Acid secretion in stomach Flashcards
Where are oxyntic glands located
Inside surfaces of body and fundus of stomach
Where are pyloric glands located
Antral portion of stomach
What do oxyntic glands secrete
HCl
Pepsinogen
Intrinsic factor
Mucus
What do pyloric glands secrete
Mucus
Gastrin
What is an oxyntic gland composed of
(top to bottom)
- Mucus neck cells (secrete mucus and HCO3-)
- Parietal cells (secrete HCl and IF)
- Chief cells (secrete pepsinogen and gastric lipase)
What pH is the HCl that parietal cells secrete
0.8
How is the parietal cell arranged
At rest have numberous tubulovesicles derived from smooth endoplasmic reticulum. These contain H+ pumps and aren’t connected to apical memb.
When acid secretion stimulated they merge and become deep invaginations, secretory canaliculi. Pump HCl out into lumen. canaliculi have to be damaged from the acid and recycled
What is the function of HCO3- secreted by epitelial cells
Diffuses into blood, making gastric venous blood have a higher pH
Formation and secretion of HCl
CO2 from blood + H20 is the source of H+
On apical membrane of canaliculi, H+ actively secreted via H+/K+/ATPase pump (K+ in, H+ out)
K+ and Cl - recylced out through leaky channels
Basolateral HCO- efflux
HCO3- formed with H+ leaves cell passively through basolateral membrane
Via HCO3-/ Cl- exchanger (Cl- moves in)
This Cl- influx is directly balanced through Cl- efflux in apical membrane
Pepsin production
Pepsinogen secreted by chief cells
HCl activates pepsinogen to form pepsin
Pepsin action
Works best at pH 1.8-3.5
Proteolytic enzyme that acts on proteins and polypeptides
Can degrade mucus but is made less active by alkaline pH caused by HCO- secretion
How is secretion of oxylitic glands stimulated?
Ach released by parasympathetic stimulation of vagus nerve/ enteric nerves plexus
Stimulates pepsinogen, HCl and mucus release
Intrinsic factor actions
essential for vitamin B12 absorbtion in ileum (as a Vit B12 binding protein)
secreted by parietal cells along with HCl
What stimulates HCl secretion?
Histamine
Gastrin
Acetylcholine
ALL ACT ON PARIETAL CELL
Pyloric gland
Mostly mucus cells
Secrete mucus for lubrication and protect stomach wall from digestion
Also have enteroendocrine cells which secrete gastrin (G cells) and somatostatin (D cells)
Surface mucus cells
Entire surface of stomach mucose has mucus cells in between glands. Protection as any irritation causes these to produce mucus
Stimulation of gastric acid secretion (enterochromaffin-like cells)
ECLs secrete histamine when gastrin and Ach binds to it.
This histamine binds to H2R receptor on parietal cells
3 phases of gastric secretion
- cephalic phase
- gastric phase
- intestinal phase
Cephalic phase
Begins before meal and lasts up to 30 mins into meal
shortest phase
30% of gastric secretion
parasymp - vagus nerve mediates via caeliac plexus
Ach acts directly on parietal, indirectly by causing release of gastrin from G cells, indirecly by causing ECL to release histamine
Gastric phase
Longest phase, lasting up to 2.5 hours after start of meal
60% of gastric secretion
Trigger by food causing distention in stomach and presence of aa and peptides
Gastric phase excites…?
Long vagal reflexes
local enteric reflexes
which stimulate gastrin release and nerves to increase Ach release
Intestinal phase
Triggered by presence of food in duodenum.
Stimulatory; G-cells in duo release gastrin
Inhib (predominant)
Inhibitory effects of intestinal phase
-Somatostatin released from D cells in duo
- Acid mediates secretin release into blood, inhibiting release of gastrin and reducing parietal response to gastrin
- fatty acids in duo causes the release of gastric inhibitory polypeptide (GIP) and cholecystokin (CKK) which inhibit parietal acid secretion
Secretin, CCK and GIP are entrogastrones
