RGI.12 Alimentary Canal, Ingestion, Oesophagus, Stomach, Ulcers

Digestive System Overview

• Primary Function: Provides the body with water, nutrients, and electrolytes.
• Accessory Organs: Liver and Pancreas
• Gut Physiology Processes:
  1. Transportation (Ingestion)
  2. Physical and Chemical Treatment
     • Mechanical processing
     • Digestion
     • Secretion
     • Excretion
  3. Absorption
• Egestion: Follows absorption.

Ingestion

• Initial Digestion: Starts in the mouth through mastication and mixing with saliva.
• Salivary Glands: Three pairs – Parotid, Submandibular, Sublingual.
• Salivation: Initiated by taste, smell, sight, or thought of food.

Saliva

• Secretion Volume: Approximately 1.2 liters/day.
• Components:
  • Mucus: From glycoproteins called mucins, providing lubrication crucial for oral health.
  • Electrolytes: Na+, Cl-, K+, and bicarbonate (HCO_3) ions.
  • Enzymes:
    • Salivary amylase: Breaks down carbohydrates (Starch → Maltose).
    • Lingual lipase: Minor role in fat digestion.
  • Antibodies
• Two Types of Secretions:
  1. Mucus secretion: For lubrication.
  2. Serous secretion: Contains salivary amylase.
• pH: 6.0 - 7.4, optimal for salivary amylase activity.

Control of Salivary Secretions

• Autonomic Nervous System: Salivary glands have sympathetic and parasympathetic innervation.
• Medulla Oblongata: Salivation controlled by salivatory nuclei in the medulla oblongata (parasympathetic system).
• Stimulation: Speeds up saliva secretion.
• Sympathetic System: Role is unclear.

Pharynx

• Function: Common passageway for food, liquids, and air between the mouth and esophagus (throat).
• Epiglottis: Prevents food from entering the trachea during swallowing.
• Swallowing (Deglutition) Stages:
  1. Voluntary swallowing: Involving the tongue.
  2. Pharyngeal involuntary process: Series of reflex actions.
  3. Esophageal involuntary stage: Food bolus moved by peristalsis.

Peristalsis

• Definition: Contraction of circular muscles above a bolus with relaxation below it.
• Occurrence: Esophagus, stomach, and small intestine.
• Control: Autonomic nervous system.

General Structure of GIT

• Layers:
  • Mucosa
    • Inner Tissue: Epithelium
  • Muscularis mucosa
    • Inner Tissue: Smooth muscle
  • Submucosa
    • Inner Tissue: Connective tissue, Lymph nodes
  • Muscularis externa
    • Inner Tissue: Smooth circular, Smooth longitudinal
  • Serosa
    • Outer Tissue: Mesothelium, Loose connective tissue

Esophagus

• Description: Hollow muscular tube from pharynx to stomach.
• Muscle Composition:
  • Upper third: Striated and smooth muscle.
  • Lower third: Smooth muscle only.
• Epithelium: Stratified squamous cells.
• Dimensions: Approximately 25 cm long, 2 cm diameter at widest point.
• Passage: Through the thoracic cavity, entering the abdominal cavity via the esophageal hiatus in the diaphragm.

Hiatus Hernia

• Hernia: Abnormal exit of tissue or organ through the cavity wall.
• Hiatus Hernia: Stomach pushes into the thorax through the esophageal opening in the diaphragm.

Stomach - Anatomy

• Function: Dilated part of the alimentary canal where food is retained for physical and chemical treatment.
• Location: C-shaped organ on the left side of the abdominal cavity, partially hidden by the liver.
• Regions: Divided into four anatomical regions, but three histological regions: Cardiac, Fundus and Body, Pyloric region.
• Gastroesophageal Sphincter: Weak sphincter at the entry to the cardiac region.
• Rugae: Inner folds characterizing the body and fundus.

Stomach - Anatomy

• Gastric Glands: Located in fundus and body, secrete enzymes and hydrochloric acid.
• Gastric Pits: Gastric glands are developed at the base of deep tubular glands in the mucosa.
• Gastric Juice Secretion: Base of gastric pit has 1-7 gastric glands that secrete gastric juice.
• Muscularis Externa: Oblique muscle layer inside the circular layer.
• Pyloric Stomach: Deep irregular gastric pits terminating in the pyloric sphincter.

Gastric Glands

• Gastric Juice Composition: Secretions of two primary cell types:
  • Parietal (Oxyntic) Cells: Secrete hydrochloric acid (HCl); large, rounded cells with an intracellular canaliculus system.
  • Chief (Zymogen) Cells: Clustered at gland base, secrete pepsin (proteolytic enzyme).

Gastric Glands

• Other Cell Types:
  1. Mucous Cells: Cover the luminal surface and extend into the glands.
  2. Enteroendocrine Cells (G-cells): Found at the base of the gland, especially in the pylorus; secrete gastrin.
  3. Stem Cells: Located in the neck of the gland; differentiate into all cell types.
• Pyloric Stomach Lining: Deep irregular pits lined mostly with mucous cells.

Stomach Cell Types & Function

Stomach - Physiology

• Chyme Formation: Food mixes with gastric juices via peristaltic waves (3/min), forming a liquid paste called chyme.
• Enzymes: Pepsin (digests proteins) and gastric lipase (digests fats).
• Pepsin Secretion: Secreted as inactive pepsinogen, converted to active pepsin by acid conditions and existing pepsin.
• Pepsin Action: Endo peptidase that hydrolyzes peptide bonds of aromatic amino acids, resulting in peptides averaging 7 amino acid residues.
• Gastric Lipase: Not highly active.
• Rennin:
  • Not found in adult humans.
  • Present in newborns to clot milk.
  • In calves, acts on milk protein casein.

Stomach - Physiology

• HCl Production Mechanism: Poorly understood but involves intracellular canaliculi.
• H+ Transport: Actively transported into the lumen of the canaliculi.
• Gastric Juice Secretion Rate: 2 liters/day, with an acidic pH of 2-3.
• Secretion Mechanisms:
  1. Neurogenic Reaction: Vagus X pathway associated with salivation also initiates gastric juice secretion.
     • Food in the stomach causes distension, detected by stretch receptors.
     • Signals to the brain trigger digestive hormone release for sustained gastric stimulation.
  2. Hormonal: Enteroendocrine cells in pyloric mucosa secrete gastrin.
     • Gastrin is carried to gastric glands in the fundus and body, stimulating gastric acid release.

Stomach - Physiology

• Absorption: Very little absorption occurs; limited to lipid-soluble substances like alcohol and some drugs.
• Chyme Retention: Liquid chyme remains for up to 4 hours.
• Chyme Release: Intermittently admitted into the small intestine in small amounts via pyloric sphincter relaxation.
• Control: Inhibitory feedback signals from the duodenum through nervous (enterogastric reflex) and hormonal reactions.

Stomach - Physiology; Gastric Ulcers

• Prevalence: Gastric ulcers may affect up to 10% of adult males, with a male:female ratio of 2:1.
• Symptoms: Epigastric pain 1 to 3 hours after eating.
• Cause: 90% of recurrent ulcers are caused by Helicobacter pylori.
  • Action: Burrows into the gastric epithelium, destroying the mucosal layer and exposing it to acid.
  • Treatment: Revolutionized by the discovery; often treated with antibiotics.