Gas Transport – CO2

Master answer

PaCO₂ is determined by the balance of production and elimination.

CO2 production:

• Normally 200ml/min (compared to average oxygen intake of 250ml/min)
• Byproduct of aerobic metabolism
• Produced in the cell mitochondria via TCA
  • Entry to TCA as acetyl CoA (from CHO, FA, aa)
  • TCA products
    • CO₂
    • Reduced compounds FADH and NADH
    • H⁺ ions
  • 2 molecules of CO₂produced for each 1 molecule of ATP
• Amount of CO2 produce per molecule of substrate varies with the substrate.
  • 1.5 Mole CO₂ per mole of Glucose
  • Respiratory quotient
    • R = CO₂ production / O₂ consumption
    • Depends on fuel substrate
    • 0.7 for fat, 1.0 for CHO
    • 0.8 for typical western diet

CO2 Storage and transport

• ~ 120 litres predominantly as bicarbonate ion
• CO₂ is carried in the blood in 3 forms
  • dissolved (5%)
  • bicarbonate (70-80%)
  • carbamino compounds (20-25%).

Elimination:

• Small amount dissolved in sweat
• Most via lungs
  • PaCO₂ ~ PACO₂
  • PACO₂ determined by alveolar ventilation equation

Factors which ↑PaCO₂

• ↑Shunts or ↑V/Q scatter → ↑PaCO₂
• ↑ Alveolar CO₂ → ↑PaCO₂
  • ↑ Production (VCO₂)
    • During vigorous exercise
    • Moderate exercise doesn’t effect pCO₂ and may actually result in ↓pCO₂ due to ↑MV
    • Hypermetabolic states (Sepsis, Fever, MH, neurolept malignant syndrome)
    • Altered (ie pure carb) diet
  • ↑ Barometric pressure/↓Pressure of water
  • ↑ Inspired CO₂
  • Determined predominantly via alveolar ventilation which depends on:
    • Dead space (DS):
      • PE, low CO state, ↑d PEEP → ↑West’s zone     
      • ↑DS → ↓Alveolar Vent → ↑Alveolar CO2 → ↑PaCO2
    • Respiratory Rate: ↓RR → ↓Alveolar Vent → ↑Alveolar CO2 → ↑PaCO2
    • Tidal Volume (Tv): ↓Tv → ↓Alveolar Vent → ↑Alveolar CO2 → ↑PaCO2

Gladwin 2016

Answer by Gladwin