The stomach, a vital organ in digestion, has distinct regions that work together to break down food. From the cardia to the pylorus, each part plays a unique role in mixing, churning, and processing what we eat.

The stomach's self-protection mechanisms are crucial for its function. A protective mucus layer, bicarbonate secretion, and tight junctions between cells all work to prevent the stomach from digesting itself while still allowing it to break down our food.

Anatomy and Physiology of the Stomach

Anatomical regions of stomach

• Cardia surrounds the esophageal opening, allowing food to enter the stomach through the gastroesophageal sphincter
• Fundus forms a dome-shaped region above and to the left of the cardia, collects gas and aids in belching (eructation)
• Body comprises the main central region, expands to accommodate food and secretes gastric juice
• Pylorus connects the stomach to the duodenum, regulating the passage of chyme
  • Pyloric antrum is the wider, proximal part of the pylorus, mixes chyme with gastric juice
  • Pyloric canal is the narrower, distal part of the pylorus, controls the release of chyme into the duodenum via the pyloric sphincter
• Lesser curvature is the shorter, concave curvature on the right side, provides a pathway for blood vessels and nerves (gastric artery and vagus nerve)
• Greater curvature is the longer, convex curvature on the left side, allows for stomach expansion and contraction
• Mucosa is the innermost layer containing glands and gastric pits, secretes gastric juice and absorbs certain substances (alcohol, aspirin)
  • The mucosa forms stomach rugae, which are folds that increase surface area and allow for expansion
• Submucosa is a connective tissue layer with blood vessels and nerves, supports the mucosa and allows for stomach expansion
• Muscularis externa consists of three layers of smooth muscle (longitudinal, circular, and oblique), responsible for peristalsis and segmentation
• Serosa is the outermost layer of connective tissue, provides a slippery surface for stomach movement within the abdominal cavity

Cell types in gastric glands

• Foveolar cells (surface mucous cells) secrete mucus to protect the stomach lining from acid and enzymes
• Mucous neck cells secrete mucus, contributing to the protective mucus layer
• Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor
  • HCl lowers pH in the stomach to activate pepsinogen and kill bacteria, providing a acidic environment for protein digestion
  • Intrinsic factor binds to vitamin B12 (cobalamin), enabling its absorption in the ileum
• Chief cells secrete pepsinogen, the inactive precursor of pepsin, which is a protein-digesting enzyme activated by HCl
• Enteroendocrine cells secrete hormones such as gastrin, which stimulates HCl secretion by parietal cells and increases gastric motility (G cells)

Stomach's self-protection mechanisms

• Mucus layer secreted by foveolar cells and mucous neck cells forms a protective barrier against HCl and enzymes, preventing autodigestion
• Bicarbonate ($HCO_3^-$) secretion by surface epithelial cells neutralizes HCl near the stomach lining, maintaining a neutral pH
• Tight junctions between epithelial cells prevent HCl and enzymes from penetrating the mucosa, maintaining the integrity of the stomach lining
• Rapid turnover of epithelial cells ensures that damaged cells are quickly replaced, maintaining a healthy mucosal barrier
• Prostaglandins stimulate mucus and bicarbonate secretion, increase mucosal blood flow, and promote epithelial cell regeneration, enhancing the stomach's self-protection
• Failure of these mechanisms can lead to gastric ulcers, which are erosions in the stomach lining

Digestion process in stomach

1. Mechanical digestion

   • Peristalsis involves waves of smooth muscle contractions that mix and churn food with gastric juice, breaking down food particles
   • Segmentation consists of localized contractions that further mix the chyme (semi-liquid mixture of food and gastric juice), ensuring thorough digestion

2. Chemical digestion

   • Protein digestion
     • Pepsinogen is converted to active pepsin in the presence of HCl, initiating protein breakdown
     • Pepsin breaks down proteins into smaller peptides, preparing them for further digestion in the small intestine
   • Fat digestion
     • Gastric lipase (secreted by chief cells) initiates fat digestion by breaking down triglycerides into fatty acids and monoglycerides, preparing fats for further digestion by pancreatic lipase
   • Carbohydrate digestion
     • Salivary amylase (from the mouth) continues to break down starch into maltose until inactivated by the low pH of the stomach, contributing to initial carbohydrate digestion

3. Chyme is released into the duodenum through the pyloric sphincter in small amounts, controlled by hormones (cholecystokinin and secretin) and neural reflexes (enterogastric reflex) to optimize digestion and absorption in the small intestine

Gastric Function and Regulation

• Gastric motility refers to the movement of the stomach muscles, which is crucial for mixing food with gastric juice and propelling chyme towards the duodenum
• Gastric emptying is the process by which the stomach releases its contents into the small intestine, regulated by various factors including the volume and composition of the chyme
• Gastric juice, a mixture of HCl, enzymes, and mucus, plays a vital role in chemical digestion and protection of the stomach lining