Gastrointestinal System Notes

Gastrointestinal System Overview

Introduction to the Digestive System

  • Functions:
    • Provides fuel for cell function.
    • Provides building blocks for cell growth and repair.
  • Components:
    • Digestive Tract (Muscular tube):
      • Oral Cavity, Teeth, Tongue: Mechanical processing, moistening, mixing with salivary secretions.
      • Pharynx: Muscular propulsion of materials into the esophagus.
      • Esophagus: Transports materials to the stomach.
      • Stomach: Chemical breakdown of materials by acid and enzymes; mechanical processing through muscular contractions.
      • Small Intestine: Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions.
      • Anus: Terminal end.
      • Large Intestine: Dehydration and compaction of undigestible materials in preparation for elimination.
    • Accessory Organs:
      • Salivary Glands: Secrete lubricating fluid containing enzymes that break down carbohydrates.
      • Liver: Secretes bile (important for lipid digestion), stores nutrients, performs many other vital functions.
      • Gallbladder: Stores and concentrates bile.
      • Pancreas: Exocrine cells secrete buffers and digestive enzymes; endocrine cells secrete hormones.
  • Six Related Processes (Functions):
    1. Ingestion: Food enters the digestive tract through the mouth.
    2. Mechanical Processing: Physical manipulation of solid foods.
      • Involves the tongue and teeth.
      • Includes swirling and mixing motions within the digestive tract.
    3. Digestion: Chemical breakdown of food through enzymatic action for absorption.
    4. Secretion: Release of water, acids, enzymes, and buffers into the digestive tract.
    5. Absorption: Movement of substances across the digestive epithelium into the interstitial fluid of the digestive tract.
    6. Excretion: Removal of waste products from body fluids through defecation.
  • Defensive Roles:
    • Protects surrounding tissues against corrosive effects of digestive acids and enzymes.
    • Protects against bacteria using nonspecific defenses.

Overview of the Digestive Tract's Histology and Movements

  • Histological Organization:
    • The digestive tract is a histological organ featuring folds called plicae.
    • Supported by the mesentery, which contains mesenteric arteries and veins.
    • Four Major Layers:
      1. Mucosa: Innermost layer, epithelium directly exposed to food.
      2. Submucosa: Layer beneath the mucosa, contains blood vessels, lymphatics, and nerves.
      3. Muscularis externa: Muscular layer responsible for movement.
      4. Serosa (Visceral Peritoneum): Outermost layer, a connective tissue sheath.
  • Movements:
    • Peristalsis:
      • Waves of muscular contractions within the muscularis externa.
      • Propels food through the digestive tract in one direction.
    • Segmentation:
      • Churning and fragmenting movements of digestive materials.
      • Results in thorough mixing of contents with intestinal secretions.
      • Does not propel material in any specific direction.

Oral Cavity

  • Description: The first part of the digestive tract to receive food.
  • Lining: Mucous membrane of stratified squamous epithelium.
  • Functions:
    • Senses and analyzes materials before swallowing.
    • Mechanically processes material with the tongue and teeth.
    • Lubricates material with mucous and salivary secretions.
  • Structures:
    • Hard and Soft Palates: Form the roof of the oral cavity.
    • Salivary Glands:
      • Produce saliva in large quantities, especially at mealtimes.
      • Functions of Saliva:
        • Lubricates the mouth.
        • Dissolves chemicals that stimulate taste buds.
        • Reduces friction with food, aiding swallowing.

The Pharynx

  • Description: A common passageway.
  • Function: Allows for the passage of solid food, liquids, and air.

The Esophagus

  • Description:
    • A muscular tube.
    • Length: 25 cm.
    • Diameter: 2 cm.
  • Function: Conveys solid food and liquids to the stomach.
  • Pathway:
    • Begins at the pharynx.
    • Runs posterior to the trachea.
    • Passes through the mediastinum.
    • Enters the peritoneal cavity through the esophageal hiatus (an opening in the diaphragm).
    • Empties into the stomach.
  • Lining: Lined with epithelial tissue that resists abrasion, hot or cold temperatures, and chemical attack.

The Stomach

  • Location: Left upper quadrant of the abdominopelvic cavity.
  • Function: Receives food from the esophagus.
  • Four Primary Functions:
    1. Temporary storage of ingested food.
    2. Mechanical breakdown of ingested food.
    3. Breakdown of chemical bonds in foods through the actions of acids and enzymes.
    4. Production of intrinsic factor, which is necessary for the absorption of vitamin B12.
  • Anatomy:
    • Muscular, J-shaped organ.
    • Four Main Regions:
      1. Cardia: Region closest to the esophagus.
      2. Fundus: Dome-shaped region superior to the cardia.
      3. Body: The large central area, located between the fundus and the curve of the J.
      4. Pylorus: Distal part of the J, connecting the stomach with the small intestine.
    • Pyloric Sphincter: Regulates the flow of chyme (partially digested food) from the stomach into the small intestine.
  • Regulation of Gastric Activity:
    • Controlled by the central nervous system.
    • Acid and enzyme production are regulated by:
      • Reflexes within the walls of the digestive tract.
      • Hormones of the digestive tract.

The Small Intestine

  • Primary Role: Responsible for 90\% of nutrient absorption.
  • Dimensions:
    • Length: Approximately 20 feet.
    • Diameter: Starts at 4 cm near the stomach and reduces to 2.5 cm at the large intestine.
  • Three Segments:
    1. Duodenum:
      • Length: 25 cm.
      • Closest segment to the stomach.
      • Curves in a C-shape, enclosing the pancreas.
      • Receives chyme from the stomach.
      • Receives digestive secretions from the pancreas and liver.
      • Lies outside the peritoneal cavity.
    2. Jejunum:
      • Connects to the duodenum at a sharp bend.
      • Length: 8 feet.
      • Supported by a sheet of mesentery.
      • Responsible for the bulk of chemical digestion and nutrient absorption.
    3. Ileum:
      • Length: Averages 12 feet.
      • Ends at the ileocecal valve, a sphincter that controls material flow from the ileum to the cecum (of the large intestine).
  • Stabilization:
    • Fills much of the peritoneal cavity.
    • Stabilized by mesentery attached to the dorsal body wall.
    • Connective tissue in the mesentery contains blood vessels, lymphatic vessels, and nerves.
  • Intestinal Movements:
    • When chyme enters the small intestine:
      • Segmentation contractions mix chyme, mucous secretions, and enzymes.
      • Weak peristaltic contractions slowly move material forward.
  • Intestinal Hormones and Regulation:
    • Food in the stomach leads to Gastrin release (stimulates increased acid production and motility in the stomach) and GIP release.
    • Chyme in the duodenum leads to Secretin and CCK release.
    • GIP release, Secretin, and CCK influence the pancreas: Secretin and CCK stimulate the exocrine pancreas to secrete enzymes and buffers. GIP stimulates the endocrine pancreas to release insulin.
    • Bile secretion and ejection from the gallbladder are also regulated to aid digestion.
    • Overall process leads to nutrient absorption and utilization by all tissues.
  • Digestion in the Small Intestine:
    • Location of most important components of digestion.
    • Final products of digestion (simple sugars, fatty acids, and amino acids) are absorbed.
    • Most of the water content is absorbed.

The Pancreas

  • Location:
    • Lies behind the stomach.
    • Extends from the duodenum toward the spleen.
    • Length: Roughly 6 inches.
    • Located retroperitoneal (only anterior surface covered by peritoneum).
  • Description: Pinkish-gray organ with a lumpy, soft texture; easily torn.
  • Histological Organization:
    • Pancreatic Islets (Endocrine Cells):
      • Secrete insulin and glucagon.
      • Comprise Only 1\% of the cellular population of the pancreas.
    • Exocrine Cells and Ducts:
      • Make up most of the pancreas.
      • Produce pancreatic juice, a mixture of digestive enzymes and buffers.

The Liver

  • Description:
    • Largest visceral organ.
    • Firm, reddish-brown.
    • Approximately 2.5\% of total body weight.
  • Location: Lies in the right hypochondriac and epigastric regions of the abdomen.
  • Anatomy:
    • Wrapped in a tough fibrous capsule.
    • Covered by a layer of visceral peritoneum.
    • Divided into 4 unequal lobes.
  • Associated Organ:
    • Gallbladder:
      • Located in a recess under the right lobe of the liver.
      • Muscular sac.
      • Stores and concentrates bile.
      • Excretes bile into the small intestine.
  • Liver Functions (3 General Roles):
    1. Metabolic Regulation:
      • Primary organ that regulates the composition of circulating blood.
      • Exposed to all blood that leaves absorptive areas of the digestive tract before it reaches general circulation.
    2. Hematological Regulation:
      • Largest blood reservoir in the body, receiving 25\% of cardiac output.
      • Kupffer cells (macrophages) remove aged or damaged red blood cells, debris, and pathogens.
    3. Bile Production:
      • Bile is synthesized in the liver.
      • Excreted into the lumen of the duodenum (after being stored/concentrated in the gallbladder).

The Gallbladder

  • Cystic Duct:
    • Extends from the gallbladder.
    • Joins with the common hepatic duct to form the common bile duct.
  • Common Bile Duct:
    • Joins the pancreatic duct.
    • Enters the duodenum at the duodenal papilla.
    • The entrance is surrounded by the hepatopancreatic sphincter.

The Large Intestine

  • Description:
    • Horseshoe-shaped.
    • Begins at the end of the ileum and ends at the anus.
    • Lies below the stomach and liver, framing the small intestine.
    • Also referred to as the large bowel.
    • Length: Approximately 5 feet.
    • Width: 3 inches.
  • Main Functions:
    1. Reabsorption of water and compaction of intestinal contents into feces.
    2. Absorption of important vitamins freed by bacterial action.
    3. Storage of fecal material prior to defecation.
    4. Preparation of fecal material for elimination.
  • Three Divisions:
    1. Cecum: Pouch-like first part.
    2. Colon: The largest portion.
    3. Rectum: The final 6 inches; the end of the digestive tract.
  • The Colon:
    • Divided into 4 segments:
      1. Ascending colon.
      2. Transverse colon.
      3. Descending colon.
      4. Sigmoid colon.
  • The Rectum:
    • Forms the end of the digestive tract.
    • Expanded organ for the temporary storage of feces.
    • The last portion is called the anal canal.
  • Absorption in the Large Intestine:
    • Composition of Feces:
      • 75\% water.
      • 5\% bacteria.
      • Remaining contents: Indigestible materials, inorganic materials, and remains of epithelial cells.
  • Movements of the Large Intestine:
    • From the transverse colon through the rest of the large intestine, mass movements occur.
    • Mass movements:
      • Powerful peristaltic contractions.
      • Occur a few times per day.
      • Stimulated by distention of the stomach and duodenum.
      • Force material into the rectum for defecation.