Percentage of Normal dead space is:-
**Core Concept**
Normal dead space refers to the volume of air that enters the lungs but does not participate in gas exchange. It includes the anatomical dead space, where air fills the conducting airways but does not reach the alveoli, and the physiological dead space, where air enters the alveoli but does not participate in gas exchange due to poor blood flow or ventilation-perfusion mismatch.
**Why the Correct Answer is Right**
The percentage of normal dead space is approximately 150-200 mL, which represents about 20-25% of the total tidal volume. This is because the anatomical dead space is approximately 150 mL, and the physiological dead space adds an additional 50-100 mL due to ventilation-perfusion mismatch. The remaining 75-80% of the tidal volume is effective dead space, where gas exchange occurs.
**Why Each Wrong Option is Incorrect**
**Option A:** 10% is too low, as the anatomical dead space alone accounts for approximately 150 mL, which is about 20% of the tidal volume.
**Option B:** 50% is too high, as the physiological dead space adds only an additional 50-100 mL to the anatomical dead space.
**Option C:** 30% is incorrect, as it does not accurately reflect the balance between anatomical and physiological dead space.
**Clinical Pearl / High-Yield Fact**
The concept of dead space is crucial in understanding respiratory physiology and pathophysiology. A ventilation-perfusion mismatch can lead to increased physiological dead space, resulting in hypoxemia and hypercapnia.
**Correct Answer:** C. 20-25%.