The Stomach

The Stomach

Gastroesophageal Sphincter

4 Main Regions:
1. Cardia
2. Fundus
3. Body
4. Antrum/Pyloric

Pyloric Sphincter

Volume

• At rest: 50 mL or 2oz
• Max: 1-1.5L or 37-52 oz
• Rugae allow stomach to expand

Gastric Juice Glands in mucosa or submucosa:

• Cardiac glands
• Oxyntic glands (fundus/body)
• Pyloric glands

Cell Types

• Neck (mucosal) cells - mucous
• Parietal (oxyntic) cells - intrinsic factor, HCl
• Chief (peptic) cells - protein and fat enzymes
• enteroendocrine cells - gastrin which stimulates above cells

Gastric Juice

Contents:

• Abundance of HCl from Parietal Cells (low pH ~ 2)
• Converts or activates ZYMOGEN PEPSINOGEN to PEPSIN (protein digestion)
• Denature proteins (uncoiling the tertiary or secondary protein structures to expose the interior peptide bonds for pepsin)
• Releasing nutrients such as mineral from organic complexes
• Acting as a bactericide agent
• 3 Enzymes:
• Pepsinogen - secreted in granules by Chief Cells when stimulated by ACH or acid. 
• Protease - hydrolyzes protein, endopeptase (inter peptide bonds targeted)
• Lipase - secreted by Chief Cells
• Hydrolyzes fatty acids from glycerol's 3rd carbon in triacylglycerols
• 20% of lipid digestion
• a- Amylase - from salivary gland
• hydrolyzes starch
• Inactivated by low stomach pH
• Mucous
• Neck Cells - also release Bicarbonate
• Network of different glycoproteins called mucins
• Bind water and form a gel
• Coats gastric mucosa by 2 mm with bicarb embedded withing (pH 6-7)
• Production stimulated by PROSTAGLANDINS, ACH, Vagal nerve stim

Regulation of Gastric Secretions

3 Phases:
1/ CEPHALIC PHASE

Sense of food stimulates Vagal Nerve, ending with release of HCl from Parietal Cell:

•  ACH released and acts on Muscarinin receptors on Parietal Cell
• ACH also stimulates Chief Cells to release enzymes
• GASTRIN released from G-Cells binds to Gastrin receptors
• HISTAMINE released from Mast Cells binds to H2 receptors

2/ GASTRIC PHASE

Ingested food reaches stomach
Stretch receptors in stomach stimulated along with protein, coffee, alcohol enhances gastric secretions
Protein stimulates stomach:

• Stimulate chemoreceptors that initate submocosal nerve activity
• Promote gastrin release
• activating the parasympathetic nervous system

3/ INTESTINAL PHASE

After food leaves stomach
Reduction in chyme volume in stomach and in pH triggers release of somatostatin by D Cells in the pancreas.
Somatostatin

• Paracrine hormone, diminishes Parietal Cell, G Cell, and enterochromafin like cell secretions.

Inhibition of gastric emptying:

• Hyperosmolar chyme
• Acidic chyme
• Fat containing chyme

Regulation of Gastric Motility and Gastric Emptying

Peristalsis strongest in antrum, start from pacemaker cells (interstitial cells of Cajal) found in outer circular muscles, about 3 per min.

Selected Disorders of the Stomach

PUD

• ZES -gastin producing tumors / hypergastrinemia
• H. pylori
• ETOH, ASA, NSAID
• RX:
• H2RAs
• PPI