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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
How does sodium potassium ATPase keep Na+ levels in the cell low?
3 Na+ exported and 2 K+ imported against electrochemical gradient using E from ATP hydrolysis by ATPase Keeps Na+ cytosol concentration low
26
What affects bidirectional fluid flux under the principles of enterocyte transport?
the tonicity of chyme entering duodenum
27
What is the polarity of enterocytes like?
they are polarised with an apical and basolateral membrane
28
How is the free flow of gut lumen contents restricted?
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
29
By what two routes can the transfer of products occur?
transcellular routes | paracellular routes
30
How are products transfered via transcellular routes?
occurs through the enterocyte itself | may be against conc grad and require ATP
31
How are products transfered via paracellular routes?
occurs between cells | doesn't require E
32
What are the three different types of transport proteins involved in transcellular electrolyte transport?
pumps - against concentrations channels carriers
33
What are the three different types of electrolyte transport?
Passive Solvent drag Active
34
How does passive electrolyte transport work?
down electrochemical gradient through ion channels or carriers or permeable tight junctions
35
How does solvent drag electrolyte transport work?
water follows Na+ gradient via osmosis, taking other ions
36
How does active electrolyte transport work?
requires ATP the Na+/K+ ATPase pump depletes cellular Na + and draws Na + across apical membrane from gut lumen via channel or cotransporter
37
What is an example of solvent drag electrolyte transport?
in the upper intestine where tight junctions more permeable
38
What happens when chyme enters the duodenum from the stomach?
rapid osmotic equilibration of the chyme occurs forms isotonic chyme into hypertonic chyme from hypotonic chyme
39
What happens to hypertonic and hypotonic chyme after undergoing rapid osmotic equilibration?
hyper - water is secreted into it | hypo - water is absorbed from it
40
What volume of chyme enters the LI?
~2 L
41
What volume of chyme is excreted from the LI?
100-200 ml
42
What is the role of tight junctions in the LI?
they prevent back-diffusion of ions into the lumen | this allows for a more complete NaCl absorption, so there is limited loss in faeces
43
What substances are secreted in the LI?
HCO3- and K+
44
Why is sodium absorption important?
bc it drives the absorption of ions, organics and water
45
By what process are Na+ ions absorbed?
basolateral active transport into the interstitial space occurs
46
How are Na+ ions absorbed?
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
What are the mechanisms by which Na+ is taken up from the gut lumen?
passive diffusion | apical membrane carrier proteins
48
What are the four different pathways for passive diffusion?
Organic substrate Na/glucose co-transporter Na/amac co-transporter Na/H antiport Na/Cl symport (ileum)
49
How is Na+ and Cl- absorption linked?
Na+ absorption with nutrients provides an electrochemical gradient for Cl- absorption to occur
50
What is special about Na+ absorption with nutrients?
it is electrogenic
51
What is the effect of Na+ absorption with nutrients being electrogenic?
leads to net negative charge in the lumen and net positive charge in the paracellular spaces)
52
What is the dominant mechanism for Cl- absorption in the SI that occurs in the proximal ileum?
Cotransport with Na+
53
What is the dominant mechanism for Cl- absorption in the SI that occurs in the distal SI?
Counter-transport in exchange for HCO3-
54
Why is the mechanism for Cl- absorption in the SI most dominant in the distal SI?
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
What happens during the counter transport of bicarbonate and Cl- ions?
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
What are the routes by which chloride ions are absorbed in the SI?
down an electrochemical gradient cotransport with na+ Counter-transport in exchange for HCO3-
57
What is the process of Na+ absorption in the LI driven by?
Na+/K+ ATPase in the basolateral membrane
58
How do the Na+ ions in the lumen enter the cells?
Na+ channels (facilitated diffusion) Na+/H+ transporter No glucose/amino acid carrier activity
59
Why is HCO3- secreted during Cl- absorption in the SI?
Cl- absorption is associated with acid production by bacteria so HCO3- acts as a buffer
60
What is the effect of tight junctions during chlorine absorption in the SI?
to ensure no ion backflow into lumen
61
What is the effect of Na+ and Cl- movement during chlorine absorption in the SI?
creates osmotic gradient for transcellular water movement
62
What happens when K+ ions become concentrated in lumen as water is absorbed from gut?
it may be absorbed down a conc grad or secreted when lumen concentration low (net secretion)
63
What substances regulate absorption? (GAS)
glucocorticoids, aldosterone, somatostatin
64
What is aldosterone release triggered by?
dehydration | released from the adrenal cortex
65
What is the effect of aldosterone?
it acts on the LI to upregulate Na absorption from the gut lumen
66
How does aldosterone cause the LI to upregulate Na absorption?
it increases in Na+ channel expression in the apical membrane it stimulates the Na+/K+ ATPase pump
67
What is the effect of the stimulation of Na+ channels and the Na+/K+ ATPase pump on Na absorption?
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
What is the effect of glucocorticoid and somatostatin action?
they increase water and NaCl absorption by upregulation of Na+/K+ ATPase pump
69
How are enterocytes in crypts and villi different?
they express different combinations of transport proteins - indicates a differing role
70
What is the role of enterocytes on the SI villi?
they are absorptive and dominate nutrient transport
71
What are the different locations of enterocytes?
on the SI villi | in the crypts
72
What are the roles of enterocytes in the crypts?
they are secretory with minimal nutrient transport | they secrete water and electrolytes in the SI and the LI
73
What is the secretion of water and Na+ and Cl- ions driven by?
by Cl- ion channels in the apical membrane
74
What type of cells are responsible for the secretion of water and Na+ and Cl- ions?
crypt enterocytes
75
What ions are driven across the basolateral membrane by the Na+/K+ ATPase pump?
Na+, Cl- and K+ ions
76
What is the role of Na+/K+ ATPase pumps?
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
How do Cl- ions leave crypt enterocytes and enter the intestinal lumen?
via apical Cl- channels including CFTR
78
How do Cl- ions draw Na+ ions into the lumen?
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
What causes cystic fibrosis?
a deletion in gene for CFTR channel
80
What are the genetics of cystic fibrosis?
autosomal recessive
81
Why is the CFTR channel important?
bc it is the main Cl- channel in the apical membrane of the gut, pancreatic and airway epitheliums
82
What are the effects of cystic fibrosis?
the secretion of sticky mucus and high viscosity of luminal contents
83
How does cystic fibrosis present?
with intestinal obstruction and meconium ileus in newborns
84
What is the cause of a meconium ileus?
when the meconium in a child's intestine is even thicker and stickier than normal meconium, creating a blockage in the ileum
85
What condition is meconium ileus associated with?
cystic fibrosis
86
What is the effect of cholera toxins?
they permanently switch on enterocyte secretion via cAMP
87
What is the cholera toxin secreted by?
bacteria vibrio cholera
88
What does bacteria vibrio cholera bind to in the body?
the cell receptor on apical membrane of crypt cells
89
How does the bacteria vibrio cholera affect adenylate cyclase?
it irreversibly upregulates it
90
What is the role of adenylate cyclase?
it's responsible for the excess formation of cAMP from ATP | this stimulates Cl- secretion via the CFTR Cl- channels
91
What pump does the bacteria vibrio cholera rely on?
the Na+/K+ ATPase pump
92
What is the effect of the bacteria vibrio cholera physiologically?
a massive Cl- efflux, Na+ and water (via osmosis) into the gut lumen, particularly in the jejunum
93
Is cholera permenant?
yes | the effects are only reduced after enterocyte turnover
94
How is cholera treated?
oral rehydration therapy
95
What does oral rehydration therapy involve?
promotes fluid absorption by coupling Na+ with glucose in solution
96
Which membrane carrier protein is preserved in most diarrhoeal diseases?
SGLT-1
97
What is the SGLT-1 membrane carrier protein specific for?
Na+-glucose co transport
98
How does the SGLT-1 membrane carrier protein work?
it binds two Na+ to one glucose it transports them into the cell Cl- follows for electrochemical balance
99
What is the effect of the action by the SGLT-1 membrane carrier protein?
it replaces salt and water loss from secretory diarrhoea
100
What is lactose intolerance caused by?
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
What are the two components of lactose?
glucose + galactose
102
How does somatostatin regulate absorption?
upregulates the Na+/K+ ATPase pump = more h2o & NaCl absorbed inhibits nutrient absorption
103
What is one of the largest and possibly one of the oldest gene families?
ATP-binding cassette transporters
104
Where are CFTR channels located?
mainly in the apical membrane of epithelium
105
What do CFTR channels have a role in regulating?
transepithelial salt and liquid movement
106
What substances are contained in meconium?
cells, protein, fats, and intestinal secretions, like bile
107
What is the consistency of meconium?
sticky, thick, dark green
108
When is meconium typically passed?
in the first few hours after birth or while still in the womb
109
Where do vibrio cholerae naturally live?
in brackish or saltwater where they attach themselves easily to the chitin-containing shells of crabs, shrimps, and other shellfish
110
What happens when vibrio cholera bind to the cell receptor on apical membrane of crypt cells?
they irreversibly upregulate adenylate cyclase
111
What are the short term effects of profuse diarrhoea caused by cholera?
dehydration and resultant circulatory shock
112
Is cholera dangerous?
yes, it's life threatening
113
What happens if cholera isn't treated?
it leads to permanent effects or death

M104 T2 (15 decks)

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