digestive

Chapter 26 Lecture Notes: Digestive System

Digestive System Overview

  • Definition: The digestive system provides the means to break down ingested nutrients, which are required for survival as they are unusable in their original form, and allows for their absorption.
  • Functions:
    • Ingestion: Introduction of solid and liquid nutrients into the oral cavity, marking the first step in the digestive process.
    • Motility: Involves voluntary and involuntary muscular contractions for mixing and moving materials through the GI tract.
    • Secretion: The process of producing and releasing fluid products (e.g., digestive enzymes, acid, bile) that facilitate digestion.
    • Digestion: Breakdown of ingested food into smaller structures, which encompasses:
    • Mechanical Digestion: Physical breakdown of materials by chewing and mixing.
    • Chemical Digestion: Utilization of specific enzymes to break chemical bonds, changing large molecules into smaller ones.
    • Absorption: Transportation of digested molecules, electrolytes, vitamins, and water from the GI tract into the blood or lymph.
    • Elimination: Expulsion of indigestible components that are not absorbed.

Digestive Enzymes

  • Enzymes: Catalysts that speed up chemical reactions; they are proteins that remain unchanged and are not consumed in the reactions they catalyze.

Movement of Materials Through the Digestive Tract

  • Muscularis (Externa): Functions to propel contents within the GI tract.
  • Types of Movement:
    • Peristalsis: An alternating contraction sequence of inner and outer layers, propelling ingested materials through the tract.
    • Mixing (Segmentation): A back-and-forth motion that blends ingested materials with secretions without directional movement.
    • Mass Movements: Strong peristaltic-like contractions that force fecal matter from the transverse colon through the rest of the large intestine.

Regulation of Digestive Processes

  • Receptors: Monitor changes in the GI tract and its contents; located throughout the mucosa and submucosa.
    • Mechanoreceptors: Detect stretch or pressure.
    • Chemoreceptors: Detect specific chemicals in the lumen content.
  • Nervous Control:
    • Sensory input is relayed to the central nervous system (CNS).
    • Autonomic motor output is relayed through facial, glossopharyngeal, and vagus nerves to effectors (e.g., salivary glands, pancreas, muscularis). This coordinated activity involves both long and short reflexes.
  • Hormonal Control: Involves three primary hormones:
    • Gastrin (from stomach): Stimulates stomach secretions and motility.
    • Secretin (from small intestine): Stimulates secretion of alkaline fluid from the pancreas.
    • Cholecystokinin (CCK) (from small intestine): Stimulates gallbladder contraction and pancreatic enzyme release.

Upper Gastrointestinal Tract: Overview of the Organs

  • Comprises the oral cavity, pharynx, esophagus, and stomach.
Oral Cavity and Salivary Glands
  • Mechanical Digestion: Begins in the oral cavity; saliva is secreted from salivary glands in response to food and contains salivary amylase (enzyme initiating starch digestion).
  • Formation of Bolus: Saliva mixes with ingested materials to form a bolus for swallowing.
  • Components of Saliva: 1.0-1.5 L daily produced; primarily water (99.5%), containing solutes like salivary amylase, mucin, and lysozyme (antibacterial enzyme).
    • Functions include moistening food, dissolving flavor chemicals for taste, cleansing the oral cavity, and initiating digestion of carbohydrates and lipids.
Pharynx
  • Shared passage for food and air, connects to the respiratory and digestive systems.
  • Muscles cooperate with those of the oral cavity and esophagus to assist in swallowing (deglutition).
Esophagus
  • Function: Carries solid food and liquids from the oral cavity and pharynx to the stomach.
  • Transit Time: Approximately 2 seconds for liquids and 9 seconds for bolus; poorly lubricated boluses may require additional peristaltic waves.
Stomach
  • Involved in mixing food with gastric juices and initiating digestion of proteins and fats; ingested materials remain here for 2-6 hours.
  • Absorption is limited to small, nonpolar substances.
  • Histology: Lined with simple columnar epithelium, featuring gastric pits and glands that secrete various substances including:
    • Surface Mucous Cells: Secretes alkaline mucus to prevent ulceration of the stomach lining.
    • Parietal Cells: Produce hydrochloric acid (HCl), intrinsic factor (necessary for vitamin B12 absorption), and are responsible for the gastric environment's low pH.
    • Chief Cells: Produce pepsinogen (converted to pepsin) and gastric lipase.
    • G-cells: Secrete gastrin that regulates stomach secretions and motility.
Motility in the Stomach
  • Gastric Mixing: Churning process that converts bolus into chyme.
  • Gastric Emptying: Movement of chyme from stomach to duodenum; regulated by neural and hormonal signals.
    • Organized into three phases: cephalic, gastric, and intestinal.

Clinical Views in Digestive Health

  • Reflux Esophagitis: Inflammation due to acidic chyme refluxing into the esophagus, commonly presenting as heartburn.
  • Gastroesophageal Reflux Disease (GERD): Chronic condition resulting from reflux esophagitis that can lead to esophageal tissue erosion and cancer risk.
  • Peptic Ulcers: Solitary erosions in the stomach or duodenum, causing severe gastrointestinal distress.
  • Vomiting: Rapid expulsion of gastric contents, controlled by the vomiting center in the medulla.

Lower Gastrointestinal Tract Overview

  • Comprises the small intestine (duodenum, jejunum, ileum) and large intestine.
  • Function: Continues digestion, aids in absorption, and eliminates unusable material.
  • Accessory Organs: Include the liver (bile production), gallbladder (bile storage), and pancreas (digestive enzyme secretion).
Small Intestine
  • Structure: Long tube; location is medial in the abdominal cavity. Nutrient absorption occurs here, taking at least 12 hours for material to pass through.
  • Secretions: Intestinal juice from intestinal glands, protective alkaline mucus from submucosal glands, and various digestive enzymes.
  • Motility: Involves segmentation and peristalsis.
  • Nutrient Absorption: 90% absorption rate; adaptations include plicae, villi, and microvilli that drastically increase surface area.
Accessory Organs and Ducts
  • Liver: Produces bile, which contains bile salts and cholesterol, aiding fat digestion.
  • Gallbladder: Stores and concentrates bile.
  • Pancreas: Dual-function as an endocrine (insulin production) and exocrine (pancreatic juice secretion) organ; produces essential digestive enzymes.
Clinical Views of the Liver and Gallbladder
  • Cirrhosis: Replacement of liver cells with scar tissue due to chronic damage, leading to liver dysfunction and portal hypertension.
  • Gallstones: Formation of hard deposits in the gallbladder that can cause pain and obstruct flow of bile.
Large Intestine
  • Function: Absorbs water and electrolytes, compacts material into feces, and stores it until elimination.
  • Histology: Lined with simple columnar epithelium, containing numerous goblet cells and lacking villi.
  • Bacterial Action: Indigenous microbiota contribute to the breakdown of undigested material and produce essential vitamins.
  • Motility: Includes haustral churning and mass movements stimulated by food intake, coordinated by the gastrocolic reflex.

Nutrient and Their Digestion

  • Carbohydrates: Broken down into monosaccharides by salivary amylase in the oral cavity and pancreatic amylase in the small intestine.
  • Proteins: Digestion starts in the stomach with pepsin; further breakdown occurs in the small intestine with enzymes like trypsin and chymotrypsin.
  • Lipids: Digestion initiated in the stomach with lingual and gastric lipase, then completed in the small intestine with pancreatic lipase facilitated by bile salts for emulsification.