Gastrointestinal System

Gastrointestinal System Overview

  • Nutrients must be broken down before entering the bloodstream.
  • Digestion: Enzymes in the GI tract break down large molecules into smaller ones.
  • Motility: Mixes and propels contents through the digestive tract.
  • Secretions: Enzyme-containing fluids enter the GI tract to aid digestion.
  • Absorption: Small molecules are transported into the bloodstream.

Divisions of the Gastrointestinal System

  • GI Tract: Hollow tube, approximately 15 feet long.
  • Accessory Glands:
    • Examples: Parotid gland, pancreas.
    • Secrete fluids and enzymes into the GI tract to promote digestion and absorption.

Pathway Through the Gastrointestinal System

  • Mouth → pharynx → esophagus → stomach → small intestine → colon → rectum → anus

Wall of the Gastrointestinal Tract

  • The wall consists of mucosa, submucosa, muscularis externa, and serosa.

Mucosa

  • Mucous membrane that separates the GI lumen from the internal environment.
  • Enterocytes: Epithelial cell types.
    • Absorptive cells: Aid absorption and transport nutrients to the bloodstream.
    • Exocrine cells: Secrete enzymes or fluids into the lumen.
    • Goblet cells: Secrete mucus.
    • Endocrine cells: Secrete hormones into the bloodstream.
  • The mucosa comprises three layers:
    • Epithelial lining: Lines the inside of the lumen.
    • Lamina propria: Connective tissue that contains Peyer’s patches.
      • Nerves regulate digestion, secretion, and motility.
    • Muscularis mucosae: Thin smooth muscle layer containing longitudinal and circular muscle fibers.
      • Functions in mixing luminal contents.

Submucosa

  • Thick connective tissue layer providing distensibility (stretching) and elasticity (returning to shape).
  • Contains large blood and lymphatic vessels.
  • Submucosal plexus (Meissner's plexus): Controls glandular secretions and blood flow.
    • First part of the enteric nervous system.

Muscularis Propria

  • Two layers of smooth muscle:
    • Inner circular layer: Changes diameter.
    • Outer longitudinal layer: Changes length.
  • Myenteric plexus (Auerbach's plexus)
    • Regulates muscle contractions for peristalsis.
    • Second part of the enteric nervous system.

Serosa

  • Outermost layer of the GI tract.
  • Inner layer: Connective tissue.
  • Outer layer: Epithelial tissue (mesothelium).
  • Mesothelium:
    • Secretes lubricant, allowing organs to slide past each other.
    • Continuous with mesenteries.
  • Mesenteries:
    • Clear thin membranes that interconnect abdominal organs and anchor them to the peritoneum.
  • Peritoneum:
    • Membrane lining the inside of the abdominal cavity.

Functional Anatomy of GI Tract

  • Mouth:
    • Digestion begins with mastication (chewing).
    • Secretion of saliva acts as a lubricant.
    • Salivary amylase:
      • Digests starch and glycogen (stored form of glucose).
  • Pharynx:
    • Common passageway for air and food.
    • Food enters the stomach via the esophagus.
  • Esophagus:
    • Muscular tube from the pharynx to the stomach.
    • Upper one-third: skeletal muscle.
    • Lower two-thirds: smooth muscle.
    • Upper esophageal sphincter:
      • Skeletal muscle between the pharynx and esophagus.
    • Lower esophageal sphincter:
      • Smooth muscle between the esophagus and stomach.

The Stomach

  • Stores food after it is swallowed.

    • Empty volume: 50 mL
    • Full volume: 1000 mL (great ability to expand).

  • Secretes gastric juice.

    • HCl, pepsinogen, intrinsic factor, mucus.

  • Releases food into the intestine slowly.

  • Stomach Anatomy

    • Fundus, body, antrum.

  • Contractions:

    • Mix chyme (digested food + enzymes).
    • Chyme passes through the pylorus.
    • Gastric emptying:
      • Moving food from the stomach to the duodenum.

  • Stomach folds:

    • Expand to accommodate food.
    • Rugae:
      • Folds that flatten on expansion.

Gastric Pits

  • Located in the stomach lining and lead to gastric glands.
  • Secretory products:
    • Pepsinogen:
      • Secreted by chief cells.
      • Precursor for pepsin.
      • Pepsin digests proteins.
    • Hydrogen ions:
      • Secreted by parietal cells.
      • Maintain the acidic environment of the stomach.
    • Intrinsic factor:
      • Secreted by parietal cells.
      • Necessary for the absorption of vitamin B12.
    • Gastrin:
      • Secreted from G cells (hormone).
      • Stimulates gastric acid secretion, promotes gastric motility, and regulates gastric emptying.
    • Mucus:
      • Secreted from neck cells.

Stomach/Small Bowel

  • Acidic environment of the stomach:
    • pH = 2
    • Necessary for activating pepsinogen, denaturing proteins, and killing bacteria.
  • Gastric mucosal barrier:
    • Protective layer of mucus and bicarbonate secreted from neck cells and goblet cells.
  • Small Intestine:
    • Coiled, hollow tube, 8–10 feet long, between the stomach and large intestine.
    • The primary site of digestion and absorption.
    • Three divisions:
      • Duodenum
      • Jejunum
      • Ileum

Small Intestine - Secretions

  • Secretions into the duodenum:
    • Pancreatic juice:
      • Enters the duodenum containing
        • Digestive enzymes.
        • Bicarbonate to neutralize acidic chyme.
    • Bile:
      • Enters the duodenum.
      • Secreted from the liver and stored in the gallbladder.
      • Contains bile salts to aid in fat digestion.

Small Intestine - Absorption

  • Absorption is completed within the first 20% of the intestine’s length.
  • Large surface area for absorption:
    • Villi:
      • Increase the surface area of the epithelium.
      • Contain blood vessels and a lacteal for nutrient absorption.
    • Microvilli:
      • Increase the surface area of epithelial cells, forming a brush border.

Crypts of Lieberkuhn

  • Epithelial cells in crypts secrete bicarbonate-rich fluid.
    • Secreted in the proximal small intestine and absorbed in the distal small intestine.

The Liver - Role in Absorption

  • Absorbed nutrients travel in the blood to the liver, then enter general circulation.
  • Liver functions:
    • Detoxifies substances.
    • Processes certain nutrients.
    • Synthesizes proteins.
  • Hepatic portal system:
    • Vasculature delivers absorbed nutrients to the liver before they enter circulation.
      • Nutrients are absorbed from the small intestine into mesenteric veins.
      • Mesenteric veins → hepatic portal vein → liver.
    • Nutrients travel from the liver to the heart (general circulation) via the hepatic vein.
    • The hepatic artery provides the liver with fresh blood to supply oxygen.

The Liver - Secretion and Processing

  • Liver functions:
    • Secretes Bile.
    • Processes nutrients
      • Glucose → glycogen
      • Amino acids → fatty acids
      • Triglycerides → cholesterol synthesis
      • Lipoprotein synthesis (transports hydrophobic materials in the bloodstream).
    • Removal of old red blood cells
      • Catabolizes hemoglobin
      • Generates bilirubin
    • Eliminates wastes and toxins
      • Bilirubin is eliminated as bile pigments.
    • Synthesizes plasma proteins (e.g., albumin).

Biliary System

  • Comprises the liver, gallbladder, and associated ducts.
    • The liver synthesizes bile, and the gallbladder stores bile.
  • Common bile duct:
    • Transports bile from the liver and/or gallbladder to the duodenum.
    • Joins the pancreatic duct to form the ampulla of Vater.
  • Sphincter of Oddi
    • Regulates the flow from the pancreas and gallbladder to the duodenum.

Pancreas: Endocrine Portion

  • Has exocrine and endocrine portions.
  • Endocrine portion:
    • Releases hormones: insulin and glucagon.
      • Insulin:
        • Lowers blood sugar levels.
        • Promotes the formation of glycogen.
      • Glucagon:
        • Raises blood sugar levels.
        • Stimulates the release of glucose from glycogen stores in the liver.

Pancreas: Exocrine Portion

  • Exocrine portion:
    • Produces pancreatic juice.
      • Rich in bicarbonate.
      • Enzymes present:
        • Pancreatic amylase (breaks down sugars into simple monomers).
        • Pancreatic lipases (emulsification of fats)
        • Proteases (breakdown proteins)
        • Nucleases (breakdown nucleic acids).

Accessory Glands

  • Products are secreted via ducts into the lumen of the GI tract.
  • Four types of glands:
    • Salivary glands (secrete saliva).
    • Pancreas (secretes pancreatic juice).
      • Acini are "ball-like" formations of secretory glands.
    • Liver (secretes bile).
  • Characteristics of saliva:
    • Rich in bicarbonate ions.
    • Contains mucus (reduces friction).
    • Enzymes present:
      • Salivary amylase initiates the digestion of carbohydrates.
      • Lysozyme has antimicrobial action by breaking down bacterial cell walls

Large Intestine

  • Consists of the cecum, colon, and rectum.
  • Ileocecal sphincter:
    • Located between the ileum and colon.
  • Teniae coli:
    • Bands of longitudinal smooth muscle.
  • Colon:
    • Hollow tube from the small intestine to the rectum.
    • Functions:
      • Concentration of wastes into feces.
      • Absorption of most water.
      • Storage of feces until defecation.
    • Four sections:
      • Ascending colon
      • Transverse colon
      • Descending colon
      • Sigmoid colon

Digestion and Absorption - Overview

  • Carbohydrates.
  • Proteins.
  • Lipids.
  • Absorption of vitamins, minerals, and water.

Carbohydrates

  • Typical diet: 500 grams/day carbohydrate.
  • Most consumed as disaccharides or polysaccharides such as:
    • Sucrose, lactose, maltose, starch.
  • Glycogen is stored glucose; cellulose is fiber and cannot be digested.

Carbohydrate Digestion

  • Only monosaccharides are absorbed.
  • Disaccharides and polysaccharides must be digested to monosaccharides.
  • Digestion of starch
    • Enzymes of digestion: salivary amylase, pancreatic amylase.
      • End-product: disaccharides (maltose) and limit dextrins.
    • Enzymes of digestion:
      • Dextrinase → limit dextrins → glucose
      • Glucoamylase → polysaccharides → glucose
      • Sucrase → sucrose → fructose + glucose
      • Lactase → lactose → galactose + glucose
      • Maltase → maltose → 2 glucose
    • Location of enzymes: brush border of the small intestine.
    • Carbohydrates are absorbed in the small intestine.

Proteins

  • Typical diet: 125 grams/day protein.
  • Consumed in the diet.
  • Protein digestion products:
    • Amino acids, dipeptides, tripeptides.
  • Proteases
    • Endopeptidases:
      • Split polypeptides at interior peptide bonds into small peptide fragments.
    • Exopeptidases:
      • Cleave amino acids from one end of the polypeptide.
    • Zymogens:
      • Inactive proteases stored in zymogen granules located in the pancreas and activated by proteolysis.

Protein Digestion

  • Protein digestion begins in the stomach.
    • Enzyme: pepsin (breaks down proteins).
      • Inactive (secreted) form = pepsinogen.
      • Activated by acid.
  • Pepsin activation and activity:
    • Chief cells (located in the stomach) secrete pepsinogen.
    • Parietal cells (located in the stomach) secrete HCl. HCl cleaves pepsinogen to pepsin.
  • Pancreatic proteases:
    • Break down proteins into peptides: trypsin, chymotrypsin, carboxypeptidase.
  • Brush border proteases:
    • Enterokinase proteins are broken down into peptides by enzymes, enabling absorption in the small intestine.

Lipids

  • Typical diet: 50 grams/day lipids, 90% triglycerides.
  • Lipids face a special problem in digestion and absorption; they are not water-soluble and do not mix with stomach or intestinal contents, forming fat droplets.
  • Lipid digestion:
    • Enzymes of digestion: lipases secreted from the pancreas.
    • Bile salt = bile acid conjugated to glycine or taurine increases the surface area of droplets to break each large droplet into smaller droplets.
    • Emulsification occurs in the duodenum.

Lipid Digestion and Absorption

  • Pancreatic lipases act on fat droplets.
  • Some triglycerides and monoglycerides are absorbed directly by nearby cells.
  • Monoglycerides are synthesized into triglycerides and packaged into chylomicrons.
  • Chylomicrons are too large for general circulation and enter lymphatics via lacteals (lymphatic vessels of the small intestine), then enter general circulation and undergo absorption by cells for energy use or storage.

Peristalsis

  • Coordinated, rhythmic contraction and relaxation of muscles in the GI tract.
    • Propels food and other contents through the digestive system.
    • Enteric Nervous System (ENS):
      • Controls peristalsis independently of the central nervous system.
      • Detects stretch and chemical changes to initiate peristaltic activity.
  • Myenteric Plexus
    * Found between muscle layers
    * Regulates smooth muscle contraction for peristalsis
  • Autonomic Nervous System (ANS):
    • Sympathetic/parasympathetic branches via the vagus nerve.
    • Hormonal and Local Factors: Hormones like gastrin and motilin influence peristalsis.