The stomach lining
(gastric mucosa/mucus membrane)
This 1 mm thick, velvety smooth, protective
mucus membrane lines the stomach.
It contains mucus secreting glands and
columnar epithelial cells.
Secreting gastric glands are
located at the bottom of deep depressions in the stomach wall, called gastric
pits. The gastric glands contain 2 types of secretory cells ---
parietal cells,
which secrete hydrochloric acid (HCl) and bicarbonate (HCO3-) and
chief cells, which secrete
pepsinogen, which requires a very acidic ( low pH) to be activated into its
active form pepsin, the enzyme for digesting protein in the stomach.
Stomach lining integrity and repair.
TGF-alpha produced by gastric epithelial cells stimulate
epithelial cell proliferation. TGF-alpha also enhances mucus secretion
and inhibits stomach acid production. Other cytokines such as fibroblast
growth factor and hepatocyte growth factor have been shown to enhance
healing of gastrointestinal ulcers in experimental models.
Gastric / Stomach acid
Predominately composed of hydrochloric
acid (HCl). Also water, electrolytes
(sodium, potassium, calcium, phosphate, sulfate, and bicarbonate), and
other organic substances, such as mucus, enzymes (pepsins), and
protein. Has a pH of about 1.5 - 3.5
Primary purpose is for partial digestion
of proteins in the stomach. Unravels proteins and
activates digestive pepsin enzymes to break down (hydrolize) the long
chains of amino acids in food proteins (peptides) into shorter
polypeptide chains and so make their complete digestion easier in the small
intestine. Sufficient HCl
keeps the pepsin enzyme working at its best.
Of interest, some absorption of water,
alcohol and weakly acidic drugs occurs in the stomach. E.g.
Aspirin is a weakly acidic drug, and being mostly un-ionised in the acid
stomach, is more easily absorbed by the stomach mucosal cells via
diffusion. However, overall, most absorption takes place in the small
intestine. ~20% of water and alcohol is absorbed from the stomach.
Gastric gland parietal and chief cells
Parietal cells
Parietal cells produce hydrochloric Acid
(HCl), bicarbonate
(HCO3-), intrinsic factor
(needed for red blood cell maturation and B12 absorption)
and most of the water in gastric juice.
HCl production is controlled by the hormone GASTRIN to produce HCl only when pH goes above ~1.5, and slows
due to negative feedback around a pH of 2. GASTRIN increases in response
to gastric distention (via vagus nerve) and insufficent acidity when food enters stomach. Parietal cells produce an average 1.35 liters of HCl / day
to create a stomach acid average pH of ~2.5 (i.e. ~250,000 times more acidic than blood).
Parietal cells secrete hydrogen ions to be combined with
chlorine ions to form HCl, which drains into the bottom of the gland and
passes into the stomach by ATP-powered magnesium-dependent proton pumps
(H+/K- ATPase). HISTAMINE and ACETYLCHOLINE also control HCl production
Parietal cells produce bicarbonate ions
(HCO3-) in exchange for chlorine ions
(Cl-) used for HCl production. After
eating, bicarbonate is secreted into the blood by gastric mucusal cells
resulting in a slight elevation of blood pH known as the "alkaline
tide". This process maintains a balanced pH inside the parietal cell.
Prostaglandins
(cell-localized, hormone-like substances present in most all tissues / body fluids)
and PPIs omeprazole (Losec™ or
Prilosec™) and lansoprazole (Prevacid™)
inhibit acid secretion by the parietal cells.
Prostaglandins are members of the eicosonoid family of lipid molecules.
In particular, the prostaglandins PGE2 and PGI2 (prostacyclin) are well
recognized cell-protectors. Their other protective modus operandi is
that they stimulate mucosal mucus and bicarbonate secretion, increase
mucosal blood flow, and limit back diffusion of acid into the
stomach lining epithelium.
Chief cells
Chief cells produce
pepsinogen (converted to the pepsin enzyme for digesting
protein). Like HCl, production is also controlled by food
consumption (via the vagus nerve) and the hormones/neurotransmitters: GASTRIN, HISTAMINE and
ACETYLCHOLINE.
Gastric mucosa and mucus cells (foveolar cells)
Foveolar cells
produce mucus
to protect the stomach lining (gastric mucosa)
from corrosive stomach acid. Located
in the neck of the gastric pits, these surface cells cover the entire
gastric mucosa to provide a thick mucus layer, which both lubricates the
lining to help food masses travel through the stomach more easily and
protects against damage. This protective mechanism prevents the
lining from being digested by its own protein "breakdown" enzymes, also
aided by the parietal cell production of bicarbonate, which is secreted
by gastric mucus cells to result in
a neutral pH close to the lining. Mucus is a slimy material composed of
mainly mucin cells and inorganic salts suspended in water.
Abundant
carbohydrates on mucin molecules bind to bacteria, which helps to
prevent them colonizing in the epithelial lining.
Gastric mucosal lining
also contains acid-resistant lipoproteins
Parietal cell bicarbonate production
brings bicarbonate to the surface for secretion by gastric mucous secreting
cells.
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