Which of the following statements is not true?

Options

A. The partial pressure of `O_(2)` in deoxygenated blood is 40 mm Hg.
B. The partial pressure of `O_(2)` in oxygenated blood is 95 mm Hg
C. The partial pressure of `O_(2)` in alveolar air is 104 mm Hg
D. The partial pressure of `CO_(2)` in alveolar air is 45 mm Hg

Answer

To determine which statement is not true regarding the partial pressures of gases in the blood and alveolar air, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Partial Pressure**:
- Partial pressure refers to the pressure exerted by a single type of gas in a mixture of gases. In the context of respiration, we often refer to the partial pressures of oxygen (O2) and carbon dioxide (CO2) in different locations in the body.

2. **Evaluating Option 1**:
- **Statement**: "Partial pressure of oxygen in deoxygenated blood is 40 mmHg."
- **Analysis**: This statement is true. The partial pressure of oxygen in deoxygenated blood is indeed around 40 mmHg.

3. **Evaluating Option 2**:
- **Statement**: "Partial pressure of oxygen in oxygenated blood is 95 mmHg."
- **Analysis**: This statement is also true. The partial pressure of oxygen in oxygenated blood is approximately 95 mmHg.

4. **Evaluating Option 3**:
- **Statement**: "Partial pressure of oxygen in alveolar air is 104 mmHg."
- **Analysis**: This statement is true. The partial pressure of oxygen in alveolar air is about 104 mmHg.

5. **Evaluating Option 4**:
- **Statement**: "Partial pressure of carbon dioxide in the alveolar air is 45 mmHg."
- **Analysis**: This statement is false. The partial pressure of carbon dioxide in the alveolar air is typically around 40 mmHg, not 45 mmHg.

### Conclusion:
- The statement that is not true is **Option 4**: "Partial pressure of carbon dioxide in the alveolar air is 45 mmHg."