F7ii: Describe the production of CO2 in the body. What are the physiological reasons why PaCO2 may be high?

  • CO2 = gas produced from aerobic metabolism
  • By CITRIC ACID CYCLE → in matrix of mitochondria
Citric Acid Cycle
  • Glucose broken down to 2 x Pyruvate
  • Pyruvate oxidised to Acetyl CoA
  • Acetyl CoA (2C) combines with Oxaloacetate (4C) to form citric acid (6C)
  • Citric Acid undergoes a series of reactions to form oxaloacetate again → cycle repeats
    1. 1 ATP for every citric acid entering cycle
    2. For each citric acid, 8 hydrogen atoms with high-E yielding ē’s are transferred to FAD + NAD+ (hydrogen carriers)
    3. 2CO2 are produced
  • This cycle repeats as long as there is O2 & Pyruvate

Reasons for ↑PaCO2

  • PaCO2 = partial pressure of CO2 in arterial blood
    • PaCO2 35 – 45mmHg
    • CO2 stored in blood as HCO3– / dissolved ~120L (x 10 that of O2!)

∴factors influencing PaCO2:

  • PATM – deep sea diving → ↑PATM → ↑PaCO2
  • FiCO2 – air composed of 0.038% Co2 ∴FiO2 should be 0
  • CO2 output
    • CO2 produced from Citric Acid Cycle
    • Oxidative phosphorelation
    • ↑production
      • Fever
      • Hyperthyroidism
    • Alveolar ventilation
      • PaCO2 is inversely proportional to alveolar ventilation
      • Causes of ↑PaCO2 relate to hypoventilation

1) 1° Hypoventilation (less than normal ventilation)

  • Central Depression (opioids) & ↓ventilatory response to PaCO2
  • Muscle weakness
  • Pain
  • Bronchospasm

2) Relative Hypoventilation (normal level of ventilation, inadequate due to …)

  • ↑FiCO2 (rebreathing anaesthetic circuit)
  • ↑CO2 production (fevers)
  • ↑dead space
  • Impaired diffusion (carbonic anhydrase inhibitors)

3) V/Q Abnormalities – shunt