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Flashcards in GI secretion and digestion Deck (20)

Secretion – General Properties

Tightly regulated processes (receptors)

Enzymes are usually released in their pro-enzyme or zymogen form

Involved in both the maintenance and integrity of the inner surface of the GI tract (Cl-, mucus, HCO3-, etc) in addition to digestive processes

Chloride secretion via a CFTR channel is one mechanism for water/ion secretion in the small intestine and colon


Acid secretion (purpose, what factors are released/activated, what drives it?)

Kills bacteria (disinfects food at pH 1.0)

Begins protein digestion - denatures proteins and activates pepsinogen (active form = pepsin)

Acid producing parietal cells also secrete intrinsic factor when secreting acid (vitamin B12 absorption)

Energy consuming process – H+/K+ ATPASE pumps across the luminal surface against a significant gradient


Defences in stomach

HCl secretion is risky, so the body has mechanisms in place to confine acidity to the stomach as much as possible

Mucus layer and alkaline (HCO3-) layer at the cell surface (surface mucus cells) protects the stomach lining – prostaglandins can increase mucus production

Tight junctions between cells prevent acid from infiltrating the layers of the wall

Rapid cell turnover maintains surface integrity


Parietal cell

Produces Intrinsic Factor and HCl


Vit B12

Important for RBC production

Binds salivary R protein in stomach

Pancreatic proteases remove R protein in duodenum

IF from stomach then binds B12 in duodenum

IF/B12 complex binds to receptor in terminal ileum for absorption (receptor is for IF)


Phases of HCl Secretion

Interdigestive (basal) phase – between meals following circadian rhythm (highest in the evening and lowest in the morning prior to waking

Cephalic phase – mostly neural regulation

Gastric phase – initially neural followed by endocrine (gastrin) and neural regulation

Intestinal phase – mostly endocrine regulation


How do the three parietal cell secretagogues induce acid secretion?

structural changes push pumps to surface to secrete acid

increased vagal release of ACh (and possibly VIP)


Describe the mechanism of gastric acid generation and secretion, including the role of K+, Cl /HCO3, carbonic anhydrase and H+-K+ ATPase



Peptic ulcer disease risk factor


Tumors (Zollinger Ellison Syndrome)

Helicobacter pylori



Process of breaking down food into an absorbable form

Some digestion occurs in the mouth and stomach but most of the final digestion occurs in the intestinal lumen or at the surface of the absorptive cells (enterocytes)



Process by which molecules of food are transported across the enterocyte membrane (transcellular/cellular) or between cells (paracellular) and into the blood or lymph


Dietary carbs

Only simple monomeric sugars can be absorbed

Polymers can't



digests carbs at the alpha 1,4 bonds

major enzyme in saliva and pancreatic secretions


Enterocyte Surface Enzymes that Covert Small Polysaccharides to Sugar Monomers

Isomaltase (alpha-dextrinase) – converts alpha-limit dextrins to glucose

Maltase – converts maltose and maltotriose to glucose

Lactase – converts lactose to glucose and galactose

Sucrase – converts sucrose to glucose and fructose

Trehalase – converts trehalose to glucose


Lactose intolerance

Missing brush border enzyme, lactase

Causes gas and diarrhea due to colonic bacterial digestion of lactose

Areas where dairy is not part of staple have a higher prevalence


Enterocyte uptake of sugar monomers

Sodium transporters

Glucose binds, facilitating binding of Na, undergoes another conformational change that brings the Na into the cell, which causes another transformational change releasing the glucose

then transporter faces outside again and repeats


SGLT1 Transporter

Requires sodium as a co-transporter

Transports glucose & galactose across the apical membrane of the enterocyte

can operate in the setting of secretory diarrhea (increased cAMP/cholera) so is important in oral rehydration



Not Na+ dependent

Transporter at basolateral surface that transports glucose & galactose and fructose



Fructose transport across the apical surface

Sodium independent


Regulation of Carbohydrate Absorption

Increased carbohydrate consumption upregulates transporters and increases the uptake of simple sugars

Decreased carbohydrate consumption downregulates transporters and decreases the uptake of simple sugars