% of O2 inhaled-
**Core Concept**
The question pertains to the percentage of oxygen in the air we inhale, which is a fundamental concept in respiratory physiology. Atmospheric air is composed of various gases, with oxygen being a critical component for human respiration. The percentage of oxygen in inhaled air is constant under normal conditions.
**Why the Correct Answer is Right**
The correct answer is based on the composition of dry atmospheric air, which contains approximately 21% oxygen. This percentage is a standard value under normal atmospheric pressure and is crucial for understanding gas exchange in the lungs. The oxygen percentage in inhaled air remains relatively constant, regardless of geographical location, as long as the air is at sea level and not significantly polluted.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the known percentage of oxygen in atmospheric air.
**Option B:** Similarly, this option does not match the established percentage of oxygen in inhaled air.
**Option D:** This option is also incorrect as it overestimates the percentage of oxygen present in the air we breathe.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the percentage of oxygen in inhaled air is approximately 21%, which is vital for calculating various respiratory parameters and understanding oxygen therapy. This knowledge is fundamental in clinical settings, especially in managing patients with respiratory diseases.
**Correct Answer:** D. 21%