Ch 23 Digestive System Functional Anatomy Pt II

Chapter 23: The Digestive System - The Esophagus and Stomach

The Esophagus

• The esophagus primarily resides in the thoracic cavity, not the abdominal cavity.

• Connection: Links the lower laryngopharynx to the upper stomach.

• Lumen: Central opening where food passes from the oral cavity to the stomach for chemical digestion.

Structure of the Esophagus

Layers of Tissue

1. Mucosa:

   • Composed of stratified squamous epithelium for friction resistance while food passes.

   • Function: Primarily protective as absorption does not occur here.

2. Submucosa: (areolar connective tissue)

   • Mainly connective tissue with modest secretions aiding the movement of food through the esophagus.

3. Muscularis Externa:

   • Upper region consists of skeletal muscle for swallowing, transitioning to smooth muscle (circular and longitudinal layers).

   • Functions:

     • Circular Layer: Squeezes the lumen to push food down.

     • Longitudinal Layer: Shortens the esophagus and opens the lumen.

     • Together, these layers facilitate peristalsis, pushing food into the stomach.

4. Adventitia:

   • Dense, irregular connective tissue surrounding muscularis externa; important since the esophagus is largely outside the abdomen.

The Stomach

• The stomach is a muscular organ that temporarily holds food and mixes it with digestive juices.

• Volume: Can hold about 50 ml when empty but can stretch significantly when full.

• Food is converted to chyme by acids and enzymes

• Joined at ends to the esophagus and small intestine

• Autonomic nervous system control

  • Sympathetic control – inhibits

  • Parasympathetic control – stimulates

• Modified muscularis  layer

  • Also contains inner most oblique layer of smooth muscle

  • Allows physical mixing, churning of food

• Modified mucosa layer

  • Simple columnar epithelium entirely foveolar cells

  • Produce HCO3 containing mucus coat

  • Joined by tight junctions

  • Surface epithelium renowned every 3-6 days (stem cells in the gastric pits)

Structure of the Stomach

1. Regions:

   • Cardia: Near the esophagus.

   • Fundus: Dome-like portion under the diaphragm.

   • Body: The largest segment of the stomach.

   • Pyloric Antrum: Leads to pyloric canal and pyloric sphincter.

2. Sphincters:

   • Pyloric Sphincter: Regulates exit from stomach to duodenum of the small intestine, prevents backflow.

   • Cardia Sphincter: Controls entry of food and gastric juices into the esophagus, preventing acid reflux.

3. Muscularis Externa:

   • Contains three layers: longitudinal, circular, and an additional oblique layer for churning and peristalsis, mixing food with gastric secretions to create chyme.

   • Function: Churning and propulsion of food towards pyloric canal for digestion.

Gastric Functionality

• Chemical Digestion: The stomach plays a crucial role in digesting proteins through pepsinogen (inactive enzyme) activated to pepsin by acid, breaking down proteins into smaller peptides.

• Rugae: Folds in the stomach lining that allow for expansion and increased surface area for contact with secretions; they do not decrease the effective surface area during contraction.

Curvatures of the Stomach

• Lesser Curvature: Smaller side adjacent to the esophagus.

• Greater Curvature: Larger side opposite the lesser curvature.

Peritoneum and Omentum

• Serosa: The outer layer of the stomach lined by peritoneum.

• Omentum: Refers to the peritoneum associated with the stomach.

Stomach Function and Structure

1. Enzyme Secretion: Pepsinogen to Pepsin

• The stomach secretes pepsinogen, an inactive enzyme precursor.

• Pepsinogen is activated by hydrochloric acid (HCl), transforming into pepsin, which breaks down proteins into smaller peptides.

• Activation of pepsinogen by acid is crucial for chemical digestion of proteins.

2. Role of Hydrochloric Acid

• Hydrochloric acid facilitates protein breakdown and activates pepsinogen.

• It creates an acidic environment necessary for digestion.

3. Churning Action of the Stomach

• The stomach contains three layers of smooth muscle that contract for churning.

• Churning mixes food with enzymes and acid, forming chyme, a semi-liquid mixture essential for digestion.

• Chyme consists of ingested food combined with gastric secretions.

4. Nervous System Influence

• The stomach's activity is influenced by the autonomic nervous system.

• Stress inhibits stomach activity, while relaxation stimulates contractions and secretions that assist digestion.

Stomach Mucosa

5. Structure of Mucosa

• The mucosa of the stomach includes

  • Surface Epithelium: Composed of simple columnar epithelium, known as foveolar cells.

  • Lamina Propria: Contains glands responsible for secreting mucus and digestive enzymes.

6. Gastric Pit and Gland Structure

• Gastric pits are located in the surface epithelium and lead to gastric glands.

• Gastric glands consist of four cell types:

  • Mucous Neck Cells: Secrete mucus for protection against acid.

  • Parietal Cells: Secrete hydrochloric acid and intrinsic factor (essential for Vitamin B12 absorption).

  • Chief Cells: Secrete pepsinogen.

  • Enteroendocrine Cells: Release signaling molecules like gastrin, histamine, and serotonin into the bloodstream.

Protection Mechanisms in the Stomach

7. Mucus and Bicarbonate Secretion

• Mucus coats the stomach lining, protecting it from acidity and enzyme degradation.

• Bicarbonate neutralizes excess acid, preventing damage to the stomach lining.

8. Cell Structure and Replacement

• Foveolar cells connected by tight junctions prevent acid from leaking into intercellular spaces.

• Stomach lining is replaced every 3 to 6 days to maintain integrity and functionality.

Summary of Gastric Secretions

• Hydrochloric Acid: Creates acidity for digestion.

• Intrinsic Factor: Vital for B12 absorption.

• Pepsinogen: Converted to pepsin for protein digestion.

• Gastrin, Histamine, Endorphins, Serotonin: Hormones and signals produced by enteroendocrine cells, influencing gastric functions.