I think solubility of CO2 is 20 to 25 times higher than that of O2 that is why it has more binding affinity with Hb than O2

Introduction:
Carbon dioxide (CO2) and oxygen (O2) are two important gases in the human body. They are transported by the blood to different parts of the body. Hemoglobin (Hb) is the protein that carries these gases in the blood.

Binding affinity:
Binding affinity is the strength of the interaction between a molecule and its receptor. Hb has a higher binding affinity for CO2 than O2. This means that Hb will bind more tightly to CO2 than O2.

Factors affecting binding affinity:
There are several factors that affect the binding affinity of Hb for CO2 and O2:

1. Partial pressure: The partial pressure of a gas is the pressure it would exert if it were the only gas present. The partial pressure of O2 is higher in the lungs than in the tissues, so O2 binds to Hb in the lungs. The partial pressure of CO2 is higher in the tissues than in the lungs, so CO2 binds to Hb in the tissues.
2. pH: The pH of the blood affects the binding affinity of Hb. When the pH decreases (becomes more acidic), Hb binds more tightly to CO2 and less tightly to O2. When the pH increases (becomes more basic), Hb binds more tightly to O2 and less tightly to CO2.
3. Temperature: The temperature of the blood affects the binding affinity of Hb. When the temperature increases, Hb binds more tightly to CO2 and less tightly to O2. When the temperature decreases, Hb binds more tightly to O2 and less tightly to CO2.

Reason for CO2 having higher binding affinity:
The reason for CO2 having a higher binding affinity for Hb than O2 is due to the fact that CO2 can form a chemical bond with Hb. CO2 reacts with water in the blood to form carbonic acid (H2CO3), which then dissociates into H+ and HCO3-. H+ binds to Hb and forms a salt bridge, which stabilizes the T state of Hb and promotes the binding of CO2. This is known as the Haldane effect. In addition, CO2 can bind to the amino acids on the surface of Hb, which further increases its binding affinity.

Conclusion:
In summary, Hb has a higher binding affinity for CO2 than O2 due to several factors, including partial pressure, pH, and temperature. CO2 can form a chemical bond with Hb and bind to amino acids on its surface, which further increases its binding affinity. This is important for the transport of CO2 in the blood and for the regulation of pH in the body.