Digestion in the stomach

• Functions of the stomach in digestion

  • Motor function
  • Secretory function
  • Source of humoral agents that regulate digestion

  
• Motor function


  • Acts as a temporary reservoir for the food
  • Mixing food with gastric secretions
  • Propels digested food to the duodenum
• Secretory function


  • Secretion of gastric acid
  • Secretion of enzymes (pepsinogen and gastric lipase)
  • Secretion of mucus and bicarbonates (to protect gastric mucosa from acidity)
• Source of humoral agents that regulate digestion

  Gastrin, Histamine, Somatostatin
• Structure of the stomach

  • Stomach three main regions – fundus, body, and antrum
  • Fundus is the upper portion of the stomach that is located above the lower esophageal sphincter
  • Below the fundus is a large, central portion of the stomach, called the body
  • Extending from the body is the antrum, the lower portion of stomach
  • The end of antrum communicates with duodenum via a pyloric sphincter (consists of smooth muscle)

  
• Layers of the gastric wall

  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa
• Muscularis externa of the stomach


  • Outer longitudinal layer
  • Middle circular layer
  • Inner oblique layer

  
• Distal part of the stomach

  Increasing amount of muscle, largest amount in the antrum
• Parasympathetic stimulation and gastrin


  Increase the force of gastric smooth muscle contractions
• Sympathetic stimulation, secretin and cholecystokinin

  Reduce the force of gastric smooth muscle contractions

  
• Contractile cells


  Responsible for stomach contractions
• Autorhythmic cells


  Responsible for originating stomach contractions

  
• Factors that support the reservoir function of the proximal part of the stomach

  • Tonic contractions (sustained increase in muscle tension)
  • Rugae (multiple folds) present in the stomach mucosa
  • Receptive relaxation of the stomach upon food entry from esophagus

  
• Tonic contraction


  Generates a sustained compressive force that reduces the volume occupied by food
• Rugae


  Large folds in the gastric mucosa that allow the stomach to expand
• Receptive relaxation


  Reduces intragastric pressure and increases the stomach volume to accommodate food
• Types of phasic contractions in the stomach

  • Propulsion
  • Retropulsion
  • Gastric emptying

  
• Propulsion


  Peristaltic waves that move gastric content from the body of the stomach down to its antrum
• Retropulsion


  Food particles too large to fit through the pyloric sphincter are forced back into the body of the stomach
• Gastric emptying


  Process by which the content of the stomach is moved to the duodenum
• Gastric emptying


  • Phasic contraction of the smooth muscle in the stomach antrum (antral pump)
  • Relaxation of the pyloric sphincter
• Factors that inhibit gastric emptying


  Presence of fat and presence of gastric acid (low pH) in the duodenum
• Fatty acids in the duodenum


  Stimulate release of hormone CCK, which inhibits gastric contractions
• Low pH in the duodenum


  Stimulates release of hormone secretin, which inhibits gastric contractions
• Gastric emptying of different types of food

  • Liquids leave faster than solids
  • Isotonic contents leave faster than hypotonic or hypertonic
  • Carbohydrate-rich foods empty faster than protein-rich
  • Fats empty slowest
• Secretory function of the stomach

  Production of gastric juice, containing hydrochloric acid, pepsin, and gastric lipase
• Salivary amylase and lingual lipase

  Continue their action in the stomach after saliva has been swallowed
• Salivary amylase


  Digestion continues in the fundus, then inactivated by gastric acid
• Lingual lipase


  Activated by gastric acid, starts to digest triglycerides
• Gastric juice


  About 2 liters secreted daily, pH 1-2, contains hydrochloric acid, pepsinogen, mucus, gastric lipase, and intrinsic factor
• Structure of the gastric gland

  • Contains three types of exocrine cells: mucous cells, parietal cells, and chief cells
  • Also contains endocrine G cells that secrete gastrin
• Mucous cells


  Secrete mucus
• Parietal cells


  Secrete hydrochloric acid and intrinsic factor
• Chief cells


  Secrete pepsinogen and gastric lipase
• G cells


  Secrete the hormone gastrin
• Gastrin


  Stimulates the parietal cells to secrete hydrochloric acid
• Gastric acid


  Secreted by parietal cells
• Pepsin


  Role in chemical digestion in the stomach
• Gastric lipase


  Role in chemical digestion in the stomach
• Gastric mucosa


  • Numerous gastric pits (electron micrograph)