CO2 Transport

• CO2 Balance:

  • We produce ~ 200 ml of CO2 per min
  • We excrete ~ 200 ml of CO2 per min
• CO2 Imbalance

  • Producing more CO2 than we excrete / inadequate excretion
  • PaCO2 retention:
  • increase in number of free Hydrogen ions = decrease in pH = acidosis
• Transportation of CO2:

  • In solution - dissolved in plasma as partial pressure of CO2
  • Bound to Haemoglobin
  • As a bicarbonate
• CO2 in solution:

  • 7 - 10% CO2 transported in plasma solution
  • Higher than amount O2 in solution
  • as CO2 is 20 times more soluble than O2 due to its molecular weight
• CO2 and Hb:

  • 20 - 30% binds to haemoglobin
  • binds to globin portion rather than haem portion
  • forming carbamino compounds
  • carbaminohaemoglobin (HbCO2)
• CO2 as bicarbonate:

  • 60 - 70% CO2 converted to HCO3-
  • Transported in this form from tissues to lungs
  • Process of turning CO2 into a bicarbonate can occur via 2 places:
  • Conversion in plasma as a very slow process
  • Most conversion is in the Red Blood Cell in presence of carbonic anhydrase, which speeds up the process
• CO2 + H2O <-> H2CO3 <-> HCO3 + H+

  • High CO2 (tissues) equation moves right
  • CO2 combines with H2O
  • HCO3 + H+ -> CO2 Tx
  • Low CO2 (lungs) equation moves left
  • H2CO3 dissociate
  • liberation of CO2 & H2O
  • excretion of CO2 as expired air
• CO2 and H+

  • Need to understand link between CO2 + Hydrogen ions
  • CO2 + H2O <-> H2CO3 <-> HCO3 + H+
  • The more CO2 in the system the more H+ there will be
  • If there are inadequate proteins / other HCO3 around hydrogen ions will not be buffered
  • Free H+ = acidosis
• Clinical Relevance:

  • Inadequate ventilation causes an increase in CO2
  • Acute hypoventilation
  • Post Upper Abdominal Surgery
  • pain inhibition
  • Diaphragmatic dysfunction
  • Chronic inadequate ventilation
  • Obstructive conditions
  • V/Q mismatch
• Haldane Effect:

  • In the tissues Hb decrease in affinity for O2 because
  • PO2 is low -> O2 unloading from Hb
  • Hydrogen ions (from CO2 ) bind to Hb
  • At same time
  • Deoxygenation of haemoglobin -> increase in Hb affinity for CO2 – Haldane effect
  • leads to O2 delivery to the tissues
  • leads to efficient CO2 loading