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 equation:

**Expiratory capacity - Inspiratory capacity + X = Functional residual capacity.**

### Step-by-Step Solution:

1. **Understand the Terms:**
- **Expiratory Capacity (EC):** The maximum amount of air that can be expelled from the lungs after a maximum inhalation.
- **Inspiratory Capacity (IC):** The maximum amount of air that can be inhaled after a normal expiration.
- **Functional Residual Capacity (FRC):** The volume of air remaining in the lungs after a normal expiration. It is the sum of the Expiratory Reserve Volume (ERV) and the Residual Volume (RV).

2. **Rearranging the Equation:**
- We start with the equation:
\[ EC - IC + X = FRC \]
- To isolate X, we can rearrange the equation:
\[ X = FRC - (EC - IC) \]
- This simplifies to:
\[ X = FRC + IC - EC \]

3. **Understanding Functional Residual Capacity:**
- FRC is defined as:
\[ FRC = ERV + RV \]
- Therefore, we can express X in terms of ERV and RV.

4. **Identifying X:**
- From the rearranged equation, we can conclude that X must be equal to the sum of the Expiratory Reserve Volume (ERV) and the Residual Volume (RV).
- Thus, we can deduce that:
\[ X = ERV + RV \]

5. **Conclusion:**
- Based on the options provided, the correct answer is:
- **Option C: Expiratory Reserve Volume + Residual Volume.**

### Final Answer:
X in the equation is **Expiratory Reserve Volume + Residual Volume.**

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