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Gastric Secretion and
Its RegulationPart I
Learning Objectives
• Stomach: Parts, Function and Gastric Glands•Gastric Secretions•Regulation•Phases Of Secretion
Most dilated partBag like organLies in epigastric, umblical & left hypogastric regions of the abdomen
Functions of stomach
• Temporary storage of food• Grinding & mixing of food• Secretion of HCl • Secretion of intrinsic factor (IF) • Secretion of pepsinogen• Absorption of water and lipid soluble substances • Performs its own controlled emptying
• Surface/Neck mucous cells-secrete protective bicarbonate ions • Parietal Cells- most numerous in
the isthmus of the glands, secrete gastric acid (HCL) + intrinsic factor.
Structure of Glands
• Chief/peptic/zymogenic cells- located towards the bases of the gastric glands. Secrete gastric lipase and pepsinogen.
• Neuroendocrine cells- part of the diffuse neuroendocrine system, secrete serotonin and other hormones
• Stem cells- divide continuously to replace all other types of cell in the glands.
Synthesize and secrete the HCl acid responsible for the acidic pH in the gastric lumen.
Synthesize and secrete the protease precursor known as pepsinogen.
Produce alkaline mucus that covers mucosa layer
Contents of Normal Gastric Juice•Volume: 1 -1.5 liters• Nature: acidic pH 0.7 to 4 Constituents: Water (99.5%); solids (0.5%)
Inorganic constituents: anions ( Cl-, PO43-, SO4
2- & HCO3- and
cations (H+, Na+, K+, Ca++, Mg ++)
Organic constituents: pepsinogen, IF, mucin, renin, gastric lipase, gelatinase, carbonic anhydrase and lysozyme.
Structure of parietal cell: resting and stimulated
•H+ - K+ ATPase in inactive form
•When activated , fuse to cell membrane & microvilli project to increase surface area for secretion
Mechanism of HCl Secretion
• Water dissociated into H+ & OH-
• Pumping of H+ out of cell by H+K+ATPase; permits OH- to accumulate & form HCO3
- from CO2
• Water passes into canaliculus by osmosis making gastric juice isotonic to plasma
Postprandial Alkaline Tide
Pyloric gland Oxyntic gland
Regulation of HCl Secretion
Pyloric gland Oxyntic gland
Potentiation
Agents stimulating the parietal cells
• Ach• Gastrin• Histamine
Agents inhibiting the parietal cells
• Acid• Somatostatin
Meal Related Phases of Gastric Secretion
Cephalic phase : 30%
Gastric phase: 60%
Intestinal phase: 10%
sight, aroma, taste, thought
Cephalic phase
Sham feeding
Gastric phase
Intestinal phase
Inter-digestive phase
Enteric hormones that inhibit gastric H+ secretion
Pepsinogen secretion• Secreted by chief cells in gastric
glands, & mucous cells• Pepsinogen : aspartic proteinases,
endopeptidases• Two types: Group I ( chief cells of
corpus)• group II ( chief cells +
mucous neck cells in all parts)
• Zymogen granule compound exocytosis pepsinogen
• Agonist acting through Ca++
M3 muscarinic receptors for AchGastrin/CCk family peptides’ receptors
Stimulated by:
Regulation of Pepsinogen Secretion
Mucus
Secreted by :• Surface mucous cells•Mucous neck cells• Glandular mucous cells
• forms a mucus gel layer with PLs, electrolytes , water
Mucus Secretion & its Regulation
•Vagal stimulation and irritation stimulate gastric mucous cells to secrete mucin
•Gastric surface cells secrete HCO3- , stimulated by Ach, Acids &
PGs.
•Entry of acid into the duodenum induces Secretin from S cells triggering HCO3
- secretion by pancreas/duodenum & neutralizes Gastric acid
Mucus protects the gastric surface epithelium by trapping an HCO3
- rich fluid near the apical border of these cells
Gastric mucosal barrier
MucusBicarbonatesEpithelium- tight junctions+ surface membranesProstaglandins Trefoil peptides
What we learned today….
• Stomach: Parts, Function and Gastric Glands•Gastric Secretions•Regulation•Phases Of Secretion
Structure of Gastric Glands and secretions
Mechanisms
sight, aroma, taste, thought -----> hypothalamus gustatory centers -----> vagal nuclei of medulla -----> vagus nerve (parasympathetic) -----> increased gastric secretion