GI hepatic excretion; digestion and absorption

Question 1

What physiologic roles does hepatic excretion play in GI tract?

Answer 1

• excretion of bilirubin, cholesterol, drugs and toxins
• promotion of intestinal lipid absorption
• delivery of IgA to small intestine

Question 2

What are the components of bile?

Answer 2

bile salts; phospholipids; cholesterol; bilirubin; water; electrolytes

Question 3

Where is bile produced and stored?

Answer 3

produced in hepatocytes;
drains into hepatic ducts
=> stored in gallbladder

Question 4

What are the primary and secondary bile acids made from?

Answer 4

primary=> cholesterol

secondary=> products of bacterial metabolism of primary bile acids in gut

Question 5

What must ALL bile acids have happen before being secreted into bile?

Answer 5

conjugated w/ glycine or taurine to form their bile salt

Question 6

What are micelles?

Answer 6

water soluble spheres w/ lipid soluble interior that form from bile salts above a *critical micellar concentration

Question 7

Why are micelles important?

Answer 7

vital in digestion, transport and absorption of lipid soluble substances from duodenum to distal ileum => allows for bile salts to be reabsorbed
=> enterohepatic recirculation

Question 8

What is enterohepatic recirculation?

Answer 8

bile salts active reabsorbed and recycled via micelles at distal ileum

Question 9

What can cause malabsorption, gallstones and *steatorrhea?

Answer 9

lack of reabsorbing mechanisms or distal ileal disease can lead to deficiency of bile salts

Question 10

What is the primary role for gallbladder?

Answer 10

concentrates and stores bile for release during meals

=> concentrates bile by water and electrolyte absorption

Question 11

During interdigestive period, how does gallbladder remain filled?

Answer 11

sphincter of Oddi is closed and gallbladder is relaxed

Question 12

When will the gallbladder release bile?

Answer 12

small peptides & FA in duodenum cause CCK secretion
=> gallbladder contraction and relaxation of sphincter of Oddi
=> ACh helps in process

Question 13

What is involved in hepatic excretion and a product of heme metabolism?

Answer 13

bilirubin

Question 14

when is bilirubin taken up and how?

Answer 14

prior to bile secretion=> taken up by hepatocytes and *conjugated w/ glucuronic acid

Question 15

What happens to bilirubin in large intestine?

Answer 15

deconjugated and metabolized by bacteria to form urobilinogens

Question 16

What gives feces its brown color?

Answer 16

stercobilins

Question 17

What is a result of elevated bilirubin?

Answer 17

jaundice

Question 18

What role does secretin play in regulation of bile secretion?

Answer 18

stimulates secretion of bile w/ high HCO3 content from biliary ductules => does NOT alter bile salt output

Question 19

What is the main regulator of bile acid secretion?

Answer 19

secretion of bile salts by hepatocytes is directly proportional to hepatic portal vein concentration of bile salts

Question 20

What is digested and absorbed in small intestine?

Answer 20

carbs, proteins, lipids

Question 21

What aids the small intestine to absorb nutrients?

Answer 21

brush border of small intestine increases surface area

Question 22

Regarding carbohydrate digestion, how are they absorbed?

Answer 22

converted to monosaccharides to be absorbed

Question 23

Where does carb digestion start? How does it work?

Answer 23

mouth has salivary amylase => hydrolyzes 10-20% of ingested starch

=> hydrolyzes only a(1:4)-glycosidic linkages to maltose, maltotriose and a-limit dextrins

Question 24

Where is pancreatic amylase found in high [ ]? what is its function?

Answer 24

duodenal lumen

=>rapidly hydrolyzes starch to oligosaccharides, maltose, maltotriose, a-limit dextrins

Question 25

what are found in high concentrations in brush border of mid-jejunum and proximal ileum?

Answer 25

maltase; a-dextrinase; lactase; sucrase; isomaltase

Question 26

action of alpha-dextrinase

Answer 26

cleaves terminal a-1,4 bonds => produces free glucose

Question 27

action of lactase

Answer 27

converts lactose to glucose and galactose

Question 28

action of sucrase

Answer 28

converts sucrose to glucose and fructose

Question 29

action of maltase

Answer 29

converts maltose and maltotriose to 2 and 3 glucose units

Question 30

What are the monosaccharide end products? where are they absorbed?

Answer 30

glucose, galactose, fructose => absorbed from small intestine; jejunum primarily

Question 31

What does a lactase deficiency cause? consequence

Answer 31

inability to digest lactose into glucose and galactose | => increase osmotic load => osmotic diarrhea; flatulence

Question 32

How are glucose and galactose absorbed at luminal membrane?

Answer 32

compete for transport across brush border by Na+ dependent coporter (SGLT-1) =>Na+ then pumped out by Na-K ATPase =>sugars move against concentration gradient into enterocyte

Question 33

how is fructose absorbed at luminal membrane?

Answer 33

GLUT-5 transporter

Question 34

How is glucose, galactose and fructose absorbed at basolateral membrane?

Answer 34

transported across basolateral membrane via GLUT-2 transporter

Question 35

How are lipids digested in the stomach?

Answer 35

* pulverized (decrease size, increase surface area); | * CCK slows gastric emptying to allow enough time for digestion and absorption in small intestine

Question 36

how are lipids digested in small intestine?

Answer 36

bile acid micelles emulsify fat; | pancreatic lipases digest fat

Question 37

What do pancreatic enzymes do to fat in the small intestine?

Answer 37

fats hydrolyzed to FFA, monoacylglycerols (cholesterol, fat soluble vitamins A,D,E,K) that collect in micelles

Question 38

What is the role of micelles in lipid absorption?

Answer 38

carry products of fat digestion in aqueous fluid of gut lumen to brush border => diffuse into enterocyte

Question 39

Role of enterocytes in lipid absorption?

Answer 39

*re-esterify FA to form TAG, phospholipids and cholesteryl esters => incorporated into apo-proteins and chylomicrons

Question 40

role of chylomicrons in lipid absorption?

Answer 40

*released by exocytosis into intercellular spaces => enter lacteals of lymph system => enter venous circulation via thoracic duct

Question 41

What are the outcomes of glycerol from lipid absorption?

Answer 41

end up in portal blood to be 1) oxidized for energy 2) stored as glycogen

Question 42

What does not require micelle formation for absorption? Why?

Answer 42

TAGs w/ medium and short chain FA => hydrolyzed quickly - undergo little re-esterification - absorbed directly into portal venous system

Question 43

What causes abetalipoproteinemia?

Answer 43

deficiency of apoprotein B causing an inability to transport chylomicrons out of intestinal cells

Question 44

Where and how does protein digestion begin?

Answer 44

pepsin in the stomach =>functions best at pH 2; irreversibly deactivated at pH 5 => deactivated in duodenum

Question 45

is pepsin essential for protein digestion?

Answer 45

no => it is not an essential enzyme

Question 46

How are proteins digested in the small intestine? are the associated enzymes required?

Answer 46

pancreatic proteases activated by brush border peptidases yes=> essential enzymes => trypsin, chymotrypsin, elastase, carboxypeptidases A and B

Question 47

How are amino acids from protein digestion absorbed at the luminal membrane

Answer 47

*Na+ dependent AA cotransport

Question 48

how are dipeptides and tripeptides absorbed at luminal membrane?

Answer 48

*H+ dependent cotransport mechanism

Question 49

How are dipepetides and tripeptides absorbed at basal membrane?

Answer 49

hydrolyzed to AA intracellularly

Question 50

how are AA transported through basal membrane?

Answer 50

to the blood by facilitated diffusion

Question 51

What is Hartnup disease?

Answer 51

disorder that neutral AA cannot be absorbed

Question 52

Where does water and electrolyte absorption take place?

Answer 52

primarily in small intestine

Question 53

How and where is Na absorbed?

Answer 53

* proximal intestine=> Na/H; Na/glucose; Na/aa; Na/Cl cotransports and passive diffusion * colon=> aldosterone stimulates passive diffusion

Question 54

How and where is Cl absorbed?

Answer 54

proximal intestine and colon via | => Na/Cl cotransport; Cl/HCO3 exchange; passive diffusion

Question 55

Where and how is K absorbed or secreted?

Answer 55

absorbed in small intestine=> passive diffusion secretion in colon=> via aldosterone

Question 56

Where and how is Ca+ absorbed?

Answer 56

small intestine => vit D dependent carrier

Question 57

Where is H20 absorbed?

Answer 57

isoosmotic absorption in gallbladder and small intestine; permeability is lower in colon

Question 58

Where and how is iron absorbed?

Answer 58

duodenum primarily=> absorbed as free Fe+2 or as heme iron

Question 59

Once absorbed, what happens to Fe2+?

Answer 59

bound to transferrin in blood

Question 60

What are the different types of vitamins that are absorbed?

Answer 60

fat soluble=> A,D,E,K water soluble Vitamin B12

Question 61

Where are fat soluble vitamins (ADEK) absorbed?

Answer 61

incorporated into micelles and absorbed

Question 62

How are water soluble vitamins absorbed?

Answer 62

via Na+ dependent co-transporters

Question 63

How is vitamin B12 absorbed?

Answer 63

occurs in ileum => transported and bound to intrinsic factor

Question 64

What occurs if there is a decrease in intrinsic factor?

Answer 64

this might occur due to a gastrectomy => PERNICIOUS ANEMIA

Question 65

Where and how does water and electrolyte secretion occur?

Answer 65

secretion in crypts => Cl- main ion secreted occurs via cAMP regulated channels in luminal membrane

Question 66

If cholera toxin is present in the GI tract, what will result and why?

Answer 66

SECRETORY DIARRHEA => it stimulates adenylate cyclase => increase cAMP => open Cl channels => Na and H2O follow => secretory diarrhea