BIO10004 WK10

• Layers of the digestive tract


  • Mucosal lining
  • Arteries and veins
  • Lymphatics
• Structure of the stomach


  • Mucosa
  • Muscularis externa
  • Serosa
• Mechanical digestion and propulsion

  Crushing of food and propelling food along the digestive tract
• Chemical digestion


  Chemical breakdown of food into organic and inorganic molecules suitable for absorption by the digestive epithelium
• Secretion


  Release of water, acid, enzymes, buffers and salt by the epithelium of digestive tract, glandular organs and gallbladder
• Absorption


  Movement of organic molecules, electrolytes, vitamins, minerals and water across digestive epithelium into the interstitial fluid for distribution to body cells
• Layers of the digestive tract


  • Mucosa
  • Submucosa
  • Muscular layer
  • Serosa
• Mucosa


  The inner lining, or mucosa, of the digestive tract is a mucous membrane consisting of an epithelium, moistened by glandular secretions, a lamina propria of areolar tissue, and a muscular muscularis mucosae
• Digestive epithelium


  The mucosal epithelium is either simple or stratified, depending on its location and the stresses placed on it
• Lamina propria


  A layer of areolar tissue that also contains blood vessels, sensory nerve endings, lymphatic vessels, smooth muscle cells, and scattered lymphatic tissue
• Submucosa


  A layer of dense irregular connective tissue that binds the mucosa to the muscular layer, containing numerous blood vessels and lymphatic vessels, and in some regions also containing exocrine glands that secrete buffers and enzymes into the lumen of the digestive tract
• Submucosal plexus


  Lies along the inner border of the muscular layer, containing smooth muscle cells that play an essential role in mechanical digestion and in moving materials along the digestive tract
• Serosa


  A serous membrane that covers the muscular layer along most portions of the digestive tract enclosed by the peritoneal cavity
• Peristalsis


  Waves of muscular contractions that move a bolus along the length of the digestive tract
• Segmentation


  Cycles of contraction in the small intestine and some portions of the large intestine that churn and fragment the bolus, mixing the contents with intestinal secretions
• Local environmental factors


  Such as the pH, volume, or chemical composition of the intestinal contents can have a direct effect on digestive activity in that segment of the digestive tract
• Neural mechanism


  Sensory receptors in the walls of the digestive tract trigger peristaltic movements, and the visceral motor neurons in the myenteric plexus control smooth muscle contraction and glandular secretion, including local reflexes that operate entirely outside the control of the central nervous system
• Hormonal mechanism


  Hormones produced by enteroendocrine cells in the digestive tract can enhance or diminish the sensitivity of the smooth muscle cells to neural commands
• Functions of the oral cavity


  • Sensory analysis of food before swallowing
  • Mechanical digestion through the actions of the teeth, tongue, and palatal surfaces
  • Lubrication by mixing with mucus and saliva
  • Limited chemical digestion of carbohydrates and lipids
• Regions of the pharynx


  Nasopharynx, oropharynx, and laryngopharynx
• Pharyngeal muscles


  Work with muscles of the oral cavity to start swallowing, pushing the bolus into the esophagus
• Esophagus


  Actively moves ingested materials down toward the stomach, approximately 25 cm in length and 2 cm in diameter at its widest point
• Esophageal mucosa and submucosa

  Packed into large folds that extend the length of the esophagus, allowing for expansion during the passage of a large bolus
• Esophageal muscularis mucosae


  Consists of an irregular layer of smooth muscle
• Esophageal submucosa


  Contains scattered esophageal glands that produce mucus to reduce friction between the bolus and the esophageal lining
• Esophageal innervation


  Parasympathetic and sympathetic fibers from the esophageal plexus, with resting muscle tone in the circular muscle layer in the superior 3 cm normally preventing air from entering the esophagus
• Buccal phase


  Compression of the bolus against the hard palate, followed by retraction of the tongue to force the bolus into the oropharynx and seal off the nasopharynx
• Pharyngeal phase


  Elevation of the larynx and folding of the epiglottis to direct the bolus past the closed glottis, while the uvula and soft palate block passage back to the nasopharynx
• Esophageal phase


  Contraction of pharyngeal muscles forces the bolus through the entrance to the esophagus, where a peristaltic wave pushes it toward the stomach
• Gastric pits and glands

  In the fundus and body of the stomach, each gastric pit communicates with several gastric glands, dominated by parietal cells and chief cells that secrete about 1500 mL of gastric juice each day
• Functions of gastric acid


  Kills most ingested microorganisms, denatures proteins and inactivates most enzymes in food, and helps break down plant cell walls and connective tissues in meat
• Gastrin


  Produced by G cells in the gastric pits of the pyloric antrum, stimulating secretion by parietal and chief cells as well as contractions of the gastric wall
• Digestion in the stomach


  Salivary amylase and lingual lipase continue digestion of carbohydrates and lipids until the pH falls below 4.5, and preliminary digestion of proteins by pepsin increases as the pH approaches 2.0
• Absorption in the stomach


  Nutrients are not absorbed in the stomach due to the mucus covering the epithelial cells, lack of specialized transport mechanisms, and incomplete digestion
• Phases of gastric control


  • Cephalic phase
  • Gastric phase
  • Intestinal phase
• Pancreas


  Lies posterior to the stomach, primarily an exocrine organ that produces digestive enzymes and buffers, with arterial blood supply from the pancreatic and pancreaticoduodenal arteries and venous drainage through the splenic vein
• Pancreatic secretions


  An alkaline mixture of digestive enzymes, water, and ions, secreted into the small intestine to break down ingested materials into small molecules suitable for absorption
• Pancreatic enzymes


  • Alpha-amylase (carbohydrase)
  • Lipase (breaks down lipids)
  • Nucleases (break down RNA or DNA)
  • Proteolytic enzymes (break apart proteins)
• Liver


  The largest visceral organ, weighing about 1.5 kg, located primarily in the right hypochondriac and epigastric regions
• Functions of the liver


  Metabolic regulation, hematological regulation, and bile production