A decrease in the pH of human blood would :

Options

A. decrease breathing rate
B. increase heart rate
C. decrease the amount of `O_2` unloaded from haemoglobin
D. decrease cardiac output

Explanation

### Step-by-Step Solution:

1. **Understanding the Role of Hemoglobin**: Hemoglobin is a respiratory pigment in the blood that binds to oxygen in the lungs to form oxyhemoglobin. This complex then transports oxygen to body tissues where it is needed.

2. **Factors Influencing Oxyhemoglobin Formation**: The formation of oxyhemoglobin is influenced by several factors:
- **Partial Pressure of Oxygen (PO2)**: Higher concentrations of oxygen favor the binding of oxygen to hemoglobin.
- **Partial Pressure of Carbon Dioxide (PCO2)**: Lower concentrations of carbon dioxide favor the formation of oxyhemoglobin.
- **Temperature**: Lower temperatures favor the formation of oxyhemoglobin.
- **Concentration of Hydrogen Ions (H+)**: A lower concentration of H+ ions (higher pH) favors the formation of oxyhemoglobin.

3. **Impact of Decreased pH**: If the pH of blood decreases, it indicates an increase in the concentration of H+ ions. This condition is known as acidosis. An increase in H+ ions promotes the dissociation of oxygen from hemoglobin, meaning that hemoglobin will release oxygen more readily.

4. **Conclusion**: Therefore, a decrease in the pH of human blood will lead to a decrease in the amount of oxygen that is unloaded from hemoglobin to the tissues. This means that less oxygen will be available for the tissues, which can affect their function.

5. **Correct Answer**: The correct answer to the question is that a decrease in the pH of human blood will decrease the amount of oxygen unloaded from hemoglobin (Option C).