3.4 Transport of Gases

The transport of gases refers to the movement of oxygen and carbon dioxide between the lungs and the body tissues through the blood. Oxygen is carried from the lungs to the tissues, and carbon dioxide is carried from the tissues to the lungs.

Definition

Transport of Gases: The transport of gases is the process by which respiratory gases (oxygen and carbon dioxide) are carried in the blood between the lungs and the body tissues.

Oxygen Transport: Oxygen is transported from the lungs to the tissues. Approximately 97% is carried by red blood cells as oxyhaemoglobin, while 3% is transported as dissolved oxygen in the plasma.

Carbon Dioxide Transport: Carbon dioxide is transported from the tissues to the lungs in three main ways: as bicarbonate ions (70%), as carbaminohaemoglobin (23%), and dissolved in plasma (7%).

Main Idea

• Oxygen is carried from lungs to tissues mainly as oxyhaemoglobin.
• Carbon dioxide is carried from tissues to lungs mainly as bicarbonate ions.
• The transport of gases depends on partial pressures and chemical reactions in the blood.

Core Concept

• Oxygen transport = 97% as oxyhaemoglobin; 3% dissolved in plasma
• Carbon dioxide transport = 70% as bicarbonate ions; 23% as carbaminohaemoglobin; 7% dissolved in plasma
• **Oxyhaemoglobin** = Hb + 4O₂
• Carbaminohaemoglobin = CO₂ + globin part of haemoglobin
• **Bicarbonate ions** = CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
• Chloride shift = Hamburger's phenomenon
• Carbonic anhydrase = enzyme that catalyzes the reaction
• Partial pressure = determines direction of diffusion

Detailed Explanation

Transport of Oxygen in Blood

Approximately 97% of oxygen is carried by the red blood cells as oxyhaemoglobin, while 3% is transported as dissolved oxygen in the plasma.

• At its high partial pressure, oxygen binds with haemoglobin.
• This binding is reversible and occurs in the lungs in the presence of the enzyme carbonic anhydrase.
• Each molecule of haemoglobin can bind with four molecules of oxygen to form oxyhaemoglobin.

Chemical Equation: Hb + 4O₂ Carbonic anhydrase→ Hb₄O₂

• The ability of haemoglobin to bind with oxygen is called the oxygen carrying capacity of blood.
• The oxygen carrying capacity of blood is directly proportional to the partial pressure of oxygen (PO₂).
• Maximum oxygen carrying capacity of arterial blood is 20 ml/100 ml of blood (100% saturated), which is achieved at 100 mmHg PO₂.
• The 5 ml of O₂ is released to the tissues by each 100 ml blood.
• Oxygen carrying capacity is sensitive to a variety of environmental conditions like:
• Rise in body temperature
• Drop in pH of blood
• Partial pressures of carbon dioxide and oxygen

• The amount of haemoglobin is 15 gms/100 ml of blood.
• Since 1 gm Hb can combine with 1.34 ml of O₂, therefore 100 ml blood combines with 20 ml O₂ (100% saturated).
• Normally each 100 ml of arterial blood contains 19.4 ml O₂ (i.e., it is 97% saturated; PO₂ is 95 mmHg).
• While 100 ml of venous blood contains 14.4 ml O₂ (i.e., it is 75% saturated; PO₂ is 40 mmHg).

Transport of Carbon Dioxide in Blood

• In the form of bicarbonate ions
• In the form of carbaminohaemoglobin
• Dissolved in plasma

(I) As Bicarbonate Ions

Approximately 70% of carbon dioxide is carried in the blood as bicarbonate ions.

• Carbon dioxide diffuses into the blood.
• Enters the red blood cells.
• Combines with water to form carbonic acid in the presence of enzyme carbonic anhydrase.

Chemical Equation: CO₂ + H₂O Carbonic anhydrase→ H₂CO₃

4. Carbonic acid (H₂CO₃) is an unstable compound and dissociates to form hydrogen ions and bicarbonate ions.

Chemical Equation: H₂CO₃ Carbonic anhydrase→ H⁺ + HCO₃^-

• Accumulation of H⁺ ions increases acidity in the blood, i.e., it leads to a decrease in pH.
• This does not occur since haemoglobin buffers the hydrogen formed.
• The hydrogen ion readily associates with oxyhaemoglobin (Hb₄O₂) to form haemoglobinic acid (HHb) and oxygen is released to the tissue.

Chemical Equation: Hb₄O₂ + H⁺ → HHb + 4O₂

• From inside the erythrocytes, negatively charged HCO₃⁻ ions diffuse to the plasma.
• This is balanced by the diffusion of chloride ions (Cl⁻) in the opposite direction.
• This is achieved by special bicarbonate-chloride carrier proteins that exist in the RBC membrane.
• This protein moves the two ions in opposite directions, maintaining the balance of ions on either side.
• This is called the chloride shift or Hamburger's phenomenon.

• The exchange of bicarbonate ions (HCO₃⁻) out of the red blood cell and chloride ions (Cl⁻) into the red blood cell.
• Maintains electrical neutrality.

• The process reverses.
• Bicarbonate ions combine with hydrogen ions to release carbon dioxide and water.
• Carbon dioxide is exhaled.

(II) As Carbaminohaemoglobin

About 23% of carbon dioxide is carried as carbaminohaemoglobin.

• CO₂ combines with the globin part of haemoglobin.
• The reaction depends upon the partial pressure of CO₂.
• When the PCO₂ is higher in the tissues than blood, formation of carbaminohaemoglobin occurs.
• When the PCO₂ is higher in the blood than tissues, as in the case of lungs, carbaminohaemoglobin releases its CO₂.

(III) As Dissolved CO₂ in Plasma

Only 7% of carbon dioxide is carried this way. This is a rather inefficient way to carry carbon dioxide, but it does occur.

Summary Table of Gas Transport

Comparison of Oxygen and Carbon Dioxide Transport

Intermediate Level

Oxygen Transport

Carbon Dioxide Transport

Chloride Shift

Factors Affecting Oxygen Transport

Formula / Mathematical Form

Oxygen Content: O₂ Content = (1.34 × Hb × SaO₂) + (0.003 × PO₂)

• 1.34 = ml O₂ per gram of haemoglobin
• Hb = haemoglobin concentration (g/dL)
• SaO₂ = arterial oxygen saturation
• 0.003 = solubility coefficient of O₂ in plasma
• PO₂ = partial pressure of oxygen

Carbon Dioxide Content: CO₂ Content = HCO₃^- + HbCO₂ + dissolved CO₂

Solved Example

• Haemoglobin = 15 g/dL
• SaO₂ = 97%
• PO₂ = 95 mmHg

Required: Oxygen content

Formula: O₂ Content = (1.34 × Hb × SaO₂) + (0.003 × PO₂)

Substitution: O₂ Content = (1.34 × 15 × 0.97) + (0.003 × 95)

Calculation: O₂ Content = 19.5 + 0.285 = 19.785 ml O₂/dL

Final Answer: The oxygen content is approximately 19.8 ml O₂/dL.

Important Points

• 1. Oxygen is transported mainly as oxyhaemoglobin (97%).
• 2. Carbon dioxide is transported mainly as bicarbonate ions (70%).
• 3. Carbonic anhydrase catalyzes the reaction.
• 4. Chloride shift maintains electrical neutrality.
• 5. Haemoglobin can bind four molecules of oxygen.
• 6. Oxygen carrying capacity is directly proportional to PO₂.
• 7. Maximum oxygen carrying capacity is 20 ml/100 ml blood.
• 8. Arterial blood is 97% saturated.
• 9. Venous blood is 75% saturated.
• 10. Carbaminohaemoglobin carries 23% of CO₂.
• 11. Dissolved CO₂ carries 7% of CO₂.
• 12. The process reverses in the lungs.
• 13. Bohr effect: decreased pH → decreased O₂ binding.
• 14. Increased temperature → decreased O₂ binding.
• 15. Carbon monoxide binds haemoglobin more strongly than oxygen.

Remember

• - Oxygen transport = 97% oxyhaemoglobin; 3% dissolved
• - Carbon dioxide transport = 70% bicarbonate; 23% carbaminohaemoglobin; 7% dissolved
• - **Oxyhaemoglobin** = Hb + 4O₂
• - **Carbaminohaemoglobin** = CO₂ + globin
• - **Bicarbonate** = CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
• - Chloride shift = Hamburger's phenomenon
• - Carbonic anhydrase = enzyme
• - Partial pressure = determines direction of diffusion
• - Arterial blood = 97% saturated
• - Venous blood = 75% saturated
• - Carbon monoxide = binds haemoglobin more strongly

Common Mistakes

• 1. Thinking oxygen is carried mainly as dissolved gas — Oxygen is carried mainly as oxyhaemoglobin.
• 2. Thinking carbon dioxide is carried mainly as carbaminohaemoglobin — Carbon dioxide is carried mainly as bicarbonate ions.
• 3. Confusing oxyhaemoglobin and carbaminohaemoglobin — Oxyhaemoglobin carries O₂; carbaminohaemoglobin carries CO₂.
• 4. Forgetting the role of carbonic anhydrase — It catalyzes the reaction.
• 5. Mixing up the chloride shift — HCO₃⁻ out; Cl⁻ in.
• 6. Thinking oxygen binds to the globin part — Oxygen binds to the haem part.
• 7. Thinking carbon dioxide binds to the haem part — Carbon dioxide binds to the globin part.
• 8. Forgetting the Bohr effect — Decreased pH → decreased O₂ binding.
• 9. Thinking carbon monoxide is harmless — Carbon monoxide binds haemoglobin more strongly than oxygen.
• 10. Mixing up the percentages — O₂: 97% oxyhaemoglobin; CO₂: 70% bicarbonate.

Quick Revision

The transport of gases refers to the movement of oxygen and carbon dioxide between the lungs and body tissues through the blood. Oxygen is transported mainly as oxyhaemoglobin (97%) in red blood cells, with a small amount (3%) dissolved in plasma. Carbon dioxide is transported mainly as bicarbonate ions (70%) in plasma, with smaller amounts as carbaminohaemoglobin (23%) and dissolved in plasma (7%). The enzyme carbonic anhydrase catalyzes the reaction between CO₂ and water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate ions. The chloride shift maintains electrical neutrality by exchanging bicarbonate ions out of red blood cells and chloride ions into them. In the lungs, the process reverses, and carbon dioxide is exhaled. Factors such as partial pressure, temperature, and pH affect the binding of oxygen to haemoglobin.

60-Second Revision

• Oxygen transport = 97% oxyhaemoglobin; 3% dissolved
• Carbon dioxide transport = 70% bicarbonate; 23% carbaminohaemoglobin; 7% dissolved
• Oxyhaemoglobin = Hb + 4O₂
• Carbaminohaemoglobin = CO₂ + globin
• Bicarbonate = CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
• Chloride shift = Hamburger's phenomenon
• Carbonic anhydrase = enzyme
• Partial pressure = determines direction of diffusion
• Arterial blood = 97% saturated
• Venous blood = 75% saturated
• Warning: Oxygen binds to the haem part; carbon dioxide binds to the globin part

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