Lecture 11

Question 1

What all must the GI system expend for digestion?

Answer 1

Has to secrete an abundance of protein & electrolytes, 6-7 litres of fluid
This costs a lot of ATP to power (expensive process)

Need to recover nutrients, electrolytes, and fluids that are in a meal and that were secreted while digesting a meal and excrete other products that aren’t needed
Need to rightly control the rate of secretion and absorption to preserve health

Question 2

What is the cost of the day to day business eating?

Answer 2

Average person who eats 2L per day
Salivary glands secrete 1.5L
Gastric secretions amount to 2.0L
Pancreatic secretions amount to 1.5L
Bile secretions amount to 0.5L
Small intestine secretes 1.0L
Almost all is recovered, with only about 0.1L of material undergoes defecation 

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Question 3

Where does absorption occur?

Answer 3

Small intestine- where nutrients, Na, K, Cl, and water are absorbed, high surface area of 200m squares enables the absorption of 6.5L per day and up to 20L to day

Large intestine- nutrients, are not absorbed in this location, absorbs mainly electrolytes and water, with a normal day to day absorption rate of almost 2L, can be as high as 5L

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Question 4

What are the secretion functions of the GI tract?

Answer 4

Most secretion is done by the small intestine, might be adaptive function for removing noxious stimuli
Large intestine is the only location is the only location where K secretion occurs
Bicarbonate is secreted in both locations

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Question 5

What is cellular heterogeneity and what does it separate?

Answer 5

Separates functions:

1. Absorption performed by villous absorptive cells in the small intestine & surface absorptive cells in the large intestines
2. Secretion seems to be performed by crypt cells

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Question 6

What is segmental heterogeneity and what does it separate?

Answer 6

Separates functions:

1. Mechanism of absorption can change depending on the location in the intestine
2. Example: bicarbonate induced alkali conditions stimulates Na absorption, but only in the duodenum, whereas electrogenic mediated Na uptake only occurs in the distal colon

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Question 7

What are the 3 role of tight junctions?

Answer 7

1. Neighbouring cells form tight junctions, which prevent unwanted material in the intestinal lumen from passing through
2. Electrolytes and water can pass through this path, which is called Paracellular transport
3. Otherwise everything passes through Transcellular pathways

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Question 8

What are the cellular mechanisms of water and electrolyte transport?

Answer 8

Multiple mechanisms
Solute movement is the driving force for fluid (water) movement, resulting from diff osmotic gradients
Fluid movement can move solvent (solvent drag) in some cases
Na, Cl, K movement have different mechanisms

If it’s paracellular than it is a passive process, if it is transcellular than at least one of the membranes (basolateral or apical) requires an active transport mechanism

Question 9

What do Na/glucose or Na/amino acid cotransporters do in the cellular mechanisms of Na absorption?

Answer 9

Most significant contributor to Na absorption during post prandial phase, but make little contribution during the interdigestive phase
Secondary active transport since it powers the uptake of the nutrients glucose and amino acids
Not regulated by second messengers such as cAMP

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Question 10

What do Na/H exchangers do in the cellular mechanisms of Na absorption?

Answer 10

Main function is to neutralize the pH when it becomes too high (as a result of pancreatic duct secretions of bicarbonate)
The Na gradient powers the secretion of acid into the lumen (this process can be inhibited by the diuretic amiloride)

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Question 11

What do Na/H & Cl/HCO3 exchangers do in the cellular mechanisms of Na absorption?

Answer 11

Occurs in small and large intestine
Driven by parallel co exchangers that are powered by pH
Primary mechanism of Na absorption between meals
Process is under control of cAMP, cGMP, and cytosolic calcium levels
When these 2nd messengers rise, Na absorption drops, conversely, the second messengers drop, Na absorption rises

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Question 12

What do epithelial Na channels do in the cellular mechanisms of Na absorption?

Answer 12

Channels in the distal colon that are highly specific for Na
It moves down its concentration gradient to enter the cells
It can be enhanced by aldosterone, which increases the conductance of the channel and the activity of the Na-K ATPase
This channel can also be inhibited by amiloride

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Question 13

What does passive Cl absorption do in the cellular mechanisms of Cl absorption?

Answer 13

Can be paracellular or transcellular
Passive process resulting from an electrochemical gradient
Driven by electrogenic Na absorption

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Question 14

What do Cl-HCO3 exchangers do in the cellular mechanisms of Cl absorption?

Answer 14

Electrochemical process in which Cl is exchanged for HCO3
Occurs in the absence of Na/H exchanger

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Question 15

What do Na/H & Cl/HCO3 exchangers do in the cellular mechanisms of Cl absorption?

Answer 15

Occurs in small and large intestine
Driven by parallel co exchangers that are powered by pH
Primary mechanism of Na absorption between meals
Process is under control of cAMP, cGMP, and cytosolic calcium levels
When these 2nd messengers rise, Na absorption drops, conversely, the second messengers drop, Na absorption rises

Same as couple flashcards ago

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Question 16

What is the cellular mechanism of electrogenic Cl secretion?

Answer 16

Small and large intestine
Low activity in basal state, can be stimmed via secretagogues such as ACh and any agent that increases cAMP, cGMP, or cytosolic calcium levels
Low intracellular Na drives Cl uptake through the Na/K/Cl cotransporter
This raises intracellular Cl sufficiently to undergo conductance through the CTFR channel
In parallel this can promote Na to undergo paracellular transport into the lumen
Secretagogues activate this process and increase level of CTFR channels

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Question 17

What are the 2 cellular mechanisms of K absorption?

Answer 17

1. Passive K absorption- occurs in small intestine via solvent drag
2. Active K absorption- distal end of the colon can absorb K, a reduction in dietary K enhances this process, is an energy dependant process in which a proton is exchanged for K

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Question 18

What are the 2 cellular mechanisms of K secretion?

Answer 18

1. Passive K secretion- main process for K secretion, driven by an electrochemical gradient and occurs via paracellular route
   Occurs in large intestine via solvent drag
2. Active K secretion- plays a smaller role in K secretion and can be stimulated by cAMP and cytosolic calcium levels

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Question 19

What are secretagogues role in the regulation of intestinal ion transport?

Answer 19

(Neural and paracrine)
Enteric nervous system, via ACh, can stimulate Cl secretion from crypt cells
Involves change in the level of cAMP, cGMP, or intracellular calcium

Question 20

What are the absorptagogues role in the regulation of intestinal ion transport?

Answer 20

Aldosterone and angiotensin stimulate Na absorption
Angiotensin stimulates electroneutral NaCl absorption in the small intestine, whereas aldosterone stimulates electrogenic Na absorption in the distal colon

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