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Flashcards in GI Digestion and absorption Deck (17)

Protein Digestion

Stomach: Pepsin breaks down about 15% of proteins to small peptides

Small Intestine (lumen): Pancreatic proteases like trypsin, chymotrypsin, carboxypeptidase & elastase break down
proteins to oligopeptides, di/tri-peptides and amino acids

Brush Border: Peptidases break down oligopeptides into amino acids, dipeptides, tripeptides

Intracellular peptidases: Peptidases in the enterocyte can break down di/tri-peptides into amino acids


Endo vs exo pepsidases

exo- target bonds on outside
endo- target internal bonds within those proteins


Protein Uptake Pathways

Sodium dependent co-transporters that utilize the N+/K+ ATPase gradient are the major route for the different classes of amino acids. Water follows.

Sodium independent transporters of amino acids

Specific carriers for small peptides (di- and tri-) linked to H+ uptake (co-transporter; example is PEP T1)

Pinocytosis of small peptides by enterocytes (infants)


Dietary fat (percent of our caloric intake, what they're used for, and how digested)

Fats provide 30-40% of our caloric intake

Fats are essential for building cell membranes, hormones, bile acids, etc.

The body can make most lipids EXCEPT linoleic (an omega 6 fatty acid) acid converted to arachidonic acid and alpha-linolenic (omega-3)acid “essential fatty acids”

Triglycerides are the most abundant fat in our diet

Our GI tract is water based so there are challenges to fat absorption


Bile Acids

Primary bile acids are produced in the liver from cholesterol – cholic acid & chenodeoxycholic acid

Secondary bile acids are formed by bacteria in the intestines & colon

Bile acids are complexed with glycine or taurine to make bile salts

Bile is recycled during a meal by uptake in the distal ileum – enterohepatic circulation


Bile salts

Have hydrophobic end and hydrophilic end
Rips fat apart into micelles which helps make digestion at the brush border much more efficient


Lipid digestion summary

Lingual and gastric lipase

Pancreatic lipase- hydrolyzes triglycerides into FFAs

Bile Salts solubilize fats into micelles. FFAs are transported to enterocytes.

Trigycerides are resynthesized and chylomicrons form


Vitamin Absorption (which are fat and water soluble and how does that change their absorption?)

Fat soluble vitamins (A, E, D, K) are absorbed along the length of the small intestines and are carried in micelles and form chylomicrons similar to dietary lipids.

Water soluble vitamins either enter the enterocyte by simple diffusion (biotin, folic acid) or via specific transporters (e.g. Vit B12)


Secretion & Absorption of Fluids

About 9 L of fluid is put into the gut each day and of this, only about 100-200 mls are lost

Movement of water follows the movement of solutes (that requires a solute gradient or active transport)

After the stomach, the small intestinal contents become iso-osmotic with respect to the blood

Water and ions can move paracellularly and/or transcellularly, depending on location

There is a net fluid secretion from cells in the intestinal crypts and a net fluid absorption from enterocytes on the villi. Villi surface area > crypt surface area


Sodium Absorption

Absorbed all along the intestine, with most absorption in the jejunum (60-80%).

Dependent on a gradient established by the Na+/K+ ATPase.

Water absorption is critically linked to Na+ absorption.

Mechanism is via Na+/glucose & galactose or
Na+/amino acid cotransport, NaCl cotransport,
Na+/H+ exchange or passive diffusion


Chloride Absorption

Passive in proximal intestines (due to loose TJs). Offsets Na+ charge in the intercellular space

In the distal ileum & colon, with less leaky TJs, Cl- is exchanged for HCO3- that is offsetting the acids produced by bacteria (HCO3- is generated by the action of carbonic anhydrase)


Potassium Absorption

Important!! Also has effects on Na.

Passive process

Paracellular movement in jejunum (due to low concentration of K+ in the intercellular space from the N+/K+ ATPase) but transcellular in colon

K+ normally high in cells due to Na+/K+ ATPase. A gradient is established as luminal water decrease on approach to the colon, with passive flux of K+ into the cells

Severe diarrhea can cause significant loss of K+ and hypokalemia


Calcium and Magnesium Absorption

Ca++ and Mg++ compete for uptake by the cells

Ca++ enters enterocyte passively down its electrochemical gradient in proximal intestines

Uptake of Ca++ in intracellular calcium stores maintains the gradient

Ca++ ATPase pumps calcium out to the blood

Remember Vit D important for Ca


Iron Absorption

Regulated absorption in the proximal intestines

Transported across apical membrane as either heme or Fe++ (receptor mediated)

Two possible fates:
- binds to apoferritin to form ferritin that
stays in the cell and is lost when the cell dies
- binds to transferrin (carrier protein), leaves
the cell and goes into the blood


Movement of Water in the Intestines

Paracellular water permeability decreases from proximal to distal in the small intestines

The colon has the lowest paracellular permeability to water because it’s trying to solidify waste and it needs to link water movement to transcellular ion movement


Pathophysiology - Diarrhea

Motility Disorders

Osmotic diarrhea: caused by impaired digestion or defects in absorption
- Lactase deficiency
- Ileal resection – bile salts not absorbed
- Celiac disease (Sprue) with gluten
sensitivity (gliaden-induced destruction of

Secretory diarrhea may be caused by Vibrio cholerae. Increases cAMP levels in cells and this in turn activates the CF chloride channel, (and thus water) on the luminal surface


Oral rehydration therapy

antibiotics plus KHCO3 to prevent hypokalemia and metabolic acidosis, glucose (or amino acids) with NaCl to facilitate the absorption of electrolytes and water