypts?

A

they are secretory with minimal nutrient transport

they secrete water and electrolytes in the SI and the LI

73
Q

What is the secretion of water and Na+ and Cl- ions driven by?

A

by Cl- ion channels in the apical membrane

74
Q

What type of cells are responsible for the secretion of water and Na+ and Cl- ions?

A

crypt enterocytes

75
Q

What ions are driven across the basolateral membrane by the Na+/K+ ATPase pump?

A

Na+, Cl- and K+ ions

76
Q

What is the role of Na+/K+ ATPase pumps?

A

to establish a Na+ electrochemical gradient across the basolateral membrane
this drives Na+, Cl- and K+ ions through Na+/K+/2Cl- cotransporters into crypt cells

77
Q

How do Cl- ions leave crypt enterocytes and enter the intestinal lumen?

A

via apical Cl- channels including CFTR

78
Q

How do Cl- ions draw Na+ ions into the lumen?

A

they provide electronegativity in the intestinal lumen
this creates an osmotic gradient for water movement into the gut lumen via paracellular routes
AAR, Na+ is drawn into the lumen

79
Q

What causes cystic fibrosis?

A

a deletion in gene for CFTR channel

80
Q

What are the genetics of cystic fibrosis?

A

autosomal recessive

81
Q

Why is the CFTR channel important?

A

bc it is the main Cl- channel in the apical membrane of the gut, pancreatic and airway epitheliums

82
Q

What are the effects of cystic fibrosis?

A

the secretion of sticky mucus and high viscosity of luminal contents

83
Q

How does cystic fibrosis present?

A

with intestinal obstruction and meconium ileus in newborns

84
Q

What is the cause of a meconium ileus?

A

when the meconium in a child’s intestine is even thicker and stickier than normal meconium, creating a blockage in the ileum

85
Q

What condition is meconium ileus associated with?

A

cystic fibrosis

86
Q

What is the effect of cholera toxins?

A

they permanently switch on enterocyte secretion via cAMP

87
Q

What is the cholera toxin secreted by?

A

bacteria vibrio cholera

88
Q

What does bacteria vibrio cholera bind to in the body?

A

the cell receptor on apical membrane of crypt cells

89
Q

How does the bacteria vibrio cholera affect adenylate cyclase?

A

it irreversibly upregulates it

90
Q

What is the role of adenylate cyclase?

A

it’s responsible for the excess formation of cAMP from ATP

this stimulates Cl- secretion via the CFTR Cl- channels

91
Q

What pump does the bacteria vibrio cholera rely on?

A

the Na+/K+ ATPase pump

92
Q

What is the effect of the bacteria vibrio cholera physiologically?

A

a massive Cl- efflux, Na+ and water (via osmosis) into the gut lumen, particularly in the jejunum

93
Q

Is cholera permenant?

A

yes

the effects are only reduced after enterocyte turnover

94
Q

How is cholera treated?

A

oral rehydration therapy

95
Q

What does oral rehydration therapy involve?

A

promotes fluid absorption by coupling Na+ with glucose in solution

96
Q

Which membrane carrier protein is preserved in most diarrhoeal diseases?

A

SGLT-1

97
Q

What is the SGLT-1 membrane carrier protein specific for?

A

Na+-glucose co transport

98
Q

How does the SGLT-1 membrane carrier protein work?

A

it binds two Na+ to one glucose
it transports them into the cell
Cl- follows for electrochemical balance

99
Q

What is the effect of the action by the SGLT-1 membrane carrier protein?

A

it replaces salt and water loss from secretory diarrhoea

100
Q

What is lactose intolerance caused by?

A

a deficiency in the enzyme lactase
lactose is not digested and remains in the lumen
this creates an osmotic gradient to cause secretion of water
results in osmotic diarrhoea

101
Q

What are the two components of lactose?

A

glucose + galactose

102
Q

How does somatostatin regulate absorption?

A

upregulates the Na+/K+ ATPase pump = more h2o & NaCl absorbed
inhibits nutrient absorption

103
Q

What is one of the largest and possibly one of the oldest gene families?

A

ATP-binding cassette transporters

104
Q

Where are CFTR channels located?

A

mainly in the apical membrane of epithelium

105
Q

What do CFTR channels have a role in regulating?

A

transepithelial salt and liquid movement

106
Q

What substances are contained in meconium?

A

cells, protein, fats, and intestinal secretions, like bile

107
Q

What is the consistency of meconium?

A

sticky, thick, dark green

108
Q

When is meconium typically passed?

A

in the first few hours after birth or while still in the womb

109
Q

Where do vibrio cholerae naturally live?

A

in brackish or saltwater where they attach themselves easily to the chitin-containing shells of crabs, shrimps, and other shellfish

110
Q

What happens when vibrio cholera bind to the cell receptor on apical membrane of crypt cells?

A

they irreversibly upregulate adenylate cyclase

111
Q

What are the short term effects of profuse diarrhoea caused by cholera?

A

dehydration and resultant circulatory shock

112
Q

Is cholera dangerous?

A

yes, it’s life threatening

113
Q

What happens if cholera isn’t treated?

A

it leads to permanent effects or death

M104 T2 (15 decks)

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