Which of these statements about the partial pressure of `CO_(2)` is true ?

Options

A. it is higher in the alveoli than in pulmonary arteries
B. it is higher in the systemic arteries than in the tissues
C. it is higher in the systemic veins than in the systemic arteries
D. it is higher in the pulmonary veins than in pulmonary arteries

Answer

To determine which statement about the partial pressure of carbon dioxide (CO₂) is true, we will analyze each statement based on the physiological understanding of gas exchange in the human body.

### Step-by-Step Solution:

1. **Understanding Partial Pressure of CO₂ in Different Locations**:
- In the **alveoli**, the partial pressure of CO₂ is approximately **40 mmHg**.
- In the **pulmonary arteries**, the partial pressure of CO₂ is approximately **45 mmHg**.
- In the **systemic arteries**, the partial pressure of CO₂ is also around **40 mmHg**.
- In the **tissues**, the partial pressure of CO₂ is greater than **45 mmHg**.
- In the **systemic veins**, the partial pressure of CO₂ is approximately **45 mmHg**.
- In the **pulmonary veins**, the partial pressure of CO₂ is about **40 mmHg**.

2. **Analyzing Each Statement**:
- **Statement 1**: "It is higher in the alveoli than in pulmonary arteries."
- **False**: Alveoli (40 mmHg) < Pulmonary arteries (45 mmHg).

- **Statement 2**: "It is higher in the systemic arteries than in the tissues."
- **False**: Systemic arteries (40 mmHg) < Tissues (>45 mmHg).

- **Statement 3**: "It is higher in the systemic veins than in the systemic arteries."
- **True**: Systemic veins (45 mmHg) > Systemic arteries (40 mmHg).

- **Statement 4**: "It is higher in the pulmonary veins than in the pulmonary arteries."
- **False**: Pulmonary veins (40 mmHg) < Pulmonary arteries (45 mmHg).

3. **Conclusion**:
- The only true statement is **Statement 3**: "It is higher in the systemic veins than in the systemic arteries."

### Final Answer:
The correct statement about the partial pressure of CO₂ is **Statement 3**: "It is higher in the systemic veins than in the systemic arteries."

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