Describe the role of plasma in the transport of carbon dioxide

Sourav PanOctober 30, 2024

Answered step-by-step

Plasma, the liquid component of blood, plays a crucial role in transporting carbon dioxide (CO₂) from tissues to the lungs for exhalation. Plasma facilitates CO₂ transport through three main forms:

1. Dissolved CO₂ in Plasma (5-10%)

• A small portion of CO₂ produced by cellular respiration dissolves directly in the plasma due to its solubility in water.
• This dissolved CO₂ is transported in its molecular form through the bloodstream and contributes to the CO₂ partial pressure in blood, aiding in the diffusion process from tissues to the lungs.

2. Bicarbonate Ions (HCO₃⁻) in Plasma (70-80%)

• Most CO₂ in the blood is transported as bicarbonate ions (HCO₃⁻) in the plasma. This occurs after CO₂ diffuses into red blood cells (RBCs), where it reacts with water, catalyzed by the enzyme carbonic anhydrase, to form carbonic acid (H₂CO₃). Carbonic acid then dissociates into bicarbonate ions (HCO₃⁻) and hydrogen ions (H⁺).
• The HCO₃⁻ ions diffuse out of RBCs and into the plasma. This exchange is facilitated by the chloride shift, where chloride ions (Cl⁻) enter RBCs to maintain ionic balance.
• Bicarbonate ions then circulate in the plasma until they reach the lungs, where they re-enter RBCs, recombine with H⁺ to form CO₂ and H₂O, and are subsequently exhaled.

3. Carbamino Compounds (including Carbaminohaemoglobin) (20-30%)

• A portion of CO₂ binds to plasma proteins and hemoglobin within RBCs to form carbamino compounds, with carbaminohaemoglobin (HbCO₂) being the most notable form.
• Although carbamino compounds mainly involve hemoglobin within RBCs, some CO₂ also binds to plasma proteins. This binding is reversible, enabling CO₂ to be released when blood reaches the lungs.