Physiology of the Digestive System: Digestion in the Mouth, Esophagus, and Stomach

1. Swallowing

• Swallowing is a complex act that involves the coordinated action of multiple muscles, including both voluntary and involuntary components.
• Stages of swallowing:
  • Oral stage: Food is chewed and mixed with saliva, then pushed to the back of the tongue.
  • Pharyngeal stage: The tongue pushes the food against the soft palate, the pharyngeal constrictors contract, the soft palate rises to block the nasal passage, the epiglottis closes, and the food is directed into the esophagus.
  • Esophageal stage: Food is moved down the esophagus through peristalsis.
• Control of swallowing:
  • Swallowing center: Located in the medulla oblongata, controls the entire swallowing process.
  • Nerves: Vagus nerve (parasympathetic) innervates the esophageal muscles.
• Notes:
  • Swallowing is a complex coordinated action that requires the synchronized activity of many muscles and nerves.
  • Swallowing is not entirely voluntary, with the pharyngeal and esophageal stages being reflexive.
  • In comatose patients, swallowing may be impaired.
• Incorrect statement:
  • Statement 954: Swallowing is a completely voluntary action (Incorrect, the pharyngeal and esophageal stages are reflexive).

2. Esophagus

• Function: Transports food from the mouth to the stomach.
• Structure: A muscular tube lined with smooth muscle, divided into three sections:
  • Body of the esophagus: The majority of the esophagus.
  • Upper esophageal sphincter: Located at the junction between the pharynx and the esophagus.
  • Lower esophageal sphincter: Located at the junction between the esophagus and the stomach.
• Esophageal peristalsis:
  • Primary peristalsis: A continuation of pharyngeal peristalsis, stimulated by distension of the esophagus, and does not require the vagus nerve.
  • Secondary peristalsis: Triggered by food stimulating the esophageal wall, requiring the vagus nerve.
• Lower esophageal sphincter: Functions as a valve to prevent food reflux from the stomach into the esophagus.
  • Influencing factors: Abdominal pressure, parasympathetic nerves, sympathetic nerves.
• Gastroesophageal reflux disease (GERD): Occurs when the lower esophageal sphincter has decreased tone, allowing gastric acid to reflux into the esophagus, causing mucosal damage.
• Notes:
  • The lower esophageal sphincter plays a critical role in preventing gastroesophageal reflux.
  • Esophageal dysmotility can lead to dysphagia (difficulty swallowing).
• Incorrect statement:
  • Statement 956: Contraction of the upper esophageal sphincter does not participate in swallowing (Incorrect, the upper esophageal sphincter prevents food reflux).

3. Saliva

• Function:
  • Moistens and lubricates food.
  • Washes away disease-causing bacteria.
  • Partially digests starch (salivary amylase).
  • Neutralizes acid produced by bacteria.
• Composition:
  • Water: The majority of the saliva.
  • Electrolytes: Na+, K+, Cl-, HCO3-.
  • Digestive enzymes: Salivary amylase.
  • Mucus: Lubricates food.
  • Antibodies: Fight infections.
• Secretion:
  • Salivary glands: Three major pairs: parotid glands, submandibular glands, sublingual glands.
  • Control: Glossopharyngeal nerve and vagus nerve (parasympathetic), feeding center in the hypothalamus.
• Notes:
  • Saliva is an important digestive fluid that plays a role in digestion and mucosal protection.
  • Decreased saliva production can lead to dry mouth, difficulty swallowing, and dental caries.
• Incorrect statement:
  • Statement 970: K+ concentration in saliva is 7 times lower than in plasma (Incorrect, K+ concentration in saliva is higher than in plasma).

4. Stomach

4.1. Mechanical Activity

• Gastric motility: Waves of smooth muscle contractions in the stomach push food from one location to another within the stomach, mixing food with gastric juice.
• Control: Myenteric and Meissner’s plexuses (enteric nervous system).
  • Parasympathetic nerves: Increase mechanical activity.
  • Sympathetic nerves: Inhibit mechanical activity.
• Gastroileal reflex: Distension of the duodenum stimulates a sympathetic reflex that decreases gastric motility, reducing the flow of food into the duodenum.
• Pyloric sphincter: Regulates the flow of food into the duodenum.
  • Influencing factors: Gastric pH, duodenal distension, hormones (gastrin, CCK, secretin, GIP).
• Notes:
  • Stomach mechanical activity is essential for food digestion, mixing with gastric juice, and pushing food into the duodenum.
  • Gastric dysmotility can lead to gastroesophageal reflux, dyspepsia, and vomiting.
• Incorrect statement:
  • Statement 980: Stimulation of the vagus nerve decreases mechanical activity (Incorrect, the vagus nerve mediates parasympathetic control, increasing mechanical activity).

4.2. Gastric Juice Secretion

• Composition of gastric juice:
  • HCl: Activates pepsin, kills bacteria.
  • Pepsin: Digests protein.
  • Mucus: Protects the gastric mucosa.
  • Intrinsic factor: Helps absorb vitamin B12.
  • Lipase: Partially digests lipids.
• Mechanism of HCl secretion:
  • Parietal cells: Transport H+ into the lumen via a proton pump.
  • CA enzyme: Facilitates HCl secretion.
  • HCO3-: Secreted into the extracellular fluid, alkalinizing the blood.
• Control:
  • Nerves: Vagus nerve (parasympathetic), stimulates gastric juice secretion.
  • Hormones:
    • Gastrin: Stimulates HCl secretion, secreted by G cells.
    • Histamine: Stimulates HCl secretion, secreted by ECL cells.
    • Acetylcholine: Stimulates HCl secretion.
    • Secretin: Inhibits HCl secretion.
    • CCK: Inhibits HCl secretion.
    • GIP: Inhibits HCl secretion.
    • Prostaglandin E2: Inhibits HCl secretion, protects the mucosa.
• Notes:
  • Gastric juice is essential for protein digestion, bacterial killing, and protecting the gastric mucosa.
  • Gastric juice secretion disorders can lead to peptic ulcers, gastritis.
• Incorrect statement:
  • Statement 995: HCl is secreted by chief cells (Incorrect, HCl is secreted by parietal cells).

4.3. Gastric Mucosal Protective Barrier

• Structure:
  • Mucus: A mucous layer covering the gastric mucosa, protecting it from the effects of HCl.
  • HCO3-: Creates an alkaline environment on the mucosal surface, neutralizing HCl.
  • Prostaglandin E2: Inhibits HCl secretion, stimulates mucus and HCO3- secretion, protects the mucosa.
• Factors causing gastric ulcers:
  • Aspirin: Inhibits COX enzymes, reducing PGE2 synthesis.
  • Corticosteroids: Stimulate HCl secretion, inhibit mucus secretion.
  • Helicobacter pylori bacteria: Causes gastritis and ulcers.
• Notes:
  • The gastric mucosal protective barrier is crucial in preventing gastric ulcers.
  • Pain relievers, corticosteroids, and Helicobacter pylori infection are risk factors for gastric ulcers.
• Incorrect statement:
  • Statement 1020: Corticosteroids directly damage gastric mucosal cells (Incorrect, corticosteroids do not directly damage cells but inhibit mucus secretion and increase HCl secretion).

4.4. Food Digestion in the Stomach

• Protein digestion:
  • Pepsin: Digests protein into short peptide chains.
  • HCl: Activates pepsin.
• Lipid digestion:
  • Lipase: Partially digests lipids into fatty acids.
  • HCl: Creates an acidic environment, enhancing lipase activity.
• Notes:
  • The stomach primarily digests protein, and lipids are partially digested.
  • Food stays in the stomach for approximately 2-4 hours for digestion.

5. Absorption

• The stomach primarily absorbs the following substances:
  • Alcohol: Rapidly absorbed through the gastric mucosa.
  • Water: Partially absorbed.
  • Amino acids: Partially absorbed.
  • Glucose: Partially absorbed.
• Notes:
  • The stomach is not the primary site of absorption in the digestive system.

6. Treatment of Peptic Ulcers

• Goals:
  • Inhibits HCl secretion.
  • Enhances the mucosal protective barrier.
  • Eradicates Helicobacter pylori bacteria.
• Treatment methods:
  • Proton pump inhibitors: Most effective inhibitors of HCl secretion.
  • H2 receptor antagonists: Inhibit histamine’s action, reducing HCl secretion.
  • Muscarinic antagonists: Inhibit acetylcholine’s action, reducing HCl secretion.
  • Mucosal protectants: Strengthen the mucosal protective barrier.
  • Antibiotics: Eradicate Helicobacter pylori bacteria.
• Notes:
  • Peptic ulcer treatment should be performed under the guidance of a physician.
  • Lifestyle changes and dietary modifications are essential to prevent peptic ulcers.

Additional Information

• The above article provides basic knowledge about the physiology of digestion in the mouth, esophagus, and stomach.
• To gain a deeper understanding of related issues, you can consult specialized materials on physiology.
• Understanding these digestive mechanisms is crucial for preventing and treating digestive system diseases.
• A healthy diet and lifestyle play a critical role in maintaining digestive health.