29.2 Introduction to the Esophagus and Stomach

The esophagus and stomach work together to start the digestion process. The esophagus moves food from your mouth to your stomach, while the stomach stores and breaks down food. Both have special muscles that control the flow of food and prevent backflow.

Digestion in the stomach happens in three phases. It starts when you see or smell food, continues when food enters the stomach, and finishes as food moves to the small intestine. The stomach produces acid and enzymes to break down food during these phases.

Anatomy and Physiology of the Esophagus and Stomach

Esophagus and stomach structure and function, including sphincters

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• Esophagus
  • Muscular tube connects pharynx to stomach
  • Transports food from mouth to stomach via peristaltic contractions
  • Upper esophageal sphincter (UES)
    • Prevents air from entering esophagus during breathing
    • Relaxes during swallowing allowing food passage (bolus)
  • Lower esophageal sphincter (LES)
    • Prevents reflux of stomach contents back into esophagus (acid, enzymes)
    • Relaxes during swallowing allowing food to enter stomach
• Stomach
  • J-shaped, muscular organ in upper left abdomen
  • Functions: food storage, mechanical digestion (churning), chemical digestion (enzymes, acid)
  • Four main regions: cardia (esophageal junction), fundus (upper portion), body (main central region), pylorus (lower portion connecting to duodenum)
  • Pyloric sphincter
    • Controls passage of partially digested food (chyme) from stomach to duodenum
    • Regulates rate of gastric emptying into small intestine

Common esophageal and stomach disorders

• ###gastroesophageal_reflux_disease_(GERD)_0###
  • Chronic condition caused by reflux of stomach contents into esophagus
  • Symptoms: heartburn, regurgitation (backflow of stomach contents), dysphagia (difficulty swallowing)
  • Complications: esophagitis (inflammation), Barrett's esophagus (precancerous changes), esophageal strictures (narrowing)
• Gastritis
  • Inflammation of stomach lining
  • Causes: H. pylori bacterial infection, NSAIDs (ibuprofen), alcohol, stress
  • Symptoms: abdominal pain, nausea, vomiting, indigestion
  • Complications: peptic ulcers (open sores), gastric cancer

Physiology of Digestion in the Stomach

Three phases of digestion and role of gastric acid secretion

• Cephalic phase
  • Initiated by sight, smell, or thought of food
  • Mediated by vagus nerve stimulation
  • Stimulates gastric acid secretion and gastric motility (contractions)
• Gastric phase
  • Initiated by presence of food in stomach
  • Mechanical digestion: stomach contractions mix and churn food
  • Chemical digestion: secretion of gastric juice
    • Hydrochloric acid ($HCl$) denatures proteins and activates pepsinogen
    • Pepsinogen converted to pepsin, breaks down proteins into peptides
    • Intrinsic factor secreted, facilitates vitamin $B_{12}$ absorption in ileum
  • Gastric acid secretion regulated by hormones (gastrin) and neural factors (vagus nerve)
• Intestinal phase
  • Occurs when chyme enters duodenum
  • Hormones secretin and cholecystokinin released, regulate pancreatic and gallbladder secretions
  • Feedback mechanisms inhibit further gastric acid secretion and slow gastric emptying