Introduction:

The air we breathe in (inspired air ) and the air we breathe out (expired air ) differ significantly in composition. These differences reflect the body’s use of oxygen (O₂) for cellular respiration and the removal of waste gases like carbon dioxide (CO₂). This chapter focuses on the changes in O₂, CO₂, and water vapor between inspired and expired air.

Differences in composition between inspired and expired air:

The composition of inspired (inhaled) and expired (exhaled) air differs due to the physiological processes that occur during respiration.

Oxygen (O2):

Inspired Air: Contains 21% O₂ (similar to atmospheric air).
Oxygen is used by cells during cellular respiration to produce ATP (energy).

Expired Air: Drops to 16–17% O₂ because:
~5% of inhaled O₂ is absorbed into the bloodstream via the alveoli.
The remaining O₂ is exhaled unused.

Carbon Dioxide (CO2):

Inspired Air: Contains 0.04% CO₂ (trace amounts in atmospheric air).

Expired Air: Rises to 4–5% CO₂ because:
CO₂ is a waste product of cellular respiration.
It diffuses from the bloodstream into the alveoli to be exhaled.

Water Vapor (H2O):

Inspired Air: Dry or low humidity (varies with environment).

Expired Air: Fully saturated with water vapor (~6.3% at body temperature).
Why? Air is humidified as it passes through the moist respiratory tract (trachea, bronchi).

Why These Differences Matter

Oxygen Depletion:

The drop in O₂ reflects its uptake by the body for energy production.

CO₂ Increase:

The rise in CO₂ shows the body efficiently removing metabolic waste.
High CO₂ levels trigger faster breathing (via receptors in the brain).

Water Vapor:

Humidification prevents dehydration of respiratory surfaces.
Expired air feels “warm and moist” compared to inspired air.