The air you breathe is _______ oxygen, and the air you exhale is _______ oxygen.

• A. 6%, 25%
• B. 21%, 16%
• C. 25%, 32%
• D. 21%, 35%

Answer: (B)

The correct answer highlights the composition of the air we breathe compared to the air we exhale. Normal atmospheric air contains approximately 21% oxygen. This is the concentration of oxygen that is readily available to us in the environment. When we inhale, our bodies take in this oxygen and utilize it for metabolic processes.

After going through the respiratory system and being used by the body, the composition of the air we exhale reflects a decrease in oxygen content due to its consumption by the cells. Typically, the air we exhale contains about 16% oxygen. This reduction is a result of the gas exchange that occurs in the alveoli of the lungs, where oxygen is absorbed into the blood, and carbon dioxide is released from the blood into the exhaled air.

This balance of oxygen in inhaled and exhaled air is crucial for understanding respiratory physiology, and it demonstrates how efficient the body is at utilizing oxygen. The other choices do not accurately represent the standard percentages of oxygen present in inhaled and exhaled air.