Biology – Transport of oxygen and carbon dioxide | e-Consult

Carbon dioxide (CO₂) transport in the blood involves several processes, each contributing to the efficient removal of CO₂ from the tissues and its transport to the lungs for exhalation. These processes can be broadly categorized into dissolved CO₂, bicarbonate ion transport, and carbaminohemoglobin formation. Carbonic anhydrase and haemoglobin play crucial roles in these processes.

A small amount of CO₂ is transported directly dissolved in the plasma. However, the majority of CO₂ is transported in the blood in three main forms:

• Bicarbonate Ions (HCO₃^-): This is the primary form of CO₂ transport. CO₂ diffuses from the tissues into red blood cells. Inside the red blood cells, the enzyme carbonic anhydrase catalyzes the reversible reaction between CO₂ and water to form carbonic acid (H₂CO₃). Carbonic acid then rapidly dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-). The bicarbonate ions are then transported out of the red blood cells into the plasma via a chloride shift mechanism (the movement of chloride ions across the membrane to maintain electrical neutrality).
• Carbaminohemoglobin: CO₂ can also bind directly to haemoglobin, forming carbaminohemoglobin. CO₂ binds to the globin portion of the haemoglobin molecule, not to the iron in the heme group. The formation of carbaminohemoglobin is facilitated by the presence of hydrogen ions (H⁺), which are produced during the conversion of CO₂ to bicarbonate. The binding of CO₂ to haemoglobin helps to remove H⁺ from the tissues, preventing a significant drop in blood pH.
• Dissolved CO₂: As mentioned earlier, a small amount of CO₂ is transported directly dissolved in the plasma. This is the least significant form of CO₂ transport.

The transport of CO₂ is crucial for maintaining acid-base balance in the body. The bicarbonate transport system allows for the efficient removal of CO₂ from the tissues and its transport to the lungs, where it is released back into the air as CO₂. The role of carbonic anhydrase is therefore essential for the rapid conversion of CO₂ to bicarbonate, facilitating this transport process. Haemoglobin's ability to bind CO₂, particularly in tissues with high CO₂ concentrations, further enhances CO₂ transport and helps to maintain a stable blood pH.