After taking a long breath, we can hold the breath as long as

Options

A. `CO_2` in the blood is more
B. `H^+` in the blood is more
C. `CO_2` in the blood is less
D. `O_2` in the blood less

Answer

To solve the question "After taking a long breath, we can hold the breath as long as," we need to analyze the options provided and understand the physiological mechanisms involved in breath-holding.

### Step-by-Step Solution:

1. **Understanding Breath-Holding**: When we take a deep breath, we fill our lungs with air, which contains oxygen (O2) and carbon dioxide (CO2). The ability to hold our breath is influenced by the levels of CO2 and O2 in our blood.

2. **Role of CO2**: The body monitors CO2 levels in the blood. An increase in CO2 concentration (hypercapnia) stimulates the urge to breathe. Conversely, if CO2 levels are low, the urge to breathe diminishes, allowing us to hold our breath longer.

3. **Analyzing the Options**:
- **Option 1: CO2 in the blood is more** - This would trigger the urge to breathe, making it difficult to hold the breath.
- **Option 2: H+ in the blood is more** - An increase in H+ ions (which occurs with increased CO2) would also stimulate breathing.
- **Option 3: CO2 in the blood is less** - Lower CO2 levels would reduce the urge to breathe, allowing for longer breath-holding.
- **Option 4: O2 in the blood is less** - While low O2 can eventually lead to the urge to breathe, the immediate trigger is more closely related to CO2 levels.

4. **Conclusion**: After taking a long breath, we can hold the breath as long as CO2 in the blood is less. This is because lower CO2 levels reduce the respiratory drive, allowing for longer breath-holding.

### Final Answer:
After taking a long breath, we can hold the breath as long as **CO2 in the blood is less** (Option 3).

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