Digestive System Lecture: The Stomach

Overview

• Focus on the stomach within the digestive system.
• Progression from the oral cavity, through the esophagus, to the stomach.

Anatomy of the Stomach

• Outer features:
  • Greater and lesser curvature.
• Layers of the stomach:
  • Serosa, muscularis externa (with an extra oblique layer), and mucosa.
  • Oblique layer unique to the stomach.
• Physiological regions within the stomach:
  • Cardia: High concentration of sodium bicarbonate to buffer acids.
  • Fundus: Anatomically the top but functionally the bottom, farthest from pyloric opening.
  • Pylorus: Differentiated from the body of the stomach.

Function and Physiology

• Physical functions:
  • Acts as a muscular bag to mush food into a toothpaste-like consistency.
  • Extensible with expansion folds called rugae.
• Chemical functions:
  • Secretions aid in digestion and protect the stomach lining.

Stomach Lining and Secretions

• Gastric Pits and Glands:
  • Gastric pits lead to gastric glands.
  • Mucous neck cells: Produce mucus to protect the stomach lining.
• Cell types and their functions:
  • Parietal cells: Produce hydrochloric acid (HCl) for digestion.
    • Functions: Activate pepsinogen, lower pH for enzyme activity, kill bacteria, and denature proteins.
  • Chief cells: Produce pepsinogen, a proenzyme activated to pepsin by HCl.
  • G cells: Produce gastrin, stimulating parietal and chief cells.
  • D cells: Produce somatostatin, inhibiting gastrin production.
  • Mast cells: Produce histamine, stimulating chief and parietal cells.
  • Intrinsic factor: Produced by parietal cells for B12 absorption.

Gastric Phases

• Cephalic phase:
  • Triggered by sight, smell, taste, or thought of food.
  • Stimulates gastric secretions and motility via the vagus nerve.
• Gastric phase:
  • Triggered by food entering the stomach.
  • Local factors such as distension and pH changes stimulate gastric secretions.
  • Positive feedback increases digestive activity.
• Intestinal phase:
  • Triggered by chyme entering the duodenum.
  • Enterogastric reflex slows gastric motility and secretions.
  • Hormones like CCK, GIP, and secretin inhibit gastric activities.

Additional Reflexes

• Gastroenteric reflex: Enhances activity along the small intestine.
• Gastroileal reflex: Controls the movement from the small to large intestine.
• Emetic reflex: Causes reverse peristalsis leading to vomiting.

These notes cover the main points about the stomach's anatomy, physiology, and digestive processes as explained in the lecture. They emphasize the physical and chemical functions, detailing the different phases of gastric activity and the roles of various cells and hormones involved in digestion.