Describe the role of haemoglobin in the transport of carbon dioxide in human body. (IFS 2022, 8 Marks)

Introduction

Haemoglobin is a crucial protein found in red blood cells that plays a vital role in the transport of gases in the human body. One of its key functions is the transportation of oxygen from the lungs to the tissues, but it also plays a significant role in the transport of carbon dioxide.

Role of Hemoglobin in Carbon Dioxide Transport:

• Binding of Carbon Dioxide to Hemoglobin (Carbaminohemoglobin Formation):
  
  • Carbon dioxide (CO₂) produced by tissues enters the bloodstream.
  • A portion of CO₂ binds directly to hemoglobin (Hb) molecules at the amino groups of the protein, forming carbaminohemoglobin (HbCO₂).
  • This binding is facilitated by the increased concentration of CO₂ in tissues, where the partial pressure of CO₂ is higher.
  • The carbaminohemoglobin formation accounts for approximately 20-25% of total CO₂ transport in the blood.
• Bicarbonate Buffering System (CO₂ → HCO₃⁻):
  
  • The majority of CO₂ is transported in the form of bicarbonate (HCO₃⁻), but the role of hemoglobin is also crucial here.
  • CO₂ diffuses into red blood cells and reacts with water to form carbonic acid (H₂CO₃), catalyzed by the enzyme carbonic anhydrase.
  • Carbonic acid dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻).
  • Bicarbonate ions diffuse into the plasma, while chloride ions (Cl⁻) enter the red blood cells to maintain electrochemical balance (this is known as the chloride shift).
  • Hemoglobin helps by buffering the H⁺ ions, preventing a decrease in blood pH, and thus stabilizing the blood's acid-base balance.
• Oxygen Release and CO₂ Uptake (The Bohr Effect):
  
  • Hemoglobin's ability to bind to CO₂ is influenced by the oxygenation state of hemoglobin.
  • As hemoglobin releases oxygen in the tissues (where the partial pressure of O₂ is low), its affinity for CO₂ increases, facilitating the uptake of CO₂ and its transport back to the lungs.
  • The Bohr effect describes this phenomenon, where the binding of CO₂ (and hydrogen ions) to hemoglobin promotes the release of oxygen and enhances the capacity for CO₂ uptake.
• Transportation of CO₂ to the Lungs:
  
  • The CO₂ carried by hemoglobin (in the form of carbaminohemoglobin) and bicarbonate ions is transported to the lungs.
  • In the lungs, where the partial pressure of CO₂ is lower, the reverse processes occur.
  • Hemoglobin releases CO₂, which is then exhaled. The bicarbonate ions also reverse the process to form CO₂, which diffuses into the alveoli for exhalation.

Conclusion

Haemoglobin plays a crucial role in the transport of carbon dioxide in the human body by facilitating the conversion of carbon dioxide into bicarbonate ions and buffering the hydrogen ions produced during this process. This process ensures efficient gas exchange and helps maintain the acid-base balance in the blood.