**Buffering Action of Haemoglobin**

**Core Concept**
Haemoglobin, the primary protein in red blood cells, plays a crucial role in oxygen transport and buffering. Its buffering action is essential for maintaining acid-base balance in the body.

**Why the Correct Answer is Right**
The buffering action of haemoglobin is mainly due to its ability to bind hydrogen ions (H+) and bicarbonate ions (HCO3-). This binding is facilitated by the presence of histidine residues in the haemoglobin molecule. When the pH of the blood decreases (becoming more acidic), the histidine residues can accept hydrogen ions, thereby reducing the concentration of free hydrogen ions in the blood and helping to stabilize the pH. This buffering action is particularly important in the lungs, where CO2 is released and the pH can drop significantly.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while haemoglobin does play a role in buffering, it is not the primary mechanism by which it achieves this function.
**Option B:** This option is incorrect because the buffering action of haemoglobin is not primarily due to its ability to bind phosphate ions.
**Option C:** This option is incorrect because the buffering action of haemoglobin is not primarily due to its ability to bind calcium ions.

**Clinical Pearl / High-Yield Fact**
Haemoglobin's buffering action is particularly important in the lungs, where the pH can drop significantly due to the release of CO2. This buffering action helps to maintain acid-base balance and prevent respiratory alkalosis.

**Correct Answer:** D. Its ability to bind hydrogen ions (H+) and bicarbonate ions (HCO3-).