Which statement 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 statement is correct regarding the respiratory system, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Evaluate the First Statement:**
- The first statement claims that "respiratory centers are not affected by carbon dioxide."
- This statement is incorrect because the respiratory centers in the brain (medulla and pons) are sensitive to the levels of carbon dioxide (PCO2) and hydrogen ions (H+). An increase in these levels signals the respiratory centers to modify the breathing rate to ensure adequate oxygen supply and carbon dioxide removal.

**Hint:** Remember that the respiratory centers respond primarily to changes in carbon dioxide levels.

2. **Evaluate the Second Statement:**
- The second statement states that "human beings have vital capacity which is double of expiratory volume."
- This statement is also incorrect. Vital capacity is the total amount of air that can be exhaled after a maximum inhalation, which includes tidal volume, inspiratory reserve volume, and expiratory reserve volume. The expiratory reserve volume is only a part of the vital capacity, not double it.

**Hint:** Recall the definitions of vital capacity and expiratory reserve volume to clarify their relationship.

3. **Evaluate the Third Statement:**
- The third statement mentions that "normally, in a human lung, there are 1000 alveoli."
- This statement is incorrect. In fact, there are approximately 300 million (25 to 30 crore) alveoli in a human lung, which are crucial for gas exchange.

**Hint:** Think about the role of alveoli in the lungs and their actual numbers.

4. **Evaluate the Fourth Statement:**
- The fourth statement indicates that "lungs work like a suction pump during inspiration."
- This statement is correct. During inspiration, the volume of the thoracic cavity increases, creating a negative pressure that allows air to flow into the lungs, similar to how a suction pump operates.

**Hint:** Consider how changes in pressure and volume during breathing facilitate air movement.

### Conclusion:
After evaluating all the statements, the correct statement is **option D**, which states that "lungs work like a suction pump during inspiration."