Functions of the stomach
Minimise ingestion of bacteria
Dissolve and partially digest macromolecules
Regulate entry of chyme into small intestine
Secrete intrinsic factor
Which function of the stomach is indispensable?
Secretion of intrinsic factor glycoprotein secreted by parietal cells, essential in absotion of B12 in the intestine
Where is the stomach located in the abdomen?
left upper quadrant, Epigastrium
What are the five regions of the stomach?
Describe the innervation of the stomach
Stomach has intrinsic innervation from the enteric nervous system:
submucosal (meissner's) plexus which is responsible for regulating blood flow and GI secretions
myenteric (auerbach's) plexus which supplies the muscle layers and controls GI motility
Extrinsic innervation comes from the vagus nerve [CN X] (parasympathetic) and the sympathetic trunk (T6-T9)
Describe the three phases of gastric motility
Receptive relaxation: enteric neurons stimulated by vagus nerve release NO and serotonin which mediate relaxation of smooth muscle in the orad region. Increases volume of the stomach to 1.5L
Mixing: Peristaltic waves move from body to antrum of the stomach. Muscle layers thicker towards the antrum which produces a powerful contraction. Pyloric sphincter closes and contents of the antrum are forced back towards the body of the stomach (retropulsion)
Emptying: controlled by co-ordinated contractions of the stomach, pylorus and small intestine. Allows controlled emptying for optimal digestion and absorption
Describe the pattern of motility that causes a rumbling stomach
Caused by MMC, which produces a distinct pattern of motility between meals.
Antrum is quiescent for 75-90 mins. Intense period of electrical and motor activity occurs (5-15mins), which results in powerful contractions that sweep the length of the stomach. The pylorus dilates and ingestible material is swept into the small intestine.
Controlled by the CNS and motilin
Describe the function and control of the pyloric sphincter
Allows the regulated emptying of gastric contents at a rate that is consistent with the ability of the duodenum to process chyme
Prevents regurgitation of duodenal contents into the stomach.
Constriction: Sympathetic nerve fibres, CCK, Gastrin, GIP, Secretin
Vagal fibres can be excitatory or inhibitory.
How is gastric emptying regulated? Why is this important?
Gastric emptying is regulated in response to the nature of the duodenal contents.
Receptors on the mucosa of the duodenum and jejunum sense acidity, osmotic pressure, fats, amino acids and peptides and release enteric hormones. These inhibit antral contractions or constrict the pyloric sphincter to decrease gastric empyting.
This is important beacuse it allows time for sufficient processing of the digested contents in the small bowel so they can be absorbed. E.g. Lipid rich chyme needs to be emulsified by bile, acidic chyme needs to be neutralised.
What are the four main components of gastric juice?
HCl: produces low pH in the stomach
Enzymes: pepsinogen and gastric lipase
Mucus: produces mucosal barrier
Intrinsic factor: essential for vitamin B12 absorption
Name the four types of secretory epithelial cells in the stomach
Mucus secreting cells (surface and neck)
Parietal cells (secrete HCl and IF)
Chief cells (secrete pepsinogen)
G cells (secrete gastrin)
Describe the functional distribution of secretory cells in the gastric mucosa
Gastric epithelium folded into pits which extend into the mucosa (straight or branched) forming gastric glands.
Structural organisation of epithleum of the stomach varies within the 5 regions. Parietal cells located in the proximal 80% of the stomach, gastrin producing cells found in the antrum.
Fundus: Pits shallower, glands long and straight. Main contributor to gastric juice
Cardia: simple tubular glands coiled at the lower end, mainly contains mucus secreting cells
Pylorus: deeper pits with branched glands, mostly secrete mucus as the majority of food has alraedy been digested
Describe the structure of a gastric gland
Columnar epithelium forms the gastric pit. Surface mucus cells secrete mucus, protection against acid and enzymes
Isthmus and neck contains mucus neck cells which secrete mucus for lubrication, and parietal cells which secrete HCl and intrinsic factor.
Base contains chief cells which produce pepsin and enteroendocrine cells which secrete hormones.
Where is this gastric gland located
Shallow pits, parietal cells, mucus neck cells, chief cells.
Secretions are major contributor to gastric juice
Where is this gastric gland located?
simple tubular glands, form coils at the lower end
Where is this gastric gland located?
Deeper pits, extending halfway down teh length of the mucosa, branched coiled glands
Describe the effects of gastrin
Principal hormone secreted from the gastric epithelium by G cells located in the antrum
Stimulates secretion of acid, pepsinogens, mucus, HCO3
Stimulation of gastric motility
Inhibition of gastric emptying
Stimulates pancreatic enzyme secretion, HCO3 secretion
Stimulation of insulin release
Stimulation of intestinal motility
Describe the effects of Histamine
paracrine hormone secreted by ECL cells scattered throughout the gastric mucosa
Stimulates secretion of acid
Increases local blood flow, which supports the increase in metabolism caused by acid secretion
What effect does somatostatin have on the stomach?
Inhibits gastrin release
Inhibits acid secretion
Synthesised by D cells in the antrum and body of the stomach
What are the three phases of gastric acid secretion?
Describe the control of each of the three phases of gastric acid secretion
Cephalic phase- initiated by the sight, smell and taste of food. Primarily mediated by the vagus nerve
Gastric phase- amino acids and peptides in the antrum elicit acid secretion by stimulating gastrin release. Distention of the stomach activates stretch receptos which release gastrin and Ach
Intestinal phase - products of digestion (amino acids and peptides) in the duodenum stimulate gastrin release.
Later, as chyme moves down the duodenum neural and endocrine responses inhibit gastric secretion (include secretin, CCK, somatostatin)
Describe the structure and function of the gastric mucosal barrier
Barrier consists of:
Mucus secreted by mucus neck cells in the gastric glands and surface epithelial cells of the stomach. Forms a 200um layer on the luminal surface
HCO3- ions secreted by epithelial cells on the gastric surface trapped within the mucus layer making it alkaline.
Keeps the surface of the epithelium near neutral pH, protects epithelial cells against acid, proteolytic enzymes and mechanical damage
Why do NSAIDs increase the risk of gastric ulcers?
NSAIDs inhibit prostaglandin synthesis
Local irritation to the mucosa stimulates production of PGs, which increase mucus and HCO3- secretion.
Control of acid secretion in the stomach
Ach is released near parietal cells from cholinergic parasympathetic nerve terminals.
Gastrin secreted from G cells travel in the blood stream to parietal cells.
Gastrin and ACh increase Ca2+ levels in parietal cells. Also stimulate release of histamine from ECL cells.
Histamine secreted by ECL cells are scattered throughout the gastric mucosa and diffuses to the parietal cells. Histamine increases cAMP/PKA which increases the H+/K+-ATPase on the plasma membrane
Mechanism of acid release by parietal cells
H+ and HCO3- are produced by carbonic anhydrase from CO2 and H2O.
Bicarbonate ions are transported out at the basolateral membrane via an anion exhanger (HCO3- out Cl- in). Cl diffuses passively into the lumen via a Cl channel.
H is secreted into the lumen by a H+/K+-ATPase on the apical membrane. K+ diffuses back into the lumen.
What is the primary secretion of zymogenic cells in the gastric glands?
Chief (zymogenic) cells secrete pepsiogens.
Pepsinogens are the inactive precursor of pepsins, acitvated by low pH when released into the gastric lumen.
Accelerates protein digestion into peptides.
What are the five regions of the stomach?
- Cardia: the superior inlet of the stomach below the lower oesophageal sphincter
- Fundus: the dilated part of the stomach, lies under the diaphragm. Dilation due to fluid or food.
- Body: major part of the stomach, mucosa arranged in gastic folds (rugae) which increase the surface area
- Antrum: has increased muscle layer which produces strong peristaltic contractions during digestion
- Pyloric canal: forms a funnel to promote outflow of chyme to duodenum
- Pylorus (pyloric sphincter): thick circular muscle controls discharge of stomach contents
Which organs are associated with the stomach bed?
liver (connected by lesser omentum)
transverse colon (connected by greater omentum)
How are gastric contractions coordinated?
Normal gastrointestinal motility is acheived through coordinated contractions of smooth muscle.
Discuss the electrophysiology of gastric motility
Contractile activity of the stomach is regulated by the electrical activity of the smooth muscle cells.
Smooth muscle cells undergo spontaneous cycles of depolarisation and repolarisation which produces a basic electrical rhythm.
The pacemaker zone in the body of the stomach generates slow waves, which are fluctuations in membrane potential spreading to adjacent sections of smooth muscle. (approximately 3 per minute) Slow waves co-ordinate contractions.
Excitatory neurotransmitters and hormones increase the amplitude of the slow wave by depolarising the membrane. If the threshold is reached then an action potential is fired which intiates gastric contraction.
Strength of contractions is determined by the number of action potentials in a slow wave
Describe the arterial blood supply of the stomach
Arterial blood supply of the stomach is derived from the coeliac trunk.
Branches to the stomach are:
Left gastric artery (lesser curvature)
Splenic artery branches (short gastric-fundus and left gastroomental-greater curvature)
Gastroduodenal artery (right gastroomental-greater curvature)
Describe the venous drainage of the stomach
The portal vein is the final common pathway for venous blood from the GI tract.
Left and right gastric veins drain directly into the portal vein
Short gastric veins and the left gastroomental vein drain to the splenic vein
Right gastroomental vein drains into the superior mesenteric vein
Symptoms of reflux oesophagitis
Haematemesis Can lead to oesophageal ulceration, peptic stricture or Barrett's oesophagus
Caused by a gastrinoma in the duodenum or pancreas. Results in ectopic production of gastrin.
Causes increases acid secretion leading to peptic ulcers. Causes parietal cell hyperplasia
Treat with PPIs and H2Rs to reduce acid secretion. Tumours surgically removed.