Digestive

Digestive System Introduction

  • Digestive System Overview
    • Breaks down food into smaller components for utilization by the body.
    • Converts food into forms such as amino acids and monosaccharides.
    • Key functions: processes food, extracts nutrients, and eliminates unabsorbed residue.

Terminology

  • As food moves through the digestive tract, specific terms are used:
    • Bolus: The form of food once chewed and mixed with saliva, present in the mouth, pharynx, and esophagus.
    • Chyme: The mixture of food and stomach enzymes found in the stomach and intestines.
    • Feces: Undigested food particles or materials not absorbed, expelled from the body through the rectum and anal canal.

Organs of the Digestive System

  • Two groups of organs:
    • Digestive Tract (GI Tract): Also known as the alimentary canal, it digests and absorbs food.
    • Components: Mouth, pharynx, esophagus, stomach, small intestine, large intestine.
    • Accessory Digestive Organs: Assist the alimentary canal in the digestive process.
    • Components: Teeth, tongue, salivary glands, liver, gallbladder, pancreas.

Major Processes During Digestive System Activity

  • Processing of food involves six essential activities:
    • Ingestion: The act of eating; food entering the body.
    • Propulsion: Movement of food through the alimentary canal.
    • Swallowing/Deglutition: Initiated voluntarily.
    • Peristalsis: Involuntary waves of muscular contraction that propel food through the digestive tract.
    • Mass Movements: Occur in the large intestine.
    • Mechanical Breakdown: Increases surface area of nutrients, prepares food for chemical digestion.
    • Methods include chewing, mixing food with saliva (via tongue), churning in the stomach, segmentation (local contractions in the small intestine), and emulsification (via bile).
    • Digestion: Enzymatic breakdown of large molecules into smaller, absorbable units.
    • Absorption: Passage of nutrients from the lumen of the GI tract through the mucosal epithelium into the blood or lymph.
    • Defecation: Elimination of indigestible substances as feces.

Peristalsis and Segmentation

  • Both are muscular contractions that occur in the digestive tract:
    • Peristalsis: Type of propulsion that moves food through the digestive tract, occurring throughout the tract.
    • Segmentation: Type of mechanical breakdown that churns food and mixes it with digestive enzymes, occurring only in the small intestine.

Gastrointestinal Tract Activities

  • Major organs and their functions in the GI tract:
    • Mouth: Ingestion, mechanical breakdown (mastication), digestion, propulsion.
    • Pharynx: Propulsion of food to the esophagus.
    • Esophagus: Propulsion of bolus to the stomach.
    • Stomach: Mechanical breakdown, digestion, propulsion of chyme into the small intestine.
    • Small Intestine: Mechanical breakdown, digestion, absorption, propulsion.
    • Large Intestine: Absorption, propulsion, defecation.

Peritoneum and Peritoneal Cavity

  • Peritoneum: The serous membrane of the abdominal cavity.
    • Visceral Peritoneum: Covers external surfaces of most digestive organs.
    • Parietal Peritoneum: Lines the body wall.
    • Mesentery: Double layer of peritoneum that supports blood vessels, lymphatics, nerves, and anchors organs.
  • Peritoneal Cavity: Space between visceral and parietal peritoneum, containing lubricating fluid.
    • Retroperitoneal Organs: Lie behind the peritoneum (e.g., kidneys, pancreas).
    • Intraperitoneal Organs: Surrounded by peritoneum (e.g., stomach, liver).

Histology of the Alimentary Canal

  • Four basic layers (tunics) in the walls of most alimentary canal organs:
    • Mucosa: Innermost layer, in contact with food.
    • Submucosa: Contains areolar connective tissue, blood and lymphatic vessels, nerve fibers.
    • Muscularis Externa: Responsible for segments and peristalsis.
    • Serosa: Outermost layer, called adventitia in certain organs.

Mucosa

  • Innermost layer in contact with luminal contents. Functions include:
    • Secrete mucus, digestive enzymes, and hormones.
    • Absorb end products of digestion.
    • Protect against infectious disease.
  • Contains three sublayers:
    • Epithelium, Lamina Propia, Muscularis Mucosae.

Submucosa

  • Located under mucosa, consists of areolar connective tissue, rich in blood and lymphatic vessels, lymphoid follicles, and nerve fibers (submucosal nerve plexus).
  • Elastic fibers allow organs to stretch and regain shape after food passes through.

Muscularis Externa

  • Composed of two layers (except stomach, which has an additional layer):
    • Inner Circular Layer.
    • Outer Longitudinal Layer.
    • Stomach has an additional Oblique Layer for churning and mixing food.
  • Regulated by the Myenteric Nerve Plexus.

Serosa

  • Covers all abdominal digestive organs, replaced by adventitia in the esophagus, pharynx, and oral cavity.
  • Retroperitoneal organs possess both serosa and adventitia.

GI Tract Regulatory Mechanisms

  • Neural Mechanisms:

    • The Enteric Nervous System (often called the gut-brain) regulates digestive activities through:
    • Two intrinsic nerve plexuses:
      • Submucosal Nerve Plexus.
      • Myenteric Nerve Plexus (between muscularis layers).
    • Sensory neurons in digestive organs respond to various stimuli to regulate motility, digestive gland activation, etc.
    • Extrinsic controls from the CNS, influenced by the senses (sights, smells, thoughts of food).

  • Hormonal Mechanisms:

    • Hormones from stomach and small intestine stimulate target cells in same or different organs (e.g., gastrin, secretin, CCK, motilin, GIP).

  • Local Mechanisms:

    • Chemicals (e.g., prostaglandins, histamine) released into interstitial fluid can stimulate nearby cells.

Blood Supply: Splanchnic Circulation

  • Arterial Supply:
    • Hepatic Artery, Splenic Artery, Left Gastric Artery, Inferior and Superior Mesenteric Arteries.
    • Hepatic Portal Circulation: Drains nutrient-rich blood from digestive organs to the liver for processing.

Lesson 2: Organs of the Alimentary Canal

  • Organs of the Alimentary Canal/GI Tract
    • Mouth/Oral Cavity: Supported by accessory organs (tongue, teeth, salivary glands).
    • Pharynx.
    • Esophagus.
    • Stomach.
    • Small Intestine: Supported by accessory organs (liver, gallbladder, pancreas).
    • Large Intestine.

Mouth - Oral Cavity

  • Structure:
    • Bounded by lips, cheeks, palate, tongue; lined with stratified squamous epithelium (oral mucosa).
  • Function:
    • Mastication: Chewing, using lips and cheeks to position food between teeth.

Palate

  • Roof of mouth composed of two parts:
    • Hard Palate: Composed of palatine bones and maxillae, slightly corrugated for friction.
    • Soft Palate: Fold of skeletal muscle, closes off nasopharynx during swallowing.

Tongue

  • Structure and Function:
    • Muscular organ that repositions food during chewing and initiates swallowing, speech, and taste.
    • Lingual Frenulum: Attaches tongue to the floor of the mouth, an overly short one can impair feeding and speech.
    • Surface contains taste buds.

Taste Buds on Tongue

  • Papillae Types:
    • Filiform Papillae: Provide friction (no taste buds).
    • Fungiform Papillae: Mushroom-shaped with taste buds, red hue from vascular core.
    • Vallate Papillae: Form V-shaped row of taste buds.
    • Foliate Papillae: Pleat-like, with taste buds on lateral aspects of the posterior tongue.

Salivary Glands

  • Types:
    • Parotid: Anterior to the ear, external to masseter muscle.
    • Submandibular: Medial to the mandible.
    • Sublingual: Under the tongue, anterior to submandibular gland.
  • Function:
    • Secretion of saliva (cleanses mouth, moistens food, begins starch breakdown via amylase), with minor glands keeping oral mucosa moist.

Composition of Saliva

  • Components:
    • 97-99.5% water, slight acidity, containing:
    • Electrolytes: Na+, K+, Cl–, PO4²–, HCO3–.
    • Digestive Enzymes: Salivary amylase, lingual lipase.
    • Mucin.
    • Metabolic Wastes: Urea, uric acid.
    • Lysozyme, antibodies (IgA), defensins, cyanide compounds.
  • Control of Salivation:
    • Stimulated by food, sending impulses via parasympathetic fibers in cranial nerves VII and IX. Strong sympathetic stimulation can lead to dry mouth (xerostomia).

Teeth

  • Adult Teeth Count:
    • 32 permanent teeth (16 on top and 16 on bottom).
  • Types:
    • Incisors: Chisel-shaped for cutting.
    • Canines: Fang-like for tearing or piercing.
    • Premolars and Molars: Flat surfaces for crushing and grinding food.
  • Deciduous Teeth:
    • 20 “baby” teeth in infants/children, falling out as they age.

Tooth Structure

  • Crown: Exposed part above the gum. Covered by enamel, the hardest substance in the body.
  • Neck: Connects root and crown.
  • Root: Embedded in jawbone, anchored by cementum.
  • Dentin: Bonelike material beneath enamel.
  • Pulp Cavity: Contains blood vessels and nerves.
  • Root Canal: Extends from pulp cavity to the apical foramen of the root.

Digestive Processes of the Mouth

  • Functions:
    • Ingestion, mechanical breakdown (mastication), digestion (via salivary amylase), and propulsion (swallowing).

Lesson 3: Pharynx

  • Commonly referred to as the throat.
  • Composed of regions used in both digestion and respiration:
    • Oropharynx, Laryngopharynx.
    • Function: Propulsion, allowing passage of food, fluids, and air.

The Esophagus

  • Structure: A flat muscular tube connecting the laryngopharynx to the stomach, containing all four basic GI tunics, collapsed when not propelling food.
  • Sphincters:
    • Upper Esophageal Sphincter: Opens during swallowing to allow food into the esophagus.
    • Lower Esophageal Sphincter: Composed mainly of smooth muscle; prevents regurgitation of stomach contents
    • Mucus cells protect the esophagus from acid reflux.

Digestive Processes: Swallowing

  • Swallowing (Deglutition): Coordinated muscle activity divided into phases:
    • Buccal Phase: Voluntary; food bolus pushed to oropharynx by the tongue.
    • Pharyngeal-Esophageal Phase: Involuntary; initiated by receptors in the oropharynx, involving the vagus nerve.
  • Phases of Deglutition:
    • 1. Buccal phase initiated by the tongue.
    • 2. Pharyngeal phase (soft palate rise, closes nasopharynx).
    • 3. Peristaltic contractions move food down the esophagus.

Esophagus: Homeostatic Imbalances

  • Gastroesophageal Reflux:
    • Acidic contents of the stomach reflux into the esophagus, possibly caused by lower esophageal sphincter dysfunction.
    • Symptoms may include heartburn due to various dietary factors. Treatment includes antacids and proton pump inhibitors.
  • Hiatal Hernia:
    • Protrusion of the stomach into thoracic cavity through the diaphragm's esophageal hiatus.

Lesson 4: Stomach

  • Description:
    • “J”- shaped muscular bag located in the left upper quadrant of the abdominal cavity.
    • Suspended by connective tissue (lesser omentum, greater omentum).
  • Chyme: The name for semi-digested food mixed with stomach enzymes.

Stomach Regions

  • Divided into four primary regions:
    • Cardia Region: Where esophagus attaches.
    • Fundus Region: Superior dome.
    • Body Region: Largest area below the fundus.
    • Pyloric Region: Narrows to form pyloric sphincter regulating emptying into the small intestine.

Stomach Histology - Muscularis Externa

  • Composed of three layers of smooth muscle (unlike other organs that typically have two):
    • Longitudinal Layer.
    • Circular Layer.
    • Oblique Layer (unique to stomach; mixes and moves food).

Stomach Histology - Mucosa Layer

  • Features:
    • Rugae: Folds allowing expansion of the stomach to hold large volumes.
    • Gastric Pits: Lead into gastric glands with secretory cells:
    • Mucous Cells: Secrete mucus to protect the lining from acids and enzymes.
    • Parietal Cells: Secrete hydrochloric acid (pH 1.5-3.5) and intrinsic factor.
    • Chief Cells: Secrete pepsinogen, activated to pepsin by HCl for protein digestion.