`O_(2)` dissociation curve is:

Answer

### Step-by-Step Solution:

1. **Understanding the Oxygen Dissociation Curve**:
- The oxygen dissociation curve illustrates the relationship between the partial pressure of oxygen (pO2) and the saturation of hemoglobin with oxygen (oxyhemoglobin saturation).

2. **Shape of the Curve**:
- The curve is described as **sigmoidal**. This means that it has an S-shaped curve, which indicates that hemoglobin's affinity for oxygen increases as more oxygen molecules bind to it.

3. **Axes of the Curve**:
- On the x-axis, we have the **partial pressure of oxygen** measured in mmHg.
- On the y-axis, we have the **percentage saturation of hemoglobin** with oxygen.

4. **Factors Affecting the Curve**:
- The oxygen dissociation curve can be influenced by several factors, including:
- **pH**: A decrease in pH (more acidic) shifts the curve to the right, indicating decreased affinity for oxygen.
- **Temperature**: An increase in temperature also shifts the curve to the right.
- **2,3-Diphosphoglycerate (DPG)**: Higher levels of DPG shift the curve to the right, while lower levels shift it to the left.

5. **Shifts in the Curve**:
- A rightward shift indicates that hemoglobin releases oxygen more readily (lower affinity).
- A leftward shift indicates that hemoglobin holds onto oxygen more tightly (higher affinity).

### Final Answer:
The oxygen dissociation curve is **sigmoidal** in shape.

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