Expiratory capacity - Inspiratory capacity + X = Functional residual capacity . X in this equation is

Options

A. Residual volume
B. Inspiratory reserve volume + Residual volume
C. Expiratory reserve volume + Residual volume
D. Vital capacity

Answer

To solve the equation given in the question, we need to identify what \( X \) represents in the context of lung volumes. The equation provided is:

**Expiratory Capacity - Inspiratory Capacity + X = Functional Residual Capacity**

### Step 1: Understand the Terms
- **Expiratory Capacity (EC)**: This is the maximum amount of air that can be exhaled after a normal tidal expiration.
- **Inspiratory Capacity (IC)**: This is the maximum amount of air that can be inhaled after a normal tidal expiration.
- **Functional Residual Capacity (FRC)**: This is the volume of air remaining in the lungs after a normal tidal expiration.

### Step 2: Rearranging the Equation
We can rearrange the equation to isolate \( X \):

\[ X = Functional Residual Capacity - (Expiratory Capacity - Inspiratory Capacity) \]

### Step 3: Substitute Known Relationships
We know that:

\[ FRC = Residual Volume (RV) + Expiratory Reserve Volume (ERV) \]

Thus, we can express \( X \) in terms of these volumes.

### Step 4: Analyze the Options
1. **Residual Volume (RV)**: This is the amount of air left in the lungs after a full exhalation. This does not fit as \( X \).
2. **Inspiratory Reserve Volume (IRV) + Residual Volume (RV)**: This is also incorrect as it does not align with the equation.
3. **Expiratory Reserve Volume (ERV) + Residual Volume (RV)**: This fits since it represents the total volume of air in the lungs after expiration, which is equal to FRC.
4. **Vital Capacity (VC)**: This is the total amount of air that can be exhaled after maximum inhalation, which is not relevant to \( X \).

### Conclusion
From the analysis, we find that:

\[ X = Expiratory Reserve Volume + Residual Volume \]

Thus, the correct answer is the third option: **Expiratory Reserve Volume + Residual Volume**.