Introduction

Air inhaled and exhaled during breathing differ in their gas composition due to gas exchange in the lungs. The principal gases involved are nitrogen (N₂), oxygen (O₂), carbon dioxide (CO₂), and water vapor (H₂O). Knowing their concentrations and order in exhaled air highlights respiratory physiology fundamentals.

Composition of Inhaled Air

Typical dry atmospheric air contains approximately:

• Nitrogen: 78%

• Oxygen: 21%

• Carbon Dioxide: 0.04%

• Water Vapor: variable; generally low in dry air

Composition of Exhaled Air

• Nitrogen: Remains largely unchanged (~78%) because the body does not metabolize nitrogen.

• Oxygen: Reduced from about 21% to roughly 16% as it is absorbed by blood for cellular respiration.

• Carbon Dioxide: Increased from about 0.04% to approximately 4-5% as it is produced by cellular metabolism and expelled via the lungs.

• Water Vapor (H₂O): Increases significantly as the air is humidified by respiratory passages.

Order of Gases in Exhaled Air by Percentage

The gases are typically ordered as:

Nitrogen > Carbon dioxide > Oxygen > Water vapor

• Nitrogen remains the most abundant due to its inert nature.

• Carbon dioxide concentration rises substantially, second in amount.

• Oxygen is next, reduced due to metabolic consumption.

• Water vapor is variable but generally present, often lower than oxygen but higher than carbon dioxide in humidified exhaled air depending on conditions.

Why Other Options Are Incorrect

• (1) N₂ > CO₂ > O₂ > H₂ (Correct order)

• (2) N₂ > O₂ > CO₂ > H₂ (Incorrect, CO₂ should exceed O₂ in exhaled air)

• (3) N₂ > CO₂ > H₂ > O₂ (Incorrect water vapor typically not greater than oxygen)

• (4) N₂ > H₂ > CO₂ > CO₂ (Incorrect, CO₂ repeated and water vapor misplaced)

Summary Table

Conclusion

The correct order of gases in exhaled air is:

(1) N₂ > CO₂ > O₂ > H₂ (water vapor)