The document summarizes the layers of the gastrointestinal tract and the mechanism of gastric acid secretion. It discusses four layers of the GI tract: the mucosa, submucosa, muscularis, and serosa. It then describes the four-step mechanism of gastric acid secretion by parietal cells: 1) active transport of ions creates a charge that allows diffusion of ions into canaliculi, 2) hydrogen ions are actively secreted in exchange for potassium ions, 3) water passes into canaliculi by osmosis, 4) carbon dioxide forms bicarbonate ions that diffuse out in exchange for chloride ions.

1. Layers of GIT
Mucosa: It is absorptive and major secretory layer. Goblet cells in mucosa secrete Mucus.
Sub Mucosa: Thick, vascular layer.it contains glands & nerve plexuses.
Muscalaris (circular muscle): Segmental contractions, peristaltic movement. Provides major
nerve supply to the entire GI tract.
Serosa: Outer complete layer & protective wall of the GI tract.

3. **Basic mechanism of secretion of Gastric HCL ??
The parietal (oxyntic) cells contain a systemof intracellular canaliculi.The Hydrochloric acid is
formed at the membranes of these canaliculi & then conducted through openings to the
exterior. Mechanism of HCL secretion consists of the following steps:
1. A) Chloride ion is actively transported from the cytoplasm of the parietal cell into the lumen
of the canaliculus.
B)Sodium ions are actively transported out of the lumen.
C)These two effects together create a negative potential of -40 to -70 millivolts in the
canaliculus.
D) Which inturn causes passive diffusion of positively charged potassium ions & a small number
of sodium ions from the cell cytoplasm also into the canaliculus.
Thus, in effect , mainly potassium chloride & much smaller amounts of sodium chloride enter
the canaliculus.
2. A) Water becomes dissociated into hydrogen ions & hydroxyl ions in cell cytoplasm.
b) The hydrogen ion is then actively secreted into the canaliculus in exchange for potassium
ions catalyzed by H+ -- K+ ATPase.
c)In addition the sodium ions are actively reabsorbed by a separate sodium pump.
d)Thus, most of the potassium& sodium ions that had diffused into the canaliculus are
reabsorbed into the cell cytoplasm & Hydrogen ions take their place in the canaliculus giving s
strong solution of HCL in the canaliculus, Which is then secreted outward through the open
end of the canaliculus into the lumen of the gland.

4. 3. A) Water passes into the canaliculus by osmosis because of the secretion of the ions into the
canaliculus.
b)Thus, the final secretion from the canaliculus contains approximately hydrochloric acid at a
concentration of 150 to 160 mEq/L, potassium chloride at a concentration of 15 mEq/L & a
small amount of sodium chloride.
4.a) Finally , carbon dioxide, either formed during metabolism in the cell or entering the cell
from the blood ,combines under the influence of carbonic anhydrase with the hydroxyl ions to
form bicarbonate ions.
b) This then diffuses out of the cell cytoplasm into the extracellular fluid in exchange for
chloride ions that enter the cell from the extracellular fluid & are later secreted into the
canaliculus.