29 Digestive and Absorptive Functions of the GI Tract Flashcards
how are carbohydrates ingested?
Starch Disaccharides (mostly lactose and sucrose) Monosaccharides (glucose and fructose)
T/F, Only monosaccharides can be absorbed?
T
Two phases of digestion?
Luminal Brush border (brush border enzymes)
where does carbohydrate absorption occur?
Apical border
Basolateral border
what happens at the apical border?
Sodium-coupled nutrient transport via SGLT-1 (sodium/glucose transporter-1):
Glucose
Galactose
Facilitated diffusion via GLUT 5:
Fructose
what happens at the basolateral border?
All monosaccharides use GLUT2
how does lactose intolerance work?
undigested lactose increases the osmolality of the gut which results in an increase in osmolality. The high osmolality leads to a concomitant increase in a a net water secretion causing diarrhea
What is Hartnup disease?
Autosomal recessive
Difficulty in absorbing non-polar amino acids like tryptophan (which is necessary in the production of serotonin, melanin, and niacin)
- Niacin deficiency results in pellagra
- -4 D’s: Dehydration, Dermatitis, Dementia, Death
What is the importance of micelles?
Enzymatic digestion takes place on the surface of emulsion droplet
Name the disease based on characteristics below:
Autosomal recessive
Difficulty in absorbing non-polar amino acids like tryptophan (which is necessary in the production of serotonin, melanin, and niacin)
Hartnup disease
what is pellagra?
4 D’s: Dehydration, Dermatitis, Dementia, Death
where does the enzymatic digestion of micelles take place?
Enzymatic digestion takes place on the surface of emulsion droplet
products of digestion and micelles form this?
mixed micelles with bile salts
what are the functions of the mixed micelles?
Keep products in suspension
Prevent them from re-esterifying
Transport them to the epithelial surface
what do micelles contain?
cholesterol, fat-soluble vitamins, monoglycerides and phospholipids with bile acids allowing for suspension
where do the micelles travel?
to the apical membrane of the enterocytes and products of fat digestion enter cell by simple diffusion
without micelle formation, what happens to the free fatty acids?
some free fatty acids can be absorbed but not cholesterol and other lipid products.
what happens to the lipid digestion products?
re-esterified with free fatty acid (FFA) within the cell cytoplasm
where are the chylomicrons formed?
in the cell (packed on the Golgi complex) and they contain a lipid core surrounded by phospholipids and Apolipoprotein B
T/F, Fat-soluble vitamins are also incorporated into chylomicrons
T
describe the path of the chylomicrons?
they exit the cell via exocytosis and enter the lymphatic system (lacteals) (not the hepatic portal system like all other absorbed nutrients) and then are returned to vascular system by thoracic duct
where do the chylomicrons not exit?
not the hepatic portal system like all other absorbed nutrients
how does fat malabsorption present?
as steatorrhea (fat in stools, often foul-smelling and floats in toilet) Give a possible cause for each condition and explain why it would lead to fat malabsorption.
What are some symptoms and cases of fat malabsorption?
- Low pH in duodenum
- pancreatic insufficiency
- Loss of surface area in intestines
- Bacterial overgrowth
- Bile salt deficiency
what would cause the low pH in duodenum? why would this lead to malabsorption?
Increased H+ secretion (gastrinoma)
decreased bicarbonate secretion (pancreas disorders)
Digestive enzymes (except pepsin) function at a neutral pH; H+ is also damaging to epithelium preventing adequate function.
what would cause pancreatic insufficiency?
why would this lead to malabsorption?
Chronic pancreatitis (decreased pancreas secretion); pancreatectomy, obstruction (tumor); impaired function (CF)
Pancreatic enzymes are critical for digestion. Gastric, salivary and intestinal enzymes are inadequate.
what would cause the loss of surface area in intestines? why would this lead to malabsorption?
Celiac disease
Large surface area is critical for complete absorption. With insufficient surface chyme is propulsed along the GI tract with inadequate absorption.
what would cause bacterial overgrowth?Why would it lead to malabsorption?
Motility disorders (slow motility), high pH, surgery
Bile salts get deconjugated in the presence of too many bacteria and lose their function and can get prematurely absorbed before the terminal ileum.
what would cause a bile salt deficiency?Why would it lead to malabsorption?
Terminal ileum disease or resection
Bile acids need to be recycled via the enterohepatic circulation (liver cannot make up for high loss rates). Bile salts are critical for suspended fat in aqueous milieu and for micelle formation and fat absorption.
what is the Shilling’s Test?
A test for pernicious anemia that is comprised of two phases
describe the two phases that we test for pernicious anemia?
Phase 1 – Replete patient’s B12 then give radiolabeled B12. If no renal excretion then B12 malabsorption. Go to phase 2
Phase 2 – Give radiolabeled B12 and intrinsic factor. If renal excretion then pernicious anemia (IF deficiency). If no excretion then look for other causes of malabsorption.
what is the difference between the phases?
Phase 1 – Replete patient’s B12 then give radiolabeled B12
Phase 2 – Give radiolabeled B12 and intrinsic factor
In the intestines, where is iron stored?
At the apical border
Inside the cell
At the basolateral membrane
at the apical border how is the heme form absorbed? what about the non-heme form?
Absorbed by the intestinal epithelium intact by facilitated transport
Fe3+ is insoluble at duodenal pH so it is reduced to Fe2+ and absorbed by active transport (Divalent metal transporter or DMT-1)
what is the effect of tea on iron?
Tannic acid in tea can form insoluble complexes with iron and prevent absorption.
what is heme released by inside the cell?
heme oxygenase
so inside the cell, what is the term used to describe the protein that complexes with Fe2+? what is the complex called?
apoferritin; ferritin
what is ferritin?
a storage form of iron and some iron stays in the enterocytes
at the basolateral membrane, where is the iron transported?
to bloodstream where it is bound to transferrin and travels to the liver.
what is the major storage site for iron?
the liver
T/F, the liver synthesizes transferrin
T
what is the main enzyme in the stomach used for protein digestion?
pepsin
what is the main enzyme in the small intestine used for protein digestion?
trypsin(activated by enterokinase) used to activate:
chymotrypsin
elastase
carboxypeptidase A/B
*all pancreatic proteases
what are the three phases of protein digestion?
Gastric
Proteins digested to peptides and oligopeptides
Lumen of intestines
Peptides digested to oligopeptides
Brush Border
Oligopeptides digested to aa’s, di-and tripeptides
what are the ultimate products of protein digestion?
Amino Acids
Di- and tripeptides
how are amino acids transported at the luminal membrane?
Cotransport with Na+
Depends on gradient created by Na+/K+ ATPase
how are Di and Tripeptides transported at the luminal membrane?
Cotransport with H+ (PEPT1)
H+ source is Na/H exchanger (NHE)
how are di and tripeptides processed in the cell? at the basolateral membrane?
into AA’s by peptidase; amino acids and transported via facilitated diffusion
what is the medical condition for patients who cannot form chylomicrons?
Abetalipoproteinemia, Fat can enter the cell but cannot be absorbed into the system. Epithelial cells appear swollen with fat on histologic section.
what could cause Low pH in duodenum?
Increased H+ secretion (gastrinoma), decreased bicarbonate secretion (pancreas disorders)
what could cause Pancreatic insufficiency?
Chronic pancreatitis (decreased pancreas secretion); pancreatectomy, obstruction (tumor); impaired function (CF)
what could cause Loss of surface area in intestines?
Celiac disease
what could cause Bacterial overgrowth?
Motility disorders (slow motility), high pH, surgery
what could cause Bile salt deficiency?
Terminal ileum disease or resection
T/F, Fat soluble vitamins are also incorporated into chylomicrons?
T
what is pellagra?
Pellagra is due to vitamin B3 (niacin) deficiency. It presents with the 3 D’s: dermatitis, diarrhea and psychiatric manifestations (Dementia, Delirium or Depression)
Causes of lipid malabsorption can cause fat soluble vitamin deficiencies, what vitamin?
ADEK
what percent of calcium is absorbed from the GI tract?
40%, mostly absorbed in the duodenum and jejunum, regulated by PTH and circulating plasma concentration and relies on Vitamin D absorption
what are the three roles of Vitamin D?
Increases luminal absorption
Upregulates Calbindin D synthesis
Increases Ca-ATPase activity
