A partial pressure gradient of oxygen exists between :

Options

A. the atmosphere and the lungs
B. lungs and metabolically active tissues
C. air at sea level and air at high altitudes
D. all of the above

Answer

To solve the question regarding the existence of a partial pressure gradient of oxygen, we can analyze the scenarios provided in the question step by step.

### Step-by-Step Solution:

1. **Understanding Partial Pressure Gradient**: A partial pressure gradient refers to the difference in the pressure of a specific gas (in this case, oxygen) between two areas. Gases move from areas of higher partial pressure to areas of lower partial pressure.

2. **Atmospheric Air vs. Deoxygenated Blood**: In the lungs, the partial pressure of oxygen (PO2) in the atmospheric air is higher than in the deoxygenated blood. This creates a gradient that allows oxygen to diffuse from the alveoli into the blood.

3. **Lungs vs. Body Tissues**: Once oxygen is in the blood, it is transported to the body tissues. The partial pressure of oxygen in the tissues is lower than in the oxygenated blood. This difference allows oxygen to diffuse from the blood into the tissues.

4. **Air at Sea Level vs. High Altitudes**: At sea level, the atmospheric pressure is higher, resulting in a higher partial pressure of oxygen. As altitude increases, the total atmospheric pressure decreases, leading to a lower partial pressure of oxygen. This creates a gradient where oxygen moves from areas of higher pressure (sea level) to lower pressure (high altitudes).

5. **Conclusion**: All the scenarios mentioned (atmospheric air vs. deoxygenated blood, lungs vs. metabolically active tissues, and air at sea level vs. high altitudes) exhibit a partial pressure gradient of oxygen. Therefore, the correct answer is option D, "all of the above."

### Final Answer:
The partial pressure gradient of oxygen exists between:
- Atmospheric air and deoxygenated blood
- Lungs and metabolically active tissues
- Air at sea level and air at high altitudes

Thus, the correct answer is **option D: all of the above**.

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