7th February 2024

Delivery and transport of oxygen and carbon dioxide

Oxygen

Oxygen is carried in the blood via two methods:

• Bound to haemoglobin (98%)
• Dissolve in blood (2%)

This is reflected in the following formula:

Oxygen content in blood (mL of oxygen/100mL blood) = [1.34 x SpO2 x Hb (g/L)] +[0.023 x PaO2 (kPa)]

Delivery of oxygen is dependent on cardiac output.

Delivery of oxygen = Cardiac output x oxygen content in blood

Carbon dioxide

Carbon dioxide is carried in blood in various forms:

• Bicarbonate ion (HCO3-) (90%)
  • Carbonic anhydrase enzymes found in abundance in red blood cell (RBC) cytoplasm catalyse the reaction CO2 + H2O ⇋ H2CO3 ⇋ H+ + HCO3-
  • H+ cannot cross the RBC membrane. H+ binds to Hb, forming HHb.
  • HCO3- in RBC is transported out of the cell down its electrochemical gradient in exchange with plasma Cl- to maintain the electrical neutrality of the cell. This process is referred to as ‘chloride shift.’
• Carbaminohaemoglobin (5%)
  • CO2 binds to haemoglobin to form carbaminohaemoglobin (do not confuse this with carboxyhaemoglobin, COHb, which is carbon monoxide bound to Hb)
  • Deoxyhaemoglobin binds more readily to carbon dioxide (Haldane effect)
• Dissolved as carbonic acid in plasma (5%)

References

Levitzky, M. G. (2018). Chapter 7: Transport of Oxygen and Carbon Dioxide in the Blood. In Pulmonary physiology (9th ed). McGraw-Hill education.