describe the role of plasma in the transport of carbon dioxide

Transport of Oxygen and Carbon Dioxide

Learning Objective

Describe the role of plasma in the transport of carbon dioxide.

Overview

In the circulatory system, gases are carried between the lungs and tissues. Oxygen (O₂) is primarily bound to haemoglobin within red blood cells, whereas carbon dioxide (CO₂) is transported in three main forms, one of which involves the plasma.

Transport of Oxygen

Oxygen is carried mainly as oxyhaemoglobin:

• Each haemoglobin molecule can bind up to four O₂ molecules.
• The binding is reversible and depends on the partial pressure of O₂ (pO₂).
• In the lungs (high pO₂) haemoglobin becomes saturated; in tissues (low pO₂) O₂ is released.

Transport of Carbon Dioxide

CO₂ produced by cellular metabolism is removed from tissues and delivered to the lungs for exhalation. It is carried in the blood in three distinct forms:

1. Dissolved CO₂ in plasma (≈7 %).
2. Carbamino compounds – CO₂ bound to the amino groups of haemoglobin (≈23 %).
3. Bicarbonate ions (HCO₃⁻) formed in plasma (≈70 %).

Role of Plasma in CO₂ Transport

Plasma, the liquid component of blood, plays a crucial role in the conversion of CO₂ to bicarbonate, the dominant transport form. The process involves several steps:

• CO₂ diffuses from tissues into the plasma.
• In plasma, CO₂ reacts with water under the catalytic action of carbonic anhydrase (present in red blood cells) to form carbonic acid:

  \$\text{CO}2 + \text{H}2\text{O} \;\xrightleftharpoons[\text{CA}]{\text{}} \;\text{H}2\text{CO}3\$

• Carbonic acid dissociates into hydrogen ions and bicarbonate ions:

  \$\text{H}2\text{CO}3 \;\rightleftharpoons\; \text{H}^+ + \text{HCO}_3^-\$

• The bicarbonate ions diffuse out of red blood cells into the plasma, where they are carried to the lungs.
• To maintain electrical neutrality, chloride ions (Cl⁻) move into the red blood cells (the “Hamburger‑Peter” exchange).

Summary Table of CO₂ Transport Forms

Why Plasma Is Essential

Plasma provides the aqueous medium required for the reversible hydration of CO₂ and the subsequent ionisation to bicarbonate. Without plasma:

• The solubility of CO₂ would be limited, reducing the amount that can be carried.
• The conversion to bicarbonate, which vastly increases the transport capacity, would be inefficient.
• The chloride shift that maintains charge balance could not occur, disrupting red blood cell function.

Key Points to Remember

• Plasma carries the majority of CO₂ as bicarbonate ions.
• Carbonic anhydrase accelerates the formation of carbonic acid, a reaction that would otherwise be too slow.
• The “chloride shift” (exchange of HCO₃⁻ for Cl⁻) is vital for maintaining electrochemical neutrality.
• Only a small fraction of CO₂ is transported dissolved directly in plasma.