Gastric Phase of Digestion

• The gastric phase of digestion involves several key processes
• It relies on receptive relaxation, entry of meal, increased gastric secretions, motility for mixing and grinding, digestion of protein, carbohydrates, and fats, and gastric emptying

Gastric Anatomy and Function

• The body of the stomach functions as a reservoir
• The antrum is responsible for mixing
• Abundant smooth muscle in the antrum facilitates gastric motility

Gastric Motor Function

• Receptive relaxation of the fundus and body accommodates the meal's volume
• Mixing and grinding involve antral peristalsis to reduce meal size and mix it with secretions
• Gastric emptying coordinates antro-pyloro-duodenal motor activity
• Gastric reservoir function regulates fundus tone

Receptive Relaxation

• Swallowing initiates the relaxation of gastric smooth muscle
• Receptive relaxation increases compliance, minimizing changes in luminal pressure between empty (50 mL) and filled (1500 mL) states
• Two phases: True receptive relaxation (swallowing) and accommodation (mechanoreceptors)

Mixing and Grinding

• The pacemaker region induces peristaltic contractions from the body to the antrum, closing the pyloric sphincter
• Pacemaker cells (ICC) generate slow waves at approximately 3 cycles per minute (BER)
• Contractile force is increased by gastric distension
• Inhibited by duodenal distension, fat, protein, acid, or hypertonic chyme in the duodenum, and increased sympathetic tone
• Smooth muscle contractions mix and propel ingested contents in the gastric lumen
• A small amount of material enters the duodenum with each wave cycle

Gastric Glands and Secretions

• Mucous cells secrete mucous, HCO3, and trefoil peptides to maintain mucosal integrity
• Chief cells secrete pepsinogen and gastric lipase
• Parietal cells synthesize and secrete hydrochloric acid and intrinsic factor
• ECL Cells secrete histamine
• Enteroendocrine cells (G cells) secrete gastrin, while D cells secrete somatostatin

Composition of Gastric Juice

• Pepsinogen is secreted by chief cells
• Pepsinogen's function is to act as an inactive protease
• Pepsin is produced from pepsinogen and functions as a protease activated by HCl
• HCl is secreted by parietal cells and helps in acid environment and activation of pepsin
• Mucus and HCO3- are secreted by mucous and goblet cells and provide a viscous, alkaline protective layer and
• Intrinsic factor is secreted by parietal cells for vitamin B12 absorption
• Gastric lipase is secreted by chief cells and helps with lipid digestion

Acid Secretion Regulation

• Potentiation occurs when two or more agents produce an effect greater than the sum of their individual effects
• Acid secretion in the stomach relies on potentiation among histamine, ACh, and gastrin
• Histamine H2 receptor blockade significantly reduces acid secretion
• Somatostatin inhibits acid secretion
• Gastrin, histamine, and ACh stimulate acid secretion in parietal cells
• H/K-ATPase inhibitors (Prilosec, Prevacid, Nexium) blocks acid secretion

Acid Production by Parietal Cells

• Carbonic anhydrase is abundant in all parietal cells
• Hydrogen ions move into the gastric lumen via primary active transport, resulting from the generation of carbonic acid

Surface Epithelium Protection

• The surface epithelium secretes a mucus layer containing bicarbonate to protect against gastric acid

Pepsinogen Conversion

• Acidity in the gastric lumen converts pepsinogen to pepsin
• Subsequent conversions result from pepsin's protease activity

Cobalamin Absorption

• Cobalamin refers to B12 vitamin
• B12 absorbed through IF intrinsic factor in stomach
• The gastric intrinsic factor helps in cobalamin absorption

Gastric Emptying Regulation

• Gastric distension increases emptying rate, which is proportional to the size of the meal
• Gastric emptying is inhibited by distension of the duodenum
• Entry of chyme into the duodenum, fat and protein digests, and acidity (pH < 3.5) inhibit gastric emptying
• Solids are more inhibitory than liquids

Gastric Emptying Factors

• Delivery of acid and nutrients into the small intestine slows gastric motility and secretion
• The slower gastric motility and secretion create adequate time for digestion and absorption in the duodenum

Gastric Content

• Bolus: Large particles, triglycerides, protein, starch, monosaccharides, disaccharides, salivary secretions, water, and ions are all present
• Chyme: Departure (chyme) includes small particles (<2 mm), emulsion, triglyceride + small amount of 2-monosaccharides and free fatty acids
• Departure (chyme) also includes, protein + small amount of peptides and amino acids, starch + 20% oligosaccharides, monosaccharides, disaccharides, and water, ions, low pH