FUNCTIONS OF STOMACH :
•Secretory functions -
Stomach secrets HCl, Pepsinogen, Mucus, Bicarbonate, Intrinsic factor and water.
•Motor functions include mixing and digestion of food.
•Endocrine functions include synthesis of hormones like Gastrin, Somatostatin.
•Stomach is not essential for survival.
SECRETORY FUNCTION :
•Parietal cell secretion – Hydrochloric acid, Intrinsic Factor
•Non-parietal cell secretion – pepsinogens, mucus, Gastrin , Somatostatin, bicarbonate
Intraluminal Hydrogen & Sodium concentrations
Corpus / proximal part of the stomach secretes acid, pepsinogen, intrinsic factor, bicarbonate and mucus
Body or corpus of stomach shows :-
•Parietal / oxyntic cells
•Chief / peptic cells
•Mucus cells
PARIETAL OR OXYNTIC CELLS
•Triangular cell
•Centrally located nucleus
•Abundant mitochondria and intracellular tubulovesicular membranes and canaliculi
•Secretes
- Hydrochloric acid
- Intrinsic factor
- Tubulo-vesicular membranes are present in the apical portion of the resting or non-stimulated parietal cell.
- It contain the H-K pump (or H,K-ATPase) that is responsible for acid secretion.
- On stimulation, cytoskeletal rearrangement causes the tubulo-vesicular membranes that contain the H-K pump to fuse into the canalicular membrane.
CHIEF CELLS OR PEPTIC CELLS
•Pepsinogens are activated to pepsins at pH of ‹ 3
•Pepsins are endopeptidases (hydrolyze interior peptide bonds )
ANTRUM OF THE STOMACH
Antrum - endocrine function
•Contains chief cells and endocrine cells ( no parietal cells )
•G cells secrete Gastrin
•D cells secrete Somatostatin
•Both are endocrine and paracrine in nature .
GASRTIC ACID SECRETION
•Gastrin, acetylcholine, histamine three secretogogues substances act synergistically to induce acid secretion.
•Gastrin is released from G cells.
•Ach released from vagal endings
•Histamine released from Enterochromaffin cells
Secretogogues :
The substances which stimulate acid production.
All the three secretogogues bind to specific G protein –coupled receptors on the parietal cell membrane.
Gastrin binds to a specific cell receptor that has been identified as the gastrin-cholecystokinin B (CCKB) receptor.
The CCKB receptor has equal affinity for both gastrin and CCK.
Directly acting Secretogogues
•Ach and gastrin also stimulate ECL cells to secrete histamine which causes direct action on parietal cells to secrete HCl.
•H2 receptor antagonists (H2 receptor blockers) – block histamine action + action of Ach and gastrin = decreases acid secretion
H-K PUMP INHIBITORS
•OMEPRAZOLE – binds with extracytoplasmic portion of the pump and controls acid secretion.
Blocking Histamine receptors on parietal cells
•H2 receptor antagonists., eg Ranitidine, Cimetidine
PHASES OF GASTRIC ACID SECRETION
1)Basal state – interdigestive phase
It follows circadian rhythm – lowest in the morning and highest in the evening.
High basal acid secretion in males than females
2)Cephalic phase
Sight, smell, taste, thought and swallowing of food release acid.
It is mediated by Vagus nerve through four events – release of Ach, release of histamine, release of GRP, inhibition of somatostatin.
3) Gastric phase
In this phase distension of stomach releases acid.
4)Intestinal phase
In this phase their is release of gastric acid after food enters intestine.
GASTRIC PHASE OF ACID SECRETION
Stimulatory pathways – vagovagal reflex, local ENS pathway, partially digested proteins
Inhibitory pathways- release of somatostatin
Gastric phase accounts for 50 – 60 % of total gastric acid secretion.
Distension of stomach initiates vagovagal reflex and leads to RECEPTIVE RELAXATION
•Vagovagal reflex = vagal afferents from distended stomach stimulate vagal efferent response in the dorsal nucleus of vagus
= four events triggering acid secretion as in cephalic phase
Partially digested proteins stimulate gastrin release from G cells.
Gastric phase - inhibition
Gastric phase
Intestinal phase of gastric acid secretion (5-10% 0f total acid secretion )
GASTRIC MOTILITY
Four events can be identified in the overall process of gastric filling and emptying:
1.Recieving and providing temporary storage of dietary food and liquids.
2.Mixing of food and water with gastric secretary products, including pepsin and acid.
3.Grinding of food so that particle size is reduced to enhance digestion and to permit passage through the pylorus.
4.Regulating the exit of retained material from the stomach into the duodenum (i.e., gastric emptying of chyme – partially digested food)
- Emptying of liquids is primarily a function of the smooth muscle of the proximal part of the stomach, whereas emptying of solids is regulated by antral smooth muscle.
APPLIED PHYSIOLOGY
PEPTIC ULCER
•Peptic ulcer occurs due to increased acid secretion.
•Pernicious anemia –
In this condition their is lack of parietal cells which leads to decrease in acid production and lack of Intrinsic factor.
•ZOLLINGER ELLISON SYNDROME
Increase in acid secretion due to gastrin secreting islet cell adenomas
No comments:
Post a Comment