I recently learned about the concept of "affinity" in regards to hemoglobin. For hemoglobin in humans, carbon dioxide has a lower affinity than oxygen, which allows gas exchange to occur in our lungs. Carbon monoxide is problematic for us because it has an even higher affinity to hemoglobin than oxygen does.

Haemoglobin and its Affinity for Different Molecules

Haemoglobin is a protein found in red blood cells that binds with oxygen and transports it to different parts of the body. However, haemoglobin also has affinity for other molecules, such as carbon dioxide and carbon monoxide.

Affinity of Haemoglobin for Different Molecules

Haemoglobin's affinity for different molecules can be measured by its binding affinity constant (Kd). The lower the Kd value, the greater the affinity of haemoglobin for that molecule.

- Oxygen: haemoglobin has a high affinity for oxygen, with a Kd value of around 0.3-0.4 mmHg. This means that haemoglobin can easily bind with oxygen molecules in the lungs, even at low oxygen concentrations, and release them in tissues where oxygen is needed.

- Carbon dioxide: haemoglobin has a lower affinity for carbon dioxide than for oxygen, with a Kd value of around 23 mmHg. This means that haemoglobin can transport small amounts of carbon dioxide, but most of it is transported in the form of bicarbonate ions dissolved in plasma.

- Carbon monoxide: haemoglobin has a very high affinity for carbon monoxide, with a Kd value of around 0.01 mmHg. This means that haemoglobin can easily bind with carbon monoxide molecules, even at low concentrations, and form carboxyhaemoglobin. This is a dangerous situation as carboxyhaemoglobin can't transport oxygen, leading to hypoxia.

Conclusion

In conclusion, haemoglobin's affinity for different molecules varies depending on the Kd value. Haemoglobin has the least affinity for carbon dioxide, followed by oxygen and carbon monoxide. The high affinity of haemoglobin for carbon monoxide is the reason why carbon monoxide poisoning can be lethal.

This is the reason CO2 is transported in dissolve form as bicarbonate. Haemoglobin has large affinity for CO thats why when CO is inhale in large quantity the person die due to lack of O2 as all haemoglobin are bind to CO.