Purpose of HCl secretion:
- Essential for digestion of food.
- Converts ferric form of iron into ferrous form for
absorption in gut.
- Kills bacteria and other ingested
microbes.
- Activates pepsinogen to pepsin.
- Provides acidic environment for the action of
pepsin.
HCl is secreted by the oxyntic or parietal cells of
mucosal epithelium of stomach (fundus and body).
Mechanism of HCl secretion:
- H+ secretion is possible in stomach due to presence of
numerous H+-K+ ATPases in apical membrane of parietal
cells.
- H+-K+ pump actively pumps H+ out of the cell into
gastric lumen.
- In exchange for H+, K+ enters the cell.
- As this is an active transport, energy is provided for
the process by hydrolysis of ATP.
- In cytosol of parietal cells, H+ is derived from
breakdown of carbonic acid (H2CO3).
- H2CO3 is formed by the combination of CO2 and H2O in a
reaction catalyzed by carbonic anhydrase.
- CO2 utilized in formation of H2CO3 is derived from
intracellular metabolisms and plasma.
- HCO3- formed by breakdown of H2CO3 is exchanged for Cl-
on the basolateral membrane of cell by HCO3- and Cl-
exchange.
- The Cl- that enters the parietal cells is transported
into gastric lumen.
- In the lumen, Cl- combines with H+ to form
HCl.
- HCO3- enters blood stream from interstitial
fluid.
- Thus, for each H+ secreted into the gastric lumen, one
HCO3- is reabsorbed into the plasma.
- As blood becomes alkaline, the urine becomes alkaline
and is called post prandial alkaline tide.
- K+ that enters the oxyntic cells by H+-K+ ATPase is
transported back into gastric lumen, which is re-utilized for
further H+ secretion.
- K+ entering the cell from basolateral membrane is also
secreted into lumen.
- If bicarbonate were not transported out of the cell,
there would be less H+ available to secrete as most of the H+ binds
with HCO3- to form H2CO3 and then back to CO2 and
H2O.
Hormones involved in digestive process:
1.Gastrin: Stimulation of gastric acid and pepsin
secretion and also stimulates growth of gastric and intestinal
mucosa (trophic action) stimulates gastric motility, exocrine
pancreatic secretions, insulin secretion and mass movement of large
intestine.
2.Secretin: Increases secretion of pancreatic juice rich
in bicarbonate and also alkaline bile secretion. It also causes
contraction of pyloric sphincter.
3. Cholecystokinin: Causes contraction of gall bladder
and increases bile release. Stimulates pancreatic secretion rich in
enzymes and causes relaxation of sphincter of oddi.
Four basic processes that occur in small
intestine:
- Intestinal secretion of mucus and enzymes helps in
digestion of food.
- Absorption of digested food through villi and
microvilli.
- Synthesis of vitamin K and Vitamin B Complex from
intestinal flora.
- Peristalsis for mixing of chyme with enzymes and for
propelling chyme towards colon.
Villi and
Microvilli:
Villi are finger like projections covered by a layer of
columnar epithelium and contains a network of capillaries and
lacteals.
Villi have numerous microvilli on their surface giving
it a brush border appearance.
Villi and microvilli increase the surface area for
absorption of digested food.
Four basic processes that occur in large
intestine:
- Secretion of water and electrolytes for lubrication of
passage of intestinal contents.
- Colonic bacteria produces gas which contributes to
flatus and colonic motility.
- Absorption of most of the water occurs here and
meanwhile solidifying the stool.
- Peristalsis for propelling the formed stools towards
rectum for defaecation.