CO2 Transport in the Blood

This article provides an overview of the transport of carbon dioxide (CO₂) in the blood including the three main methods of transport, the effect of oxygen on CO₂ carriage and the impact of CO₂ on the acid-base balance of the body.  

Production of CO₂

CO₂ is produced as a waste product of respiration within the mitochondria of cells.¹ CO₂ is then transported to the lungs for excretion via the three mechanisms discussed below.

Transport of CO₂

CO₂ is transported within the body in three forms: ²

• Dissolved CO₂
• Bicarbonate
• Carbamino compounds

Table 1. The three forms in which CO₂ is transported and the relative percentage in arterial and venous blood respectively.

Dissolved component

CO₂ is 20 times more soluble in blood than oxygen is. As a result, the dissolved component of CO₂ is more significant than the respective dissolved component of oxygen.

Bicarbonate

The majority of CO₂ is transported via bicarbonate. CO₂ diffuses down its concentration gradient out of tissues and into red blood cells (RBCs). Once inside the RBC, the enzyme carbonic anhydrase catalyses the conversion of CO₂ into carbonic acid (H₂CO₃). Carbonic acid is then hydrolysed into H⁺ ions and HCO₃– (bicarbonate). The H⁺ ion is bound by haemoglobin which buffers the process. The reverse occurs at the lungs where H⁺ dissociates from haemoglobin and combines with bicarbonate to form carbonic acid which is then converted back to CO₂ and water. This reaction is again catalysed by carbonic anhydrase.

Chloride shift

Chloride shift refers to the exchange of bicarbonate (HCO₃⁻) and chloride (Cl⁻) across the membrane of red blood cells (RBCs). CO₂ diffuses freely into RBCs but as HCO₃^– ions are charged they cannot easily cross the cell membrane to leave the RBC. As a result, a carrier protein is utilised to allow HCO₃⁻ to leave the RBC in exchange for chloride ions. This also allows the RBC to maintain its electroneutrality. 

Carbamino compounds

CO₂ directly combines with haemoglobin to form carbamino compounds. The affinity of haemoglobin to CO₂ depends on whether haemoglobin is in its oxy or deoxygenated state.

Haldane effect

The Haldane effect describes the ability of deoxygenated haemoglobin to carry more CO₂ than oxygenated haemoglobin. This occurs for two reasons:

1. Deoxygenated haemoglobin forms carbamino complexes more readily with CO₂ (hence the greater proportion of CO₂ carried via this method in venous deoxygenated blood).
2. Deoxygenated haemoglobin is a better buffer of H⁺ ions than oxygenated haemoglobin. This shifts the equilibrium in the equation for the formation of bicarbonate to the right and therefore increases the amount of HCO₃^– formed.

The Bohr effect

The Bohr effect is not strictly related to CO₂ transport but is the reverse of the Haldane effect. The Bohr effect refers to the reduction in affinity of haemoglobin for oxygen in the presence of a reduction in pH or an increase in CO₂. This phenomenon allows offloading of oxygen at the tissues where the pH is lower as CO₂ is produced there. It also facilitates the uptake of oxygen at the lungs where the CO₂ concentration is lower, and the pH is higher.

Key points

• CO₂ is produced in tissues as a waste product of respiration and is excreted in the lungs.
• CO₂is transported in the blood in three forms: dissolved, bicarbonate and carbamino compounds.
• Deoxygenated haemoglobin is able to carry more CO₂ than oxygenated haemoglobin.
• Removal of carbon dioxide is vital to maintain a normal acid-base balance.

References

1. James Doyle; Jeffrey S. Cooper. Physiology, Carbon Dioxide Transport. StatPearls. Available from: [LINK]
2. West JB. Respiratory Physiology, 7th Edn. Lippincott Williams & Wilkins. Published in 2004.
3. OpenTextBC. The respiratory system: transport of gases. Licence: CC 4.0. Available from: [LINK]