describe the role of plasma in the transport of carbon dioxide

Transport of Oxygen and Carbon Dioxide (Cambridge IGCSE/A‑Level 9700)

Learning Objective

Describe the role of plasma in the transport of carbon dioxide and explain how oxygen is carried and released.

1. Overview of Gas Transport

• O₂ and CO₂ are exchanged between the lungs and body tissues via the bloodstream.
• O₂ is carried mainly bound to haemoglobin (Hb) inside red blood cells (RBCs).
• CO₂ is transported in three quantitative forms; the dominant form (≈ 70 %) is the bicarbonate ion (HCO₃⁻) dissolved in plasma.

2. Transport of Oxygen

• Oxyhaemoglobin (HbO₂): each Hb molecule can bind up to four O₂ molecules.
• Binding is reversible and depends on the partial pressure of O₂ (pO₂).
• In the lungs (high pO₂) Hb becomes saturated; in tissues (low pO₂) O₂ is released.

2.1 Oxygen‑Dissociation Curve

• The curve is sigmoid because each Hb molecule has four binding sites that influence one another (co‑operativity).
• Steep part (low pO₂): a small rise in pO₂ produces a large increase in O₂ saturation.
• Plateau (high pO₂): Hb is already near saturation, so further increases in pO₂ have little effect.

Factors that shift the curve to the right (lower O₂ affinity, easier O₂ release):

• Increased temperature
• Increased 2,3‑BPG (bis‑phosphoglycerate)
• Decreased pH (more H⁺) – the Bohr effect
• Increased pCO₂

Right‑shift → more O₂ delivered to active tissues.

Factors that shift the curve to the left (higher O₂ affinity, less O₂ release):

• Decreased temperature
• Decreased 2,3‑BPG
• Increased pH (alkalosis)
• Decreased pCO₂

Left‑shift → O₂ remains bound to Hb, useful in the lungs or during fetal life.

3. Transport of Carbon Dioxide

3.1 Quantitative Forms of CO₂ in Blood

Form	Location	Approx. % of Total CO₂	Key Features
Dissolved CO₂	Plasma	≈ 7 %	Directly proportional to pCO₂ (Henry’s law); diffuses freely.
Carbamino‑haemoglobin (Hb‑CO₂)	Haemoglobin inside RBCs	≈ 23 %	CO₂ binds to the N‑terminal α‑amino groups of the globin chains, forming a carbamino compound.
Bicarbonate ion (HCO₃⁻)	Plasma (majority) & transiently in RBCs	≈ 70 %	Formed by hydration of CO₂ → carbonic acid → H⁺ + HCO₃⁻; requires carbonic anhydrase and the chloride shift.

3.2 Why Plasma Is Essential for CO₂ Transport

≈ 70 % of the CO₂ produced by tissues is carried in the plasma as the bicarbonate ion. This makes plasma the **principal medium** for CO₂ removal because:

• It provides an aqueous environment in which CO₂ can be hydrated to H₂CO₃ and then dissociated to H⁺ and HCO₃⁻.
• Transport of HCO₃⁻ in plasma increases the blood’s CO₂‑carrying capacity >20‑fold compared with dissolution alone.
• The plasma also participates in the chloride shift, preserving electrical neutrality and red‑cell volume.

4. Detailed Sequence of CO₂ Transport (Tissues → Lungs)

1. Diffusion into blood: CO₂ moves from metabolising cells into the plasma (Henry’s law).
2. Entry into RBCs: CO₂ diffuses across the RBC membrane into the cytoplasm.
3. Hydration (carbonic anhydrase) (inside the RBC): \[ \text{CO}_2 + \text{H}_2\text{O} \;\xrightleftharpoons[\text{CA}]{\text{}} \;\text{H}_2\text{CO}_3 \]
4. Ionisation: \[ \text{H}_2\text{CO}_3 \;\rightleftharpoons\; \text{H}^+ + \text{HCO}_3^- \] The H⁺ is largely buffered by haemoglobin, forming **haemoglobinic acid (Hb‑H⁺)**.
5. Chloride shift (Hamburger‑Peter exchange):
   • HCO₃⁻ exits the RBC into plasma.
   • Cl⁻ moves from plasma into the RBC to maintain charge balance.
6. Plasma transport: The bulk of CO₂ now travels as HCO₃⁻ dissolved in plasma toward the pulmonary capillaries.
7. Reverse reactions in the lungs:
   • Cl⁻ leaves the RBC; HCO₃⁻ re‑enters.
   • Carbonic anhydrase catalyses the reverse reaction: \[ \text{H}^+ + \text{HCO}_3^- \;\xrightleftharpoons[\text{CA}]{\text{}} \;\text{CO}_2 + \text{H}_2\text{O} \]
   • CO₂ diffuses from the RBC into the plasma, then into the alveoli, and is exhaled.

4.1 Quick‑Reference Box

5. Quantitative Relationship (Henry’s Law)

The amount of CO₂ dissolved in plasma (\(C\)) is directly proportional to its partial pressure (\(p_{\text{CO}_2}\)):

\[ C = k_H \times p_{\text{CO}_2} \] where \(k_H\) is Henry’s constant for CO₂ in blood. This explains why a rise in tissue pCO₂ leads to a proportional increase in dissolved CO₂, which then drives the hydration‑bicarbonate pathway.

6. Clinical Relevance (Brief)

• Hypercapnia – elevated pCO₂ raises H⁺, causing respiratory acidosis; the bicarbonate buffer system (plasma HCO₃⁻) mitigates the fall in pH.
• Chronic obstructive pulmonary disease (COPD) – impaired CO₂ removal stresses the plasma bicarbonate system, often leading to compensatory renal retention of HCO₃⁻.
• Electrolyte disturbances – conditions that alter plasma Cl⁻ (e.g., massive transfusion, diuretics) affect the chloride shift and can modify CO₂ transport efficiency.

7. Summary of Key Points

• ≈ 70 % of CO₂ is carried in plasma as bicarbonate ion; this is why plasma is essential for CO₂ transport.
• Carbonic anhydrase, located **inside red blood cells**, accelerates the conversion of CO₂ ↔ H₂CO₃.
• H⁺ produced during hydration is buffered by haemoglobin, forming **haemoglobinic acid (Hb‑H⁺)**.
• The chloride shift moves HCO₃⁻ into plasma and Cl⁻ into RBCs, preserving electrical neutrality and cell volume.
• Only ~7 % of CO₂ is transported dissolved directly in plasma; the rest is as carbamino‑Hb (≈23 %) and HCO₃⁻ (≈70 %).
• O₂ is carried bound to Hb; its release is controlled by the sigmoid oxygen‑dissociation curve.
• The Bohr effect (low pH/high pCO₂) and other right‑shift factors (temperature, 2,3‑BPG) promote O₂ release where metabolic demand is greatest.