Salivary, Gastric, Intestinal Secretions - Fan 2/19/16 Flashcards
why secrete in the first place?
-
provide digestive enzymes with optimal working environment
* sm int and pancreas secrete HCO3-rich alkaline fluid to neutralize stomach acid, prevent enzymes from being denatured -
passive secretion of water that is pulled in when influx of sugars, a.a.s, peptides increases osmolality of lumen
* water will diffuse back once these particles are absorbed - protection
- excretion of toxic metabolic wastes
general secretory mechanisms
- enzymes secreted via exocytosis, in response to neuroendocrine factors
- fluids (electrolytes/water) via ion movement into lumen (requires energy, one way or another),osmotic gradient that’s set up
salivary secretions
1500 mL/day
- lubrication
- assists w/ swallowing and speech
- protection
- mucus, bicarb, lysozyme (effective against gram+ bacteria), lactoferrin (effective against gram- bacteria), secretory IgA
- digestion
- amylase (active in oral cavity), lingual lipases (activated by stomach acid)
characteristics of saliva
- four major ions: Na, HCO3, K, Cl
- concentrations vary, but a few major trends…
- always lower in Na and Cl than plasma
- always higher in K than plasma
- usually rich in bicarb
2-stage model of saliva secretion
- acinar cells secrete: Na, Cl, K, bicarb, pours water into lumen [isotonic primary solution]
- secretions move through ductal area, where modifications occur
- Na reabs, Cl reabs
- leads to hypotonicity (because ductal tight junctions are super tight, water won’t be able to follow)
- K secreted, bicarb secreted
- Na reabs, Cl reabs
acinar cell: ion movements (resting)
- Cl moves from serosal side into cell via Na/Cl cotransporter
- Cl moves from cell into lumen via Cl/HCO3 ion channel (leaks both of them)
- both Cl and HCO3 are negatively charged, so K leaks from cell into lumen and from cell to serosal side via K channel
- Na moves from serosal side to lumen through cell-cell jx
- water follows!
sets up isotonic primary secretion that moves down to ductal area
acinar cell: ion movements (stimulated)
salty food will stimulate the acinar cells, lead to increased Ca within acinar cells
- stimulates anion channel (Cl/HCO3) and K channel
increase in metabolism will give you increased water and CO2 → increased HCO3 formation
ductal cells: ion movements
- Na reabs through Na/H exchanger
- Na eventually moves to serosal side via Na/K ATPase
- H reabs through H/K exchanger (K secreted)
- K leaks to either side from cell
- Cl reabs through Cl/HCO3 exchanger
- leads to increased bicarb secretion
- Cl leaks to serosal side via chloride channel (ONLY ON SEROSAL SIDE)
regulation of salivary secretion
almost 100% neurally regulated
-
salivary nucleus of medulla
- + : smell, taste, pressure, nausea
- - : fatigue, no sleep, fear, dehydration
most CNS signals come through PSNS (CN IX, X, FN VII)
-
Ach, VIP
- vasodilation (fluid delivery)
- acinar cell metabolism (more bicarb)
- channel activation (secrete K/Cl/bicarb)
- glandular growth (mainly due to vagal system)
- myoepithelial cell contraction
minor effect through sympathetics T1-T3 by way of superior cervical ganglion
- norepi
- mostly mucus secretion (not really fluid secretion)
gastric secretions
2000 mL/day
two major secretions
1. oxyntic component
- parietal cells: HCl, K, IF
- highly stimulated by feeding
2. non-oxyntic component
- mucous cells: mucus Na, HCO3
- chief cells: pepsinogen
- diffused interstitial fluids
- secreted at relatively constant rate; major component at rest, becomes v diluted by oxyntic component on feeding
gastric secretions: rest vs. stimulated
- resting: non-oxyntic components dominate: isonotic NaCl
- after stimulation: oxyntic components take over: isotonic HCl
general features of gastric secretion (Cl, K)
Cl is always the ion present in greatest quantity in gastric secretions
K concentration is always higher than plasma
functions of gastric secretion
- digestion
- acidic pH activates pepsinogen and lipases
- HCl denatures proteins for digestion by pepsin
- aiding absorption
- HCl solubilizes minerals (Ca, PO4, Fe, etc)
- IF required for B12 abs
- protection
- HCl inactivates microbes (exception: H. pylori in stomach)
- mucus, HCO3, Cl contain acidity of H
mechanism of HCl secretion
parietal cells undergo morphological change to enhance H secretion
- stimulation causes tubulovesicles rich in H pumps that are resting in parietal cell to fuse with intracellular canaliculi which eventaully opens to the lumen of the gastric pit to secrete H
HCl secretion: ion movements in resting cell
parietal cells, resting
- regular metabolism: bicarbonate made
- HCO3/Cl exchanger puts bicarb into blood, brings Cl into cell
- Cl moves into lumen via Cl channel
- Na/K ATPase moves Na out of cell, K into cell
- K moves into blood and into lumen via K channels
generally, you get accumulation of Cl (-70 to -80 mV potential difference) and K in resting lumen