Lecture 7 - Stomach Flashcards
Gastrin
Stimulated by:
- Protein
- Distention
- Gastrin releasing peptide and acetylcholine (vagal stimulation) - PS triggers e.g. smell
Inhibited by:
- Acid secretion (post -meal) - when food leaves stomach pH drops as acts as buffer
- Somatostatin (stimulated by low pH)
- Endochromaffin like cells - release histamine
- reduced stomach distention and vagal activity
Site of secretion:
- G cells of antrum, duodenum and jejunum
Actions:
Stimulates gastric acid secretion and mucosal growth
Cholecystokinin
Stimulated by:
- Protein
- Fat
- Acid - fatty acids and monoglycerides
Site of secretion:
- I cells of duodenum, jejunum and iluem
Action:
- Stimulates pancreatic enzyme and bicarbonate secretion
- Gall bladder contraction tp expel bile
- Growth of exocrine pancreas
- Inhibits gastric emptying - more time fore digestion
Secretin
Stimulated by:
- Acid (H+) - gastric juice
- Fatty acids
Stimulated by:
- S cells of the duodenum, jejunum and ileum
Stimulates:
- Pepsin secretion
- Pancreatic and biliary HCO3- secretion - neutralise chyme
- growth of exocrine pancreas
- inhibits gastric acid secretion
Paracrine
Hormone which has effect only in the vicinity of the gland secreting it
Peptides released by endocrine cells acting in the local environment and diffuse short distances
e.g somatostatin by D cells in the antrum
Neurocrine
Peptides released by neurones due to action potential
e.g. gastrin releasing peptide released by the fibres of the postganglionic vagus nerve which stimulates gastrin release
Gastric inhibitory polypeptide (GIP)
Stimulated by:
- Sugar - carbohydrates
- Protein amino acids
- fatty acids
Stimulated by:
- K cells of duodenum and jejunum
Action:
- Insulin release
- Inhibits gastric acid release
Functions of the stomach
- Short term storage of food
- Physical break down of food due to smooth muscle contractions
- Chemical breakdown of food - proteases
- Innate immunity - acidic
- Activates enzymes
How is the stomach expandable
Temporary folds - rugae
Parts of stomach
Fundus Cardia - heart sits anterior Body Antrum Pylorus
Greater and lesser curvature
Where does the lesser omentum attach to
Stomach - first 1/3rd of duodenum near pylorus
Connect the liver and stomach
Layers of smooth muscle in stomach
Outer longitudinal layer
Middle circular layer
Inner oblique layer
Innervation of stomach
Autonomic NS
Enteric NS - Miessner’s and Auerbach’s plexus
Orad
Fundus
Proximal portion of body
Relaxes to relax the lower oesophageal sphincter to receive bolus via vagovagal reflex
- Food enters without raising intra-gastric pressure too much
- Prevents reflux
- Distension of rugae
Relaxation neural reflex
- Mechanoreceptors detect distention of the stomach
- Impuslese are sent to the CNS via sensory neurones of vagus nerve
- Efferents sent via vagus nerve fibres to smooth muscle wall of orad
- Relaxation
Factors affecting gastric emptying
- Fluids and isotonic solutions empty quicker
- fat in duodenum - CCK decreases gastric emptying
- H+ in duodenum - impulses to gastric SMC via interneurones of the myenteric plexus - increased force of contraction and AP frequency
[Not affected by neural or hormonal inputs]
Caudad
Distal portion of body
Antrum
Stomach lining
Stratified columnar
Rugae - folds
Gastric pits
Epithelial cells
Cover surface and extend into pits
Parietal cells - HCL / IF Chief cells - Pepsinogen G cells - Gastrin Mucous cells D cells - somatostatin Enterochromaffin like cells - histamine (paracrine)
Structure of stomach
Large proximally to small distally
Upper stomach - sustained contractions for basal tone
Lower stomach - strong peristalsis - more muscular and narrow
Contractions every 20 seconds
Chyme
Enters duodenum 3 times a minute
Coeliac trunk branches
- Splenic
- Left gastric
- Common hepatic
[SMA below coeliac trunk]
Coeliac trunk
Splenic artery –> short gastric arteries and left gastroepiploic artery
Common hepatic –> gastroduodenal artery –> Superior pancreaticoduodenal artery
–> Right gastric comes off and anastamoses with the left gastric
–> Proper hepatic to right and left hepatic arteries
Left gastroepiploic artery
Supplies the lesser curve of the stomach and anastamoses with the right gastroepiploic
Left and right gastric artery
Supplies lesser curve of stomach
Gastroduodenal artery
Common site for ulcer to perforate artery
- peritonitis
- vomit blood
HCL
Acidic conditions activate pepsinogen to pepsin
Innate immunity
Parietal cells pump out H+ into stomach against the conc gradient via K+/H+ ATPase
Acid causes iron absorption in stomach
Pernicious anaemia
Autoimmune destruction of parietal cells
Less intrinsic factor secreted so decreases absorption of Vit B12 in ileum - macrocytic anaemia
Roles of stomach areas
Cardia - mucous secretion
Fundus and body- HCL / mucus and pepsinogen
Pylorus - gastrin / somatostatin
[lots of crossover]
Cell distribution
Parietal cells - Body Chief cells - Body G cells - Antrum Mucous cells - Antrum D cells - Antrum
Phases of digestion
Cephalic
Gastric
Intestinal
Cephalic phase
Parasympathetic stimuli e.g. smell and tasting
Vagus nerve stimulates gastric secretions before food is swallowed to parietal cells (HCL release) and G cells via GRP
Increases gut motility
30% of HCL secretion
Gastric phase
60% of HCL secretion
Distention of stomach stimulates the vagus nerve to release GRP and Ach which stimulates gastrin release from G cells and parietal cells to release HCL
Proteins and amino acids stimulate gastrin secretion from G cells
Food acts as a buffer increasing pH somewhat, which removes G cell inhibition by stomatostatin
Myenteric plexus and gastrin causes strong smooth muscle contractions
Enterochromaffin like cells also release histamine that stimulates acid secretion by parietal cells
Intestinal phase
10 % HCL production
Initially: Chyme stimulates gastrin release as partially digested proteins in the duodenum
Progress: Inhibition of G cells
- Presence of lipids activates the enterogastric reflex which decreases vagal stimulation
- Chyme stimulates CCK and secretin
Stomach body glands
oxyntic glands
Antrum glands
Pyloric glands - deeper pits
Mucin
Secreted by foveolar cells
Mucin creates a viscous mucus layer that adheres to the epithelium and prevents chemical digestion of stomach by acid
HCO3- secreted into mucus layer - neutral pH
Rich vascular blood supply - remove acid that breaches mucous layer
High epithelial cell turnover to replace damaged cells
Prostoglandins promote processes
Why does NSAIDs like aspirin and ibroprofen cause stomach ulcers?
Inhibits prostoglandins that promote HCO3- release into the mucous layer and mucous layer formation.
Therefore more likely for mucous layer to be breached and physical damage to occur to the epithelium of the stomach by gastric acid causing an ulcer.
What breaches the stomach defense
Alcohol - dissolves mucous layer
Helicobacter pylori - chronic active gastritis
NSAIDs - inhibit prostoglandins
