Overview
This lecture introduces the structure and function of the digestive system, focusing on the organs, key processes, histology, and movements involved in digestion.
Organization of the Digestive System
- The digestive system consists of the alimentary canal (digestive tract) and accessory digestive organs.
- The alimentary canal is a muscular tube running from mouth to anus.
- Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
Pathway of Food Through the Digestive Tract
- Food enters the mouth (oral cavity) where teeth, tongue, and salivary glands form a bolus.
- The bolus moves through the pharynx and esophagus to the stomach.
- In the stomach, food mixes with acid to form chyme.
- Chyme moves to the duodenum (first part of small intestine) where most digestion occurs.
- Nutrient absorption mainly happens in the jejunum (middle part of small intestine).
- The ileum (last part of small intestine) connects to the cecum of the large intestine.
- The large intestine absorbs water, compacts waste, and stores feces for excretion.
Functions and Key Terms of the Digestive System
- Ingestion: Taking food into the mouth.
- Mechanical processing: Chewing and breaking down food physically.
- Digestion: Chemical breakdown of food into amino acids, fatty acids, nucleic acids, and monosaccharides.
- Secretion: Release of acids, enzymes, and other fluids for digestion.
- Absorption: Movement of nutrients from the digestive tract into blood or lymph.
- Compaction: Dehydration and consolidation of indigestible material into feces.
- Defecation: Elimination of waste through the anus.
Histological Layers of the Alimentary Canal
- Four layers: mucosa, submucosa, muscularis externa, and serosa (or adventitia).
- Mucosa has epithelium (simple or stratified, depending on location), lamina propria, and muscularis mucosae.
- Submucosa contains dense irregular connective tissue, blood vessels, lymphatics, glands, and the submucosal neural plexus.
- Muscularis externa typically has circular and longitudinal smooth muscle layers, with the myenteric plexus between them.
- Serosa (visceral peritoneum) covers most digestive organs; organs without it have an adventitia for anchoring.
Peritoneum and Mesenteries
- The peritoneum consists of the visceral and parietal layers with the peritoneal cavity in between.
- Organs completely surrounded are intraperitoneal (e.g., stomach, jejunum, liver); those behind it are retroperitoneal (e.g., kidneys).
- Mesenteries are folds of peritoneum that suspend and anchor digestive organs (e.g., greater omentum, mesentery proper).
Movements of the Digestive Tract
- Peristalsis is a wave-like muscle contraction moving contents forward from mouth to anus.
- Segmentation is non-directional churning that mixes contents with digestive juices, mainly in the small intestine.
Key Terms & Definitions
- Alimentary canal — muscular tube from mouth to anus for digestion and absorption.
- Accessory organs — organs aiding digestion without being part of the canal.
- Bolus — chewed food mixed with saliva, ready for swallowing.
- Chyme — semi-liquid food blended with stomach acids.
- Mesentery — double-layered peritoneal folds anchoring digestive organs.
- Peristalsis — muscle contractions moving food along the tract.
- Segmentation — mixing contractions in the intestines without directional movement.
- Serosa/Visceral peritoneum — outer serous membrane covering digestive organs.
- Adventitia — connective tissue layer anchoring organs without a serosa.
Action Items / Next Steps
- Review diagrams of the digestive tract and peritoneal structures.
- Memorize histological layers and key digestive terms.
- Prepare questions for clarification in the next session.