Read the following statements about respiratory system of humans :
I. Type II alveoli help in secretion of a chemical surfactant called lecithin , which decreases the surface tension around alveoli.
II. The diffusion membrane for exchange of gases is made up of three major cellular layers.
III. Binding of oxygen with hemoglobin is primarily related to the partial pressure of `O_2`.
IV. Every 100 ml of oxygenated blood can deliver around 5 ml `CO_2` to the tissues under normal physiological conditions.
How many statement is /are correct from the above ?

Options

A. Two
B. Three
C. Four
D. One

Answer

To determine how many statements about the respiratory system of humans are correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement I**: "Type II alveoli help in secretion of a chemical surfactant called lecithin, which decreases the surface tension around alveoli."
- **Analysis**: This statement is correct. Type II alveolar cells secrete surfactant, which includes phospholipids like lecithin. This surfactant reduces surface tension in the alveoli, preventing collapse during exhalation.

**Hint**: Remember that surfactant plays a crucial role in maintaining alveolar stability.

2. **Statement II**: "The diffusion membrane for exchange of gases is made up of three major cellular layers."
- **Analysis**: This statement is incorrect. The diffusion membrane consists of three layers: the squamous epithelium of the alveoli (cellular), the endothelium of capillaries (cellular), and the basement membrane (acellular). Since one of the layers is acellular, the statement is not entirely accurate.

**Hint**: Consider the nature of the layers involved in gas exchange.

3. **Statement III**: "Binding of oxygen with hemoglobin is primarily related to the partial pressure of O2."
- **Analysis**: This statement is correct. The binding of oxygen to hemoglobin is indeed primarily influenced by the partial pressure of oxygen. Higher partial pressures of O2 lead to increased binding, forming oxyhemoglobin.

**Hint**: Think about how gases behave under different pressures.

4. **Statement IV**: "Every 100 ml of oxygenated blood can deliver around 5 ml CO2 to the tissues under normal physiological conditions."
- **Analysis**: This statement is incorrect. Every 100 ml of oxygenated blood actually contains about 5 ml of oxygen, not CO2. The amount of CO2 transported in blood is different and varies based on several factors.

**Hint**: Recall the typical gas content in oxygenated blood.

### Conclusion:
After analyzing all four statements:
- Statement I: Correct
- Statement II: Incorrect
- Statement III: Correct
- Statement IV: Incorrect

Thus, **2 statements are correct** (I and III).

### Final Answer:
The number of correct statements is **2**.