Stomach

Prof. K. Sivapalan.

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Stomach.

2013 Stomach 3

Functions of the Stomach

• Storage- receptive dilatation.• Mixing- longitudinal, circular and oblique

musculature.• Slow release of Chyme- pylorus and sphincter• Digestion- enzymes and acid.• Eliminating noxious agents- acid.• Absorption (facilitation)

– B12- intrinsic factor.– Iron and calcium- acid.

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Storage Function

• Plasticity- distention causes dilation without increase of pressure.

• It is promoted by vagal reflex during meal.

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Gastric Motility

• Longitudinal and circular muscles-– incomplete peristalsis for milking

the body of the stomach to leave solids and propel liquidized chyme into pylorus

– complete peristalsis in the pylorus for emptying [pyloric pump].

• Oblique muscles- churning movements for mixing and grinding.

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Gastric Secretion• 2.5-3.0 liters per day.• Enzymes- pepsinogen and

gastric lipase. [Chief cells]• Acid and intrinsic factor-

[parietal or oxyntic cells]• Mucus by surface and neck

cells in the fundus and the body, pylorus and cardia of the stomach.

• It forms 2-3 mm thick cover over the epithelium.

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Acid Secretion

• Canaliculi in cells with microvilli.• When activated, microvili develop

and contain H+- K+ ATP ase.• H+ in the cell is exhanged to K+ in

lumen [energy]• CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3 –

[Carbonic unhydrase]• HCO3 – diffuses into ECF and blood

[post prandial alkaline tide].• pH in cell is 7.2, in the lumen- 0.8

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Gastrin

• Secreted by G cells in pylorus [andrum]• pH above 3 in andrum stimulate secretion

and below 3 inhibit.• Vagal efferent stimulate secretion.• Actions:

– ↑ gastric motility, ↑ acid and pepsinogen secretion, ↑ tone of lower esophageal [cardiac] sphincter, trophic action on mucosa of GIT.

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Control of Acid Secretion.

• Cephalic phase:[↑]– Sight, smell, thought of food– Emotional states- anger, hostility ↑. Fear, depression ↓.– Presence of food in the mouth– Hypoglycemia

• Gastric phase:[↑]– Stretch, chemicals – local reflex.– Gastrin– Vagal reflex– Caffaine, nicotine

• Intestinal phase:[↓]– Entero-gastric reflex (distention, acid, irritation to mucosa,

digested food products)– Secretin, cholecystokinin, Gastric inhibitory peptide.

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Protection of Gastric Mucosa.

• Mucus.• Turnover of surface cells.• Tight junctions in cells.• Control of acid secretion.• Alcohol, aspirin, acid in empty stomach,

increased secretion of acid- weaken the above. ? Helicobacter Pylori in Under mucus layer

• When protection fails, Peptic Ulcer.

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Control of Gastric Motility and Emptying

• All factors that stimulate pyloric pump relax the pyloric sphincter.

• Type of food- carbohydrate fastest, protein next and fat slowest.

• Gastric factors:[↑]– Degree of distention.– Gastrin

• Intestinal factors:[↓] entero-gastric reflex– Acid [pH- 3.5-4.0]and protein products.– Distention of duodenum.– Irritation to duodenal mucosa.– High Osmolality

• hormonal– Fatty food (CCK, GIP)

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Lower Esophageal Sphincter

• Internal- circular muscles.• External- part of diaphragm.• Sling fibers of the oblique muscles form a

flap valve.• Acalasia:

– Increased tone, incomplete relaxation due to failure of myenteric plexus development.

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VomitingStimulus:• Stomach- salt water,

warm water, toxic substances.

• Abdominal viscera.• Irritation to throat.• Higher centre:

– ↑ intracranial pressure– Bad sight, smell– Feeling disgust, pain– Labyrinthine stimulus.– Emetic drugs

Vomiting centre • Situated in medulla.

Effects:– Nausea and excessive

salivation.– Glottis and nasopharynx

closed.– Lower esophageal

sphincter relax– ? reverse peristalsis from

duodenum– Pyloric sphincter closed– Raised intra abdominal

pressure- diaphragm and abdominal muscles.

– Forced expiration to empty esophagus.