**Question:** Residual volume of lung is defined as: March 2013

**Core Concept:** Residual volume (RV) is a measure of the amount of air remaining in the lungs after a maximal exhalation effort. It is a part of total lung capacity (TLC), which includes vital capacity (VC), residual volume, and total lung capacity.

**Why the Correct Answer is Right:**

Residual volume is a crucial concept in understanding lung function and physiology. It is defined as the volume of air remaining in the lungs after a maximal exhalation effort, which is performed by a person when they exhale as hard as they can. RV is a component of total lung capacity (TLC), which is the highest amount of air that can be held in the lungs. TLC includes RV, VC (vital capacity), and FRC (functional residual capacity).

**Why Each Wrong Option is Incorrect:**

1. **Option A:** This option is incorrect because residual volume is not the same as functional residual capacity (FRC). FRC is the remaining air in the lungs after a person has exhaled as much as possible and then inhaled to the largest possible amount. RV is a subset of FRC.

2. **Option B:** This option is incorrect because RV refers specifically to the amount of air remaining after maximal exhalation, while inspiratory capacity (IC) is the amount of air that can be taken into the lungs during a deep, slow inspiration. IC is a distinct measure from RV and TLC.

3. **Option C:** This option is incorrect because RV is a component of total lung capacity (TLC), but it is not the same as total lung capacity. TLC is the total amount of air that can be held in the lungs, which includes RV, VC, and FRC.

**Clinical Pearl:**

Understanding the relationship between residual volume, functional residual capacity, vital capacity, and total lung capacity is crucial for clinical applications, particularly in assessing respiratory function and diagnosing pulmonary disorders. A decrease in RV can indicate restrictive lung disease, while an increase in RV can indicate obstructive lung disease. A thorough evaluation of lung function tests is essential in diagnosing various respiratory conditions and monitoring treatment progress.