Transport of Gases (O₂ and CO₂)
Once O₂ enters blood from alveoli, it must be carried to all tissues, and CO₂ produced in tissues must be brought back to the lungs.
Blood is the transport medium, mainly using RBCs (haemoglobin) and plasma.
How Gases Travel in the Blood?
Blood acts as the transporter vehicle for both Oxygen () and Carbon Dioxide ().
A. Transport of Oxygen (): Think of Red Blood Cells (RBCs) as the main buses for Oxygen.
- 97% (Majority): Carried by RBCs (specifically by Haemoglobin).
- 3% (Minority): Travels in a dissolved state within the Plasma (the liquid part of blood).
B. Transport of Carbon Dioxide (): is more soluble than , so it has more travel options.
- 70% (Major route): Carried as Bicarbonate ions ().
- 20-25%: Carried by RBCs (bound to Haemoglobin as Carbamino-haemoglobin).
- 7%: Dissolved directly in the Plasma.
Transport of Oxygen (O₂)
Forms of transport
- ~97% as Oxyhaemoglobin (HbO₂) in RBCs
- ~3% dissolved in plasma
Why Hb is needed: O₂ has low solubility in plasma, so haemoglobin (Hb) carries most O₂ efficiently.
Oxyhaemoglobin formation
- Hb + O₂ ⇌ HbO₂
- Each Hb molecule can bind up to 4 O₂ molecules (because Hb has 4 heme groups).
Hb binds O₂ reversibly, so it can load O₂ in lungs and unload in tissues.
Oxygen Dissociation Curve (ODC)
The ODC is obtained when % saturation of Hb with O₂ is plotted against pO₂.
- It is sigmoid due to cooperative binding: binding of the first O₂ increases affinity for the next O₂.
A. At Alveoli (Loading of O₂)
Conditions (favour association):
- High pO₂
- Low pCO₂
- Low H⁺ (higher pH)
- Lower temperature
Result: Hb gets saturated → O₂ loading.
B. At Tissues (Unloading of O₂)
Conditions (favour dissociation):
- Low pO₂
- High pCO₂
- High H⁺ (low pH)
- Higher temperature
Result: Hb releases O₂ → O₂ unloading.
Bohr Effect
When pCO₂ increases and H⁺ increases (pH falls), Hb releases O₂ more easily. Meaning: Active tissues (more CO₂ + H⁺ + heat) automatically get more O₂
100 mL of oxygenated blood delivers ~5 mL O₂ to tissues under normal conditions.
Transport of Carbon Dioxide (CO₂)
CO₂ is more soluble than O₂, so it is transported in three forms:
1. As Bicarbonate (HCO₃⁻) — Major form (~70%)
This is the main pathway.
Key enzyme: Carbonic anhydrase (very high amount inside RBCs).
Reaction (reversible):
At Tissues (CO₂ pickup)
- Tissue pCO₂ is high → CO₂ diffuses into blood/RBC.
- Reaction shifts forward → more HCO₃⁻ + H⁺ formed.
Key point: CO₂ is “packed” mainly as bicarbonate for safe transport.
At Alveoli (CO₂ release)
- Alveolar pCO₂ is low → CO₂ diffuses out.
- Reaction shifts backward → HCO₃⁻ converts back to CO₂.
Key point: In lungs, bicarbonate is “unpacked” back to CO₂ for exhalation.
2. As Carbamino-haemoglobin (~20–25%)
- CO₂ binds to the amino groups of haemoglobin (not the heme iron).
- Favoured by:
- High pCO₂ (tissues)
- Low pO₂ (deoxygenated Hb binds CO₂ better)
Haldane Effect (high yield): Deoxygenated Hb carries more CO₂, so tissues load CO₂ easily.
3. Dissolved in plasma (~7%)
CO₂ is sufficiently soluble, so a small fraction is carried dissolved.
100 mL of deoxygenated blood delivers ~4 mL CO₂ to alveoli.
Comparison
O₂: mainly carried by Hb (97%) → depends strongly on pO₂, and release is helped by ↑CO₂, ↑H⁺, ↑Temp (Bohr effect).
CO₂: mainly carried as bicarbonate (70%) → needs carbonic anhydrase, and deoxygenated blood carries more CO₂ (Haldane effect).
Important Points
- O₂ transport: Hb is key.
- CO₂ transport: HCO₃⁻ is key.
- Carbonic anhydrase speeds up bicarbonate formation.
- ODC is sigmoid.
- Bohr effect helps tissues get O₂ efficiently.
Transport of Gases
1. The "Highs" and "Lows" of Transport
| Location | Alveoli (Lungs) | Tissues (Body) |
|---|---|---|
| Goal | Grab O2 / Release CO2 | Release O2 / Grab CO2 |
| pO2 | High | Low |
| pCO2 | Low | High |
| Temp/H+ | Low | High |
2. The 4-5 Rule (Delivery Stats)
- 4 mL of CO2 is delivered to lungs.
- 5 mL of O2 is delivered to tissues. (Remember: You need more O2 to live, so the number is higher: 5)
3. Percentage Breakdown
- O2: 97% (RBC) + 3% (Plasma).
- CO2: 70% (Bicarbonate) + 20-25% (RBC) + 7% (Plasma).
💡 Questions and Answers
Q1. In which form is most CO₂ transported in blood?
A1: Most CO₂ is carried as bicarbonate ions (HCO₃⁻) in plasma. This happens because CO₂ reacts with water inside RBCs (with the help of carbonic anhydrase) and becomes bicarbonate.
Key points:
- 70% as HCO₃⁻
- Needs carbonic anhydrase
Q2. How much oxygen is delivered to tissues by 100 mL of oxygenated blood?
A2: About 5 mL of O₂ is delivered to tissues by every 100 mL of oxygenated blood under normal conditions.
Key points:
- 5 mL O₂ / 100 mL blood
Q3. What conditions in tissues help Hb release oxygen?
A3: In tissues, pO₂ is low but CO₂, H⁺ (low pH), and temperature are high. These conditions make haemoglobin release oxygen easily so active tissues get more oxygen.
Key points:
- ↓pO₂ + ↑pCO₂ + ↑H⁺ + ↑Temp → O₂ release
- This is Bohr effect
Q4. Name the enzyme that helps in bicarbonate formation. Why is it important?
A4: The enzyme is carbonic anhydrase. It makes the CO₂ ↔ bicarbonate reaction very fast, so CO₂ can be transported efficiently in blood.
Key points:
- Enzyme: Carbonic anhydrase
- Makes reaction fast and reversible
Q5. What is the shape of the oxygen dissociation curve and why?
A5: It is sigmoid (S-shaped) because haemoglobin shows cooperative binding. Once one O₂ binds, Hb’s affinity for the next O₂ increases.
Key points:
- Curve: Sigmoid
- Reason: Cooperative binding