Digestive System - Part 1 Overview

Digestive System - Part 1

• Functional Anatomy: The digestive system comprises primary and accessory organs.
• Primary Digestive Organs: Mouth, pharynx, esophagus, stomach, small intestine, large intestine
• Accessory Digestive Organs: Salivary glands, liver, gallbladder, pancreas
• Organization (Primary/Accessory): A clear breakdown of organs into categories for better understanding.
• Primary Function of GIT: The digestive system has four main functions.
  • Motility: Movement of food through the digestive tract.
  • Secretion: Exocrine and endocrine secretions involved in digestion.
  • Digestion: Breakdown of food molecules into absorbable units.
  • Absorption: Passage of digested materials into the blood or lymph.
• Nervous Supply of the GI Tract: Two major types of nerve supply.
  • Enteric Nerve Supply: Located within the wall of the GI tract (esophagus to anus), inter-connected to form Meissner (submucosal) and Auerbach (myenteric) plexuses.
    • Meissner Plexus: Regulates secretory functions.
    • Auerbach Plexus: Regulates movements and involves neurotransmitters like acetylcholine, serotonin, substance P, VIP, and enkephalin.
  • Extrinsic Nerve Supply:
    • Sympathetic Fibers: Originate from spinal segments T5 to L2, pass through sympathetic chain ganglia, and terminate in celiac and mesenteric ganglia. Inhibits movements and secretions (via noradrenaline) and constricts sphincters.
    • Parasympathetic Fibers: Pass through cranial nerves (facial and glossopharyngeal for mouth, salivary glands), vagus nerve for esophagus, stomach, and upper part of large intestine, and sacral segments S2, S3, and S4 (pelvic nerves for lower large intestine). Accelerates movements and increases secretions (via acetylcholine).
• Mouth and Salivary Glands:
  • Functions of Mouth: Ingestion, chewing, mixing with saliva, taste perception, swallowing, speech, social functions.
  • Major Salivary Glands: Parotid, submandibular, sublingual
  • Minor Salivary Glands: Lingual, buccal, labial, palatal
  • Classification of Salivary Glands: Categorized as serous (thin watery saliva, e.g., parotid), mucous (thick viscous saliva, e.g., lingual), and mixed (both serous and mucous cells, e.g., submandibular).
  • Properties of Saliva: Volume (1000-1500 mL/day), reaction (pH 6.35-6.85, slightly acidic), specific gravity (1.002-1.012), and tonicity (hypotonic compared to plasma).
  • Composition of Saliva: Primarily water (99.5%), with 0.5% solids (organic, inorganic).
  • Functions of Saliva: Preparation for swallowing, taste perception, speech, regulating body temperature, regulating water balance, excretory function (excreting substances like mercury, potassium iodide, lead, thiocyanate, and viruses like rabies and mumps), and digestive function (enzymes like salivary amylase, maltase, lingual lipase).
  • Cleansing and Protective Functions of Saliva: Rinse mouth, remove debris, kill bacteria (e.g., staphylococcus, streptococcus, brucella), neutralize toxic substances, and protect the enamel.
• Esophagus:
  • Structure: Muscular tube connecting pharynx to stomach; two sphincters (upper and lower).
  • Function: Move food through peristalsis.
    • Lower Esophageal Sphincter: Prevents regurgitation.
• Stomach:
  • Parts: Cardiac, fundus, body (corpus), pyloric
  • Structure: Four layers (outer serous, muscular, submucous, inner mucus) with three smooth muscle groups (inner oblique, middle circular, outer longitudinal).
  • Glands of Stomach (Gastric Glands):
    • Fundic Glands: Chief, parietal, mucus neck, enteroendocrine (G, EC, ECL).
    • Pyloric Glands: G cells, mucus cells, enteroendocrine cells.
    • Cardiac Glands: Mucus cells, enteroendocrine cells, ECL cells, chief cells.
  • Gastric Juice: Volume (1200-1500 mL/day), reaction (highly acidic, pH 0.9-1.2 due to HCl), specific gravity (1.002-1.004).
    • Composition: Water (99.5%), solids (0.5%) with organic (pepsin, renin, gastric lipase, gelatinase, urease, mucus, intrinsic factor) and inorganic substances (HCl, Na, Ca, K, bicarbonate, chloride, phosphate, sulfate).
    • Secretion of Hydrochloric Acid (HCl): Active process using carbonic anhydrase to produce HCl. Factors stimulating HCl secretion include gastrin, histamine, and vagal stimulation; factors inhibiting HCl secretion include secretin, gastric inhibitory peptide (GIP), and peptide YY.
  • Functions of Gastric Juice: Digestive function (pepsin, gastric lipase, gelatinase, urease, gastric amylase), hemopoietic function (intrinsic factor for Vitamin B12 absorption and maintenance of healthy erythrocytes, which can result in pernicious anemia if missing), and protective function (mucus protects stomach lining from irritant substances).