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)
- Diffusion into blood: CO₂ moves from metabolising cells into the plasma (Henry’s law).
- Entry into RBCs: CO₂ diffuses across the RBC membrane into the cytoplasm.
- Hydration (carbonic anhydrase) (inside the RBC):
\[
\text{CO}2 + \text{H}2\text{O} \;\xrightleftharpoons[\text{CA}]{\text{}} \;\text{H}2\text{CO}3
\]
- 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⁺).
- Chloride shift (Hamburger‑Peter exchange):
- HCO₃⁻ exits the RBC into plasma.
- Cl⁻ moves from plasma into the RBC to maintain charge balance.
- Plasma transport: The bulk of CO₂ now travels as HCO₃⁻ dissolved in plasma toward the pulmonary capillaries.
- 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
CO₂ distribution in blood
≈ 70 % as HCO₃⁻ (plasma) – primary transport route
≈ 23 % as carbamino‑Hb (inside RBCs)
≈ 7 % dissolved directly in plasma
5. Quantitative Relationship (Henry’s Law)
The amount of CO₂ dissolved in plasma (\(C\)) is directly proportional to its partial pressure (\(p{\text{CO}2}\)):
\[
C = kH \times p{\text{CO}_2}
\]
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.