Which of these relations between two partial pressure is incorrect?

Options

A. `pO_2` in atmospheric air `gt pO_2` in alveoli
B. `pO_2` in tissues =`pO_2` in deoxygenated blood
C. `pCO_2` in oxygenated blood < `pO_2` in deoxygenated blood
D. `pCO_2` in oxygenated blood `lt pCO_2` in tissues

Answer

To solve the question regarding which of the relations between two partial pressures is incorrect, we will analyze the provided options 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. For example, the partial pressure of oxygen (O2) and carbon dioxide (CO2) in the air or in the blood.

2. **Analyzing the Options**:
- **Option 1**: The partial pressure of oxygen in atmospheric air is 159 mmHg, and it is greater than the partial pressure of oxygen in alveoli, which is 104 mmHg.
- This statement is correct because 159 mmHg > 104 mmHg.

- **Option 2**: The partial pressure of oxygen in tissues is 40 mmHg, and it is equal to the partial pressure of oxygen in deoxygenated blood, which is also 40 mmHg.
- This statement is correct because both values are equal (40 mmHg = 40 mmHg).

- **Option 3**: The partial pressure of carbon dioxide in oxygenated blood is 40 mmHg, and it is less than the partial pressure of oxygen in deoxygenated blood, which is also given as 40 mmHg.
- This statement is incorrect because 40 mmHg is not less than 40 mmHg; they are equal.

- **Option 4**: The partial pressure of carbon dioxide in oxygenated blood is 40 mmHg, which is less than the partial pressure of carbon dioxide in tissues, which is 45 mmHg.
- This statement is correct because 40 mmHg < 45 mmHg.

3. **Conclusion**:
- The incorrect relation is found in **Option 3** where it states that the partial pressure of carbon dioxide in oxygenated blood is less than that in deoxygenated blood, which is not true as they are equal.

### Final Answer:
The incorrect relation between the two partial pressures is **Option 3**.

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