Digestion and Absorption II
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- What makes lipids soluble so that they can be digested?
- bile added to the intestine in the duodenum acts as a detergent
- What is bile made up of?
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-bile acids (60%)
-water and electrolytes
-cholesterol
-phospholipids
-proteins
-wastes (bilirubin, drug metabolites) - Where are bile acids made?
- the liver
- What is the functional unit of the liver?
- the liver lobule
- What is the significance of having fenestrated sinusoids in the liver?
- they allow direct contact and exchange between the blood and the hepatocyte -- hepatocyte can remove large amts of material from the blood
- What is bile made from?
- cholesterol
- Are bile salts coming into the liver in the hepatic vein reused or discarded?
- they are reused by being imported into hepatocyte via Na+ dependent transport, then go out into bile canniliculi
- Which hormone stimulates secretion of HCO3- in the duct cells of the bile duct?
- secretin
- Where does bile empty into the duodenum?
- sphincter of oddi via the common bile duct (it is combined with pancreatic juices from the pancreatic duct here)
- How much does the gall bladder concentrate bile?
- 10-20x
- What reflexes control the contraction of the gall bladder and relaxation of the sphincter of oddi after a fatty meal?
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1) neural (local and vagal)
2) hormonal (CCK) - Whare are bile salts reabsorbed in the intestine?
-
the terminal ileum
(actively via Na linked carrier mechanisms and passively due to lipid solubility) - How often is the bile salt pool recirculated (enterohepatic circulation)?
- 2x during a normal meal
- How much bile salts are made daily?
- about 20% to match amount lost in feces daily
- Name 2 examples of primary bile acids (synthesized de novo by hepatocytes from cholesterol).
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1) cholic acid
2) chenodeoxycholic acid - What is the difference between primary and secondary bile acids?
- secondary bile acids are modified by bacteria in the intestine which dehydroxylate them
- Name some examples of secondary bile acids.
-
1) deoxycholic acid
(from cholic acid)
2) lithocholic acid
(from chenodeoxycholic acid) - Which is better at digesting fats, primary or secondary bile acids?
-
primary bile acids
- because as lose OH-, less able to digest fats (secondary bile acids have been dehydrated) - What are bile acids conjugated to and what is the result of the conjugation?
-
- conjugated to taurine or glycine
- makes more hydrophilic thus better at solubilizing fats - What is Critical Micelle Concentration?
-
a defined concentration of bile salts above which bile salts spontaneously form micelles
(it is specific to each bile salt) - How much do micelles increase the solubility of a FA?
-
1000x
(get digestion where lipase can contact lipid) - What stabilizes lipid droplets to a greater extent, an emulsion or micelles?
- micelles
- What is the main form of ingested lipids?
- TGs
- What molecule must pancreatic lipase work with to act at oil/water interface to begin hydrolyzing ester linkages?
- Co-lipase
- Explain the cycle of a micelle.
- - pancreatic lipase/co-lipase bkdn lipids into 2FFAs and 1 2-MG - these + chol, PL etc form micelles - move to gut wall - release contents to cross gut wall - return to lumen
- Where in the small intestine does most of the absorption of lipolytic products occur?
-
in the jejunum
(take as while to "get going") - What 2 things does the movement of Na+ in the gut depend on?
-
1) region of the gut
2) state of the gut (postprandial or interdigestive) - What kind of Na+ absorption predominates in the postprandial state?
-
nutrient-coupled (apical)
(Na/K ATPase (basolateral)) - What kind of Na+ absorption predominates in the interdigestive state?
-
electroneutral NaCl absorption (apical)
(Na/K ATPase (basolateral)) - How does most water enter enterocytes in the intestine?
-
thru the paracellular route due to "leaky" tight junctions
(less water absorption due to tight juctions when you move distally) - What 3 ways can change the "leakiness" of tight junctions in enterocytes?
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1) NTs
2) hormones
3) nutrients (ex. glucose) - Is water and sodium absorption in the intestine energy-dependent?
- yes - need ATP to move Na+ out of cells at basolateral surface via Na/K ATPase
- Is K+ passively or actively absorbed in the small intestine?
- passively when concentrations increase in ECF
- Does the colon secrete or absorb K+?
-
secretes at luminal conc <25mM
absorbs at luminal conc 25+ - List the state of Cl- and HCO3- in the small intestine and colon (in terms of secretion or absorption).
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proximal duodenum - secretion of HCO3-
jejunum - net absorption of both HCO3- and Cl-
ileum and colon - chloride absorbed, HCO3- secreted - What 2 things increase the absorption of Ca?
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1) PTH
2) Vitamin D - What distinguishes absorbed Ca (via intestine) and Ca for an AP?
- absorbed Ca binds to a brush border membrane carrier protein, then a cytosolic Ca binding protein in the cell
- What 2 things promote iron absorption?
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1) acidic pH in stomach
2) Vitamin C - What form of Fe is most soluble?
- Fe2+
- What is the carrier protein associated with iron in the intestinal lumen?
- transferrin
- What is a Mucosal Block in reference to iron?
- iron is stored in cells and is lost when cell is desquamated, chich prevents excess iron from entering blood (which would be toxic)
- Which water soluble vitamins are NOT passively absorbed?
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1) Vit C
2) Biotin
3) Vit B1
4) Vit B12 - Can Vit B12 be absorbed normally?
- No -- it must have intrinsic factor (which is secreted by oxyntic cells of stomac)
- Are fat soluble vitamins solubilized in micelles and chylomicrons like lipids are?
- Yes