2 - Stomach I: Structure, Motility, and Gastric Secretion

Question 1

Objectives: Describe receptive relaxation and retropulsion

Storage

Mixing

Emptying

Answer 1

• Storage
  
  • Receptive Relaxation: Proximal stomach relaxes to accomodate ingested meal
  • Mediated by Vagovagal reflex, which is initiated by distension of stomach and is abolished by vagotomy
• Mixing
  
  • Food causes distal stomach to increase contractions, result is chyme
• Emptying
  
  • Distal stomach contracts to propel food into duodenum

Question 2

Objectives: Describe receptive relaxation and retropulsion

Peristalsis

Retropulsion

Migrating Myoelectric Complexes

Answer 2

• Peristalsis
  
  • Contractions originat in pacemaker region; proveed distally
  • As they move distally, increase in velocity/force
• Retropulsion
  
  • Wave of contraction closes distal antrum and pyloric sphincter before chyme reaches them
  • Causes more chyme to be propelled back into stomach and mixed
• Migrating Myoelectric Complexes
  
  • Occur at 90 min intervals; clear stomach of residual food; cause hunger contractions

Question 3

Objectives: Describe how gastric emptying is regulated

Composition Dependent Rate

Inhibition

Answer 3

• Rate
  
  • Liquids > Solids
  • Carbs > Proteins > Fats
  • Controlled mostly by signals from duodenum
• Inhibition: Prevents flow of chyme from overworking intestine
  
  • High [H+] (low pH)
  • Fat/Protein digestion products
  • Non-isotonic solutions
  • Distension of proximal stomach
  • Pressure in proximal small intestine

Question 4

Objectives: List the components of gastric secretions and describe their functions

Oxyntic Gland Mucosa v Pyloric Gland Mucosa

Answer 4

• Oxyntic Gland Mucosa
  
  • Proximal Stomach
  • Secretes: Acid, Pepsinogen, Intrinsic Factor (IF), Mucus
• Pyloric Fland Mucosa
  
  • Distal Stomach
  • Secretes: Gastrin, (some mucus, pepsinogen)

Question 5

Objectives: List the components of gastric secretions and describe their functions

Structure of Oxyntic Gland

Answer 5

• Structure of Oxyntic Gland
  
  • Mucous neck cells secrete mucus and serve as stem cells
    
    • Differentiate into surface mucus cells, parietal cells, chief cells, or endocrine cells
  • Parietal Cells secrete acid and intrinsic factor
  • Endocrine Cells secrete regulatory products
  • Chief/Peptic Cells secrete pepsinogen

Question 6

Objectives: List the components of gastric secretions and describe their functions

HCl (“acid”)

Pepsin

Mucus

Intrinsic Factor

Answer 6

• HCl
  
  • Begins digestion of protein
  • Converts pepsinogen to pepsin
  • Kills bacteria (loss of acid can lead to bacterial infections)
• Pepsin
  
  • Stored, Secreted as INACTIVE pepsinogen
  • Gastric Acid and Pepsin convert (auto-catalyze)
  • Splits interior peptide bonds
  • (+) Vagal Stimulation increases secretion
• Mucus - Protective Coating
  
  • Soluble Form: Mucous Neck Cells
    
    • (+) Vagus Nerve stimulates
    • Not present in resting stomach
  • Insoluble/Visible Form: Surface Mucus Cells
    
    • Secreted by resting stomach
    • HCO3 trapped in layer; preserving neutrality
• Intrinsic Factor - Parietal Cells
  
  • Bind Vitamin B12
  • Required for absorption of vitamin in ileum
  • Absence: Pernicious Anemia
  • Total gastrectoy patients require injections of B12

Question 7

Objectives: Describe the mechanism of acid production

Secretion - H and Cl movement?

Ionic Composition

Hypokalemia and Vomiting

Answer 7

• Secretion
  
  • Parietal Cells secrete HCl into lumen of stomach and secrete HCO₃- into bloodstream
  • Secreted H+ into lumen exchanges for K+ in a 1:1 ratio by H-K ATPase
  • Cl- enters cell in active exchange for HCO₃-
    
    • Creates alkaline tide in venous blood (high pH)
• Ionic Composition
  
  • Low (basal) Rates - Gastric juice primarily NaCl from nonparietal secretion
  • High (stimulated) Rates - Gastric juice primarily HCl from parietal secretion
• Hypokalemia
  
  • ​K+ concentration in gastric juice always higher than in plasma; chronic vomiting may lead to hypokalemia

Question 8

Objectives: Describe how the ionic composition of gastric juice changes with the rate of secretion

Answer 8

• High Secretory Rate
  
  • HCl increase
• Low Secretory Rate
  
  • Mostly NaCl

Question 9

Describe general stomach anatomy

Proximal vs distal regions

Answer 9

• Proximal (Fundus and Proximal Body)
  
  • Orad Stomach - receives ingested meal
• Distal Region (Antrum and Distal Body)
  
  • Caudad Stomach - Contractions to mix food, propel it into duodenum

Question 10

Describe major disorders of gastric emptying

Gastroparesis - Impaired/Delayed Emptying

Increased Emptying

Dumping Syndrome

Answer 10

• Gastroparesis - Impaired/Delayed Emptying
  
  • Symptoms: Feeling full; no appetite; nausea; vomitus
  • Diabetes
  • Causes: Anticholinergics, neurological disorders
• Increased Emptying
  
  • Can cause diarrhead due to increased osmotic load in SI
  • Can cause duodenal ulcers from stomach acid
• Dumping Syndrome
  
  • Occurs when lower end of small intestine fills too quickly with undigested food from the stomach
  • Common after stomach surgery
  • Symptoms:
    
    • Early: Nausea, Vomiting, Diarrhea (too much fluid)
    • Late: Sweating, Weakness, Dizziness (hypoglycemia)