Which of the following option is correct ?

Options

A. Respiratory centres are not affected by `CO_(2)`
B. In humans, vital capacity is just double the expiratory volume
C. A human lung has `10^(3)` alveoli
D. During inspiration, the lungs act as suction pump

Answer

To determine which of the given options is correct regarding breathing and gas exchange, let's analyze each option step by step.

### Step 1: Analyze Option 1
**Statement:** Respiratory centers are not affected by CO2.
- **Explanation:** This statement is incorrect. The respiratory centers in the brain, particularly in the medulla, are highly sensitive to CO2 levels in the blood. When CO2 levels increase, it leads to a decrease in blood pH (making it more acidic), which activates the respiratory centers to increase the rate and depth of breathing.
- **Conclusion:** Option 1 is **incorrect**.

### Step 2: Analyze Option 2
**Statement:** In humans, vital capacity is just double the expiratory volume.
- **Explanation:** Vital capacity (VC) is the total amount of air that can be exhaled after a maximum inhalation. It is calculated as the sum of tidal volume (TV), inspiratory reserve volume (IRV), and expiratory reserve volume (ERV). Typical values are approximately:
- TV: 500 mL
- IRV: 3000 mL
- ERV: 1100 mL
- Therefore, VC = TV + IRV + ERV = 500 mL + 3000 mL + 1100 mL = 4600 mL.
- Since the ERV is about 1100 mL, the vital capacity is more than double the expiratory volume.
- **Conclusion:** Option 2 is **incorrect**.

### Step 3: Analyze Option 3
**Statement:** A human lung has 10 to the power 3 alveoli.
- **Explanation:** This statement is also incorrect. The human lungs contain approximately 300 million alveoli, which is significantly more than 10^3 (1,000). This large number of alveoli increases the surface area for gas exchange.
- **Conclusion:** Option 3 is **incorrect**.

### Step 4: Analyze Option 4
**Statement:** During inspiration, the lungs act as a suction pump.
- **Explanation:** This statement is correct. During inspiration, the diaphragm and intercostal muscles contract, increasing the volume of the thoracic cavity. This decrease in pressure within the lungs compared to atmospheric pressure allows air to rush into the lungs, effectively making them act like a suction pump.
- **Conclusion:** Option 4 is **correct**.

### Final Answer
The correct option is **Option 4: During inspiration, the lungs act as a suction pump.**

---