The urge to inhale in humans results from

Options

A. rising `PCO_(2)`
B. rising `PO_(2)`
C. falling `PCO_(2)`
D. falling `PO_(2)`

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the physiological mechanism that triggers the urge to inhale in humans.

2. **Role of Carbon Dioxide**: The primary factor that stimulates the urge to breathe is the concentration of carbon dioxide (CO2) in the blood. When CO2 levels rise, it leads to an increase in the partial pressure of carbon dioxide.

3. **Respiratory Center Activation**: The increased partial pressure of carbon dioxide acts as a signal to the respiratory center located in the brain. This center is responsible for regulating the breathing process.

4. **Signal Transmission**: Once the respiratory center is activated by high CO2 levels, it sends signals to the respiratory muscles (such as the diaphragm and intercostal muscles) to initiate inhalation.

5. **Chemoreceptors Involvement**: Additionally, the elevated CO2 levels activate chemoreceptors located in the carotid bodies and aortic bodies. These chemoreceptors detect changes in blood pH, which decreases as CO2 levels rise.

6. **Resulting Urge to Breathe**: The activation of these chemoreceptors leads to the sensation of the urge to breathe, prompting an inhalation response.

7. **Conclusion**: Therefore, the correct answer to the question is that the urge to inhale in humans results from the rising partial pressure of carbon dioxide.

### Final Answer:
The urge to inhale in humans results from the rising partial pressure of carbon dioxide.

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