A man connected to a body plethysmograph for estimation of FRC inspires against a closed glottis. Which of the following statements is true –
**Core Concept**
The question tests the understanding of functional residual capacity (FRC), a critical concept in respiratory physiology. FRC is the volume of air present in the lungs at the end of passive expiration, representing the balance between inward recoil of the lungs and outward recoil of the chest wall. The body plethysmograph is a device used to measure FRC by detecting changes in the volume of the closed box surrounding the subject.
**Why the Correct Answer is Right**
When the subject inspires against a closed glottis, they are forced to inhale air from the plethysmograph, increasing the volume inside the box. Since the glottis is closed, air cannot enter the lungs. This results in a decrease in the pressure inside the box, causing the box to expand and increase its volume. By measuring the change in volume of the box, the FRC can be estimated. This is because the volume of the box and the volume of air in the lungs are inversely related.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because inspiring against a closed glottis does not directly measure the residual volume (RV), which is the volume of air remaining in the lungs after maximal exhalation.
**Option B:** This option is incorrect because the statement does not accurately describe the principle behind the body plethysmograph. The plethysmograph measures changes in volume, not pressure.
**Option D:** This option is likely incorrect because it does not accurately describe the relationship between the volume of the box and the volume of air in the lungs.
**Clinical Pearl / High-Yield Fact**
To remember the principle behind the body plethysmograph, recall that the device measures the change in volume of the box, which is inversely related to the volume of air in the lungs. This is a critical concept in respiratory physiology, and understanding it is essential for accurately measuring FRC.
**Correct Answer:** C.