Anatomy and Physiology of Digestive System

Digestive System: Overview

• The digestive system consists of the gastrointestinal (GI) canal or tract and accessory or associated digestive organs.
• The GI canal is a continuous coiled hollow tube comprising the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
• Accessory organs include teeth, salivary glands, pancreas, liver, and gallbladder.

Layers of the Digestive Tract

• The digestive tract has four layers (tunics):
  • Mucosa: inner secretory and absorptive layer with simple columnar epithelium, lamina propria, muscularis mucosae, and goblet cells that secrete mucus.
  • Submucosa: very vascular, with glands and nerve plexuses.
  • Muscularis: smooth muscle responsible for peristalsis and segmentation, with a nerve plexus for autonomic nervous system control.
  • Serosa: outer binding and protective layer.

Functions of the Stomach

• The stomach stores food, churns it to mix with gastric secretions, and begins protein digestion with pepsin.
• Gastric acid kills bacteria, denatures proteins, and activates pepsinogen into pepsin.
• Intrinsic factor secretion from the stomach lining helps in vitamin B12 reabsorption later in the intestine.
• The stomach moves food into the small intestine in the form of chyme.

Digestion and Absorption in the Stomach

• Protein digestion starts in the stomach.
• Starch digestion starts in the mouth but stops in the stomach due to pH levels.
• Only alcohol and NSAIDs (aspirin) are absorbed in the stomach.

Stomach: HCl Secretion

• HCl secretion in the stomach involves the formation of carbonic acid from CO2 and H2O, which then dissociates into bicarbonate ion and H+.
• Parietal cells couple the transport of Cl- and bicarbonate ion across the plasma membrane, secreting Cl- into the gastric lumen.
• The H+/K+ ATPase pump couples the secretion of H+ into the gastric lumen while transporting K+ from the lumen of the stomach in the opposite direction.

Regulation of Gastric Secretion

• HCl secretion by parietal cells is regulated by the vagus nerve (parasympathetic), hormone gastrin, and histamine secretion.

Stomach Defenses

• The stomach lining has defenses to prevent damage from acid and pepsin, including:
  • An adherent layer of mucus with alkaline bicarbonate.
  • Tight junctions between epithelial cells to prevent underlying tissue destruction.
  • Rapid epithelial mitosis (cell division) that replaces the whole stomach epithelium every three days.

Small Intestine Structure

• The small intestine is divided into three sections:
  • Duodenum: first 25 cm (most digestion).
  • Jejunum: middle 2/5 (most absorption).
  • Ileum: last 3/5.