Fluids, Electrolytes & Nutrition: Energy Production

Metabolism

Aerobic Respiration

  • ATP Generation Through the Use of Oxygen
  • Steps:
    • Glycolysis
      • Converts Glucose to Pyruvate & ATP
      • Occurs in Cell Cytoplasm
    • Oxidative Decarboxylation
      • Pyruvate is Oxidized to Acetyl-CoA & CO2
      • Occur in Cell Mitochondria
    • Citric Acid (Krebs) Cycle
      • Acetyl-CoA is Oxidized to CO2
      • Produces NADH & FADH2
      • Occur in Cell Mitochondria
    • Oxidative Phosphorylation
      • Electron Transport Chain – Oxidizes NADH to Establish a Proton Gradient
        • Proton Gradient Used to Phosphorylate ADP & Produce ATP
      • Occur in Cell Mitochondria
  • Produces 30-32 ATP Per Glucose Molecule

Cori (Lactic Acid) Cycle

  • ATP Generation Without the Use of Oxygen
  • Steps:
    • Glycolysis
      • Same as in Aerobic Respiration
      • Converts Glucose to Pyruvate & ATP
      • Occurs in Cell Cytoplasm
    • Fermentation
      • Pyruvate is Converted to Waste Products & Removed from the Cell
        • Converted to Lactic Acid in Skeletal Muscle
      • Occurs in Cell Cytoplasm
    • Gluconeogenesis
      • Lactic Acid (From Fermentation in Muscle) is Converted Back to Glucose
      • Occurs in the Liver
      • Prevents Lactic Acidosis
  • Produces Muscle ATP Faster than Aerobic Respiration for Short Bursts
  • Net Body Consumption of 4 ATP Molecules Per Cycle
    • Only Produces 2 ATP Per Glucose Molecule Through Glycolysis
    • Consumes 6 ATP Through Gluconeogenesis
  • Lactic Acidosis:
    • Buildup of Lactic Acid Causing Acidosis
    • Cohen-Woods Classification:
      • Type A: Caused by Tissue Hypoxia
      • Type B: Non-Hypoxic Causes
        • B1: From Underlying Disease
        • B2: From Medication or Intoxication
        • B3: From Inborn Errors of Metabolism

Energy Source After Meals

Immediately After a Meal

  • Primary Energy Source: Carbohydrates (Used Rapidly)
  • Excess Stored as Fatty Acids or Glycogen

6-24 Hours After Last Meal

  • Primary Energy Source: Glycogenolysis
  • Glycogen Stores:
    • Muscle (2/3 of Stores)
      • Only Available Locally
    • Liver (1/3 of Stores)
      • Available Systemically
      • Liver Stores Last 16-36 Hours

Days After Last Meal

  • Glycogen Stores are Depleted
  • Primary Energy Source: Breakdown of Muscle & Lipids
    • Largest Source: Free Fatty Acids
    • Gluconeogenesis
      • Occurs Primarily in Liver Early & Kidney Later
      • Primary Substrate: Alanine in Liver
        • Glutamine in Kidney Once Hepatic Alanine Exhausted

Starvation (6-8 Days After Last Meal)

  • Primary Energy Source: Gluconeogenesis
    • From Muscle Protein Breakdown to Amino Acids
  • Primary Substrate: Ketone Bodies

Stress vs Starvation

  Stress Starvation
Primary Energy Source Fat & Protein (Mixed) Fat
Basal Metabolic Rate Increased Decreased
Inflammatory Mediators Increased Decreased
Ketone Production Unchanged Increased
Gluconeogenesis Greatly Increased Increased
Protein Metabolism & Negative Nitrogen Balance Greatly Increased Increased
Muscle Proteolysis Greatly Increased Increased
Hepatic Protein & Urea Synthesis Greatly Increased Increased

Cell-Specific Energy Sources

Cell-Specific Sources

  • Cells Only Able to Use Glucose for Energy:
    • Red Blood Cells (RBC)
    • White Blood Cells (WBC)
    • Adrenal Medulla
    • Peripheral Nerves
  • Brain Cells Normally Use Glucose but Can Use Ketones if Needed
  • Primary Energy Source of Neoplastic Cells: Glutamine

Bowel Energy Sources

  • Small Intestine Enterocytes: Glutamine
  • Colonocytes: Short-Chain Fatty Acids