GI hepatic excretion; digestion and absorption Flashcards
What physiologic roles does hepatic excretion play in GI tract?
- excretion of bilirubin, cholesterol, drugs and toxins
- promotion of intestinal lipid absorption
- delivery of IgA to small intestine
What are the components of bile?
bile salts; phospholipids; cholesterol; bilirubin; water; electrolytes
Where is bile produced and stored?
produced in hepatocytes;
drains into hepatic ducts
=> stored in gallbladder
What are the primary and secondary bile acids made from?
primary=> cholesterol
secondary=> products of bacterial metabolism of primary bile acids in gut
What must ALL bile acids have happen before being secreted into bile?
conjugated w/ glycine or taurine to form their bile salt
What are micelles?
water soluble spheres w/ lipid soluble interior that form from bile salts above a *critical micellar concentration
Why are micelles important?
vital in digestion, transport and absorption of lipid soluble substances from duodenum to distal ileum => allows for bile salts to be reabsorbed
=> enterohepatic recirculation
What is enterohepatic recirculation?
bile salts active reabsorbed and recycled via micelles at distal ileum
What can cause malabsorption, gallstones and *steatorrhea?
lack of reabsorbing mechanisms or distal ileal disease can lead to deficiency of bile salts
What is the primary role for gallbladder?
concentrates and stores bile for release during meals
=> concentrates bile by water and electrolyte absorption
During interdigestive period, how does gallbladder remain filled?
sphincter of Oddi is closed and gallbladder is relaxed
When will the gallbladder release bile?
small peptides & FA in duodenum cause CCK secretion
=> gallbladder contraction and relaxation of sphincter of Oddi
=> ACh helps in process
What is involved in hepatic excretion and a product of heme metabolism?
bilirubin
when is bilirubin taken up and how?
prior to bile secretion=> taken up by hepatocytes and *conjugated w/ glucuronic acid
What happens to bilirubin in large intestine?
deconjugated and metabolized by bacteria to form urobilinogens
What gives feces its brown color?
stercobilins
What is a result of elevated bilirubin?
jaundice
What role does secretin play in regulation of bile secretion?
stimulates secretion of bile w/ high HCO3 content from biliary ductules => does NOT alter bile salt output
What is the main regulator of bile acid secretion?
secretion of bile salts by hepatocytes is directly proportional to hepatic portal vein concentration of bile salts
What is digested and absorbed in small intestine?
carbs, proteins, lipids
What aids the small intestine to absorb nutrients?
brush border of small intestine increases surface area
Regarding carbohydrate digestion, how are they absorbed?
converted to monosaccharides to be absorbed
Where does carb digestion start? How does it work?
mouth has salivary amylase => hydrolyzes 10-20% of ingested starch
=> hydrolyzes only a(1:4)-glycosidic linkages to maltose, maltotriose and a-limit dextrins
Where is pancreatic amylase found in high [ ]? what is its function?
duodenal lumen
=>rapidly hydrolyzes starch to oligosaccharides, maltose, maltotriose, a-limit dextrins
what are found in high concentrations in brush border of mid-jejunum and proximal ileum?
maltase; a-dextrinase; lactase; sucrase; isomaltase
action of alpha-dextrinase
cleaves terminal a-1,4 bonds => produces free glucose
action of lactase
converts lactose to glucose and galactose
action of sucrase
converts sucrose to glucose and fructose
action of maltase
converts maltose and maltotriose to 2 and 3 glucose units
What are the monosaccharide end products? where are they absorbed?
glucose, galactose, fructose
=> absorbed from small intestine; jejunum primarily
What does a lactase deficiency cause? consequence
inability to digest lactose into glucose and galactose
=> increase osmotic load => osmotic diarrhea; flatulence
How are glucose and galactose absorbed at luminal membrane?
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
how is fructose absorbed at luminal membrane?
GLUT-5 transporter
How is glucose, galactose and fructose absorbed at basolateral membrane?
transported across basolateral membrane via GLUT-2 transporter
How are lipids digested in the stomach?
- pulverized (decrease size, increase surface area);
* CCK slows gastric emptying to allow enough time for digestion and absorption in small intestine
how are lipids digested in small intestine?
bile acid micelles emulsify fat;
pancreatic lipases digest fat
What do pancreatic enzymes do to fat in the small intestine?
fats hydrolyzed to FFA, monoacylglycerols (cholesterol, fat soluble vitamins A,D,E,K) that collect in micelles
What is the role of micelles in lipid absorption?
carry products of fat digestion in aqueous fluid of gut lumen to brush border => diffuse into enterocyte
Role of enterocytes in lipid absorption?
*re-esterify FA to form TAG, phospholipids and cholesteryl esters => incorporated into apo-proteins and chylomicrons
role of chylomicrons in lipid absorption?
*released by exocytosis into intercellular spaces => enter lacteals of lymph system
=> enter venous circulation via thoracic duct
What are the outcomes of glycerol from lipid absorption?
end up in portal blood to be
1) oxidized for energy
2) stored as glycogen
What does not require micelle formation for absorption? Why?
TAGs w/ medium and short chain FA => hydrolyzed quickly
- undergo little re-esterification
- absorbed directly into portal venous system
What causes abetalipoproteinemia?
deficiency of apoprotein B causing an inability to transport chylomicrons out of intestinal cells
Where and how does protein digestion begin?
pepsin in the stomach
=>functions best at pH 2; irreversibly deactivated at pH 5
=> deactivated in duodenum
is pepsin essential for protein digestion?
no => it is not an essential enzyme
How are proteins digested in the small intestine? are the associated enzymes required?
pancreatic proteases activated by brush border peptidases
yes=> essential enzymes => trypsin, chymotrypsin, elastase, carboxypeptidases A and B
How are amino acids from protein digestion absorbed at the luminal membrane
*Na+ dependent AA cotransport
how are dipeptides and tripeptides absorbed at luminal membrane?
*H+ dependent cotransport mechanism
How are dipepetides and tripeptides absorbed at basal membrane?
hydrolyzed to AA intracellularly
how are AA transported through basal membrane?
to the blood by facilitated diffusion
What is Hartnup disease?
disorder that neutral AA cannot be absorbed
Where does water and electrolyte absorption take place?
primarily in small intestine
How and where is Na absorbed?
- proximal intestine=> Na/H; Na/glucose; Na/aa; Na/Cl cotransports and passive diffusion
- colon=> aldosterone stimulates passive diffusion
How and where is Cl absorbed?
proximal intestine and colon via
=> Na/Cl cotransport; Cl/HCO3 exchange; passive diffusion
Where and how is K absorbed or secreted?
absorbed in small intestine=> passive diffusion
secretion in colon=> via aldosterone
Where and how is Ca+ absorbed?
small intestine => vit D dependent carrier
Where is H20 absorbed?
isoosmotic absorption in gallbladder and small intestine;
permeability is lower in colon
Where and how is iron absorbed?
duodenum primarily=> absorbed as free Fe+2 or as heme iron
Once absorbed, what happens to Fe2+?
bound to transferrin in blood
What are the different types of vitamins that are absorbed?
fat soluble=> A,D,E,K
water soluble
Vitamin B12
Where are fat soluble vitamins (ADEK) absorbed?
incorporated into micelles and absorbed
How are water soluble vitamins absorbed?
via Na+ dependent co-transporters
How is vitamin B12 absorbed?
occurs in ileum => transported and bound to intrinsic factor
What occurs if there is a decrease in intrinsic factor?
this might occur due to a gastrectomy => PERNICIOUS ANEMIA
Where and how does water and electrolyte secretion occur?
secretion in crypts => Cl- main ion secreted
occurs via cAMP regulated channels in luminal membrane
If cholera toxin is present in the GI tract, what will result and why?
SECRETORY DIARRHEA => it stimulates adenylate cyclase => increase cAMP => open Cl channels => Na and H2O follow => secretory diarrhea
