Which of the partial pressure are equal ?

Options

A. `pO_2` in atmospheric air and `pO_2` in alveoli
B. `pO_2` in alveoli and `pO_2` in oxygenated blood
C. `pCO_2` in oxygenated blood and `pO_2` in deoxygenated blood
D. `pCO_2` in oxygenated blood and `pCO_2` in tissues

Answer

To solve the question of which partial pressures are equal, we need to analyze the partial pressures of oxygen (PO2) and carbon dioxide (PCO2) in different locations within the respiratory system and blood. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand the Concept of Partial Pressure
Partial pressure refers to the pressure exerted by a particular gas in a mixture of gases. In the context of the respiratory system, we are primarily concerned with the partial pressures of oxygen (PO2) and carbon dioxide (PCO2).

### Step 2: Identify the Key Locations
We need to compare the partial pressures of gases in the following locations:
- Atmospheric air
- Alveoli (air sacs in the lungs)
- Oxygenated blood
- Deoxygenated blood
- Tissues

### Step 3: Analyze the Partial Pressures of Oxygen
1. **Atmospheric Air**: The partial pressure of oxygen (PO2) in atmospheric air is approximately 159 mmHg.
2. **Alveoli**: The PO2 in the alveoli is about 104 mmHg. This difference is necessary for oxygen to diffuse from the alveoli into the blood.

### Step 4: Analyze the Partial Pressures of Carbon Dioxide
1. **Oxygenated Blood**: The partial pressure of carbon dioxide (PCO2) in oxygenated blood is approximately 40 mmHg.
2. **Deoxygenated Blood**: The PCO2 in deoxygenated blood is also around 40 mmHg, as it is the same as in oxygenated blood when considering the exchange process.

### Step 5: Compare the Values
- **PO2 in Alveoli vs. Atmospheric Air**: Not equal (104 mmHg vs. 159 mmHg).
- **PO2 in Alveoli vs. Oxygenated Blood**: Not equal (104 mmHg vs. 95 mmHg).
- **PCO2 in Oxygenated Blood vs. Deoxygenated Blood**: Equal (both are 40 mmHg).

### Conclusion
The only pair of partial pressures that are equal are the partial pressures of carbon dioxide in oxygenated blood and deoxygenated blood, both at 40 mmHg. Therefore, the correct answer is:

**Option C: The partial pressures of carbon dioxide in oxygenated and deoxygenated blood are equal.**

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