15 - Gastric Physiology, Acid Secretion, and Digestion Flashcards
Pancreas
Mixed endocrine-exocrine organ
Primarily exocrine
Acini = 84% Ducts, 5% of pancreatic mass
Islets (endocrine pancreas) = 1% of pancreatic mass
Both ducts and acini of the exocrine pancreas are secretory
Exocrine cells of the pancreas
Polarized
Secrete from apical surfaces
Respond to signals delivered to basolateral surfaces
Acinar cells
Secrete enzymes
Duct Cells
Secrete water, bicarb, sodium
Centroacinar cells
Cells that project back into the acini
Mechanism for creating bicarb that is eventually shuttled into the lumen of the duct
CO2 + H2O = HCO3- and H+
H+ moves into bloodstream in exchange for sodium
Sodium moves into the bloodstream in exchange for potassium via the Sodium/Potassium pump (sodium is kind of just a middle man)
Bicarb moves into the lumen in exchange for chloride.
Secretin
H+ stimulates secretin’s production.
Produced by S Cells
Stimulates cAMP (works with nerves, works without nerves)
Duct cells put out a lot of water and bicarb.
Cholecystokinin (CKK)
Produced by I cells
Stimulates duct cells to release intracellular calcium (via IP3 cascade, working together with nerves)
Calcium is the signal for pancreatic enzyme release.
What is the ideal pH fro pancreatic enzymes to work?
Neutral!
This means the pancreas has to secrete bicarb along with its enzymes so that the acidic pH of the gut can be combatted!
Secretin response to acid
Acid induces secretin-releasing peptides (S-RPs) to be released.
Vagal stimulation also induces this.
S-RPs stimulate S cells
S cells release secretin
Secretin’s effect on the duct
Secretin stimulates receptors on the duct cells.
Duct cells increase cAMP production.
Carbonic anhydrase catalyzes the production of H+ and HCO3- from CO2 and H2O
Luminal Cl- channel (CTFR) opens, and is coupled to a Cl-/HCO3- antiport channel.
Basolateral Na+/H+ antiport, Na+/K+ ATPase & H+ ATPase (exporting H+
Overal trend. When the gut gets more acidic (via parietal cells), the blood
Gets more alkaline
When the gut gets more basic (via pancreatic secretion), the blood
Gets more acidic
What amplifies the absorptive surface of the gut?!
Length Plicae Villi Microvilli Glycocalyx
How big is the surface area of the intestines?
A tennis court!!!
300 square meters
Small intestinal mucosa crypt-villus organization
Crypt-villus axis is continuous (Cores of villi contain lamina propria)
Stem cells are found near the base of the crypts (generate crypt epithelial cells that line the villi)
Enterocytes mature at the crypt-villus junction
Old cells are extruded from villus tips.
It’s like an escalator.
Core of microvilli
Microfilaments
External plasma membrane surface of microvilli
Studded
Studs are clathrin, enzymes and transporters.
Terminal Web
Microfilaments in microvilli enter the cells and are linked by myosin. This is called the terminal web.
Beneath the terminal web
Intermediate Filaments (providing structure)
Celiac Pathogenesis
Gluten triggers a hypersensitivity reaction against the gut.
Crypt cells turn over crazy quicksies.
The intestines are now lined with crypt cells without the microvilli and junk!!!!!! Can’t absorb!!!!!!!!
Steatorrhea
Fat-soluble vitamin deficiencies!
5 patterns of digestion-absorption
No digestion (glucose transport)
Luminal hydrolysis of polymer to monomers (protein to AA, AA transport)
Brush border hydrolysis of oligomer to monomer (Sucrose to fructose & glucose, glucose & fructose transport)
Intracellular hydrolysis (peptide uptake, breakdown to AA, AA export)
Luminal hydrolysis followed by intracellular resynthesis (triglyceride to glycerol & fatty acids, uptake of those components, triglyceride resynthesis, triglyceride export)
Where are carbohydrates, proteins & lipids maximally absorbed?
Duodenum
Less absorbed further downstream, no more absorption once you hit the ileum.
Where are calcium, iron and folate actively absorbed?
Duodenum
Calcium also continues to be absorbed throughout small intestine.
