3.3 Composition of Inspired and Expired Air

Introduction

Breathing involves taking air into the lungs (inspired air) and pushing air out of the lungs (expired air). The air we breathe in and the air we breathe out are not the same. The body uses some of the oxygen from the inspired air and adds carbon dioxide to the expired air. This topic compares the composition of inspired and expired air.

Definition

Inspired air is the air that is breathed in (inhaled) from the environment.

Expired air is the air that is breathed out (exhaled) from the lungs.

Main Idea

We will see the composition of inspired and expired air in the given table.

Core Concept

Composition of Inspired and Expired Air

Detailed Explanation

1. Oxygen

What happens:

• Inspired air contains about 21% oxygen.
• Expired air contains about 16% oxygen.
• The difference (5%) is used by the body for cellular respiration.

Simple explanation: When you breathe in, the air has 21% oxygen. Your body uses some of this oxygen, so when you breathe out, the air has only 16% oxygen.

Why this is important:

• Oxygen is needed for cellular respiration to produce ATP (energy).
• The body extracts oxygen from the air in the lungs.

2. Carbon Dioxide

What happens:

• Inspired air contains about 0.04% carbon dioxide.
• Expired air contains about 4% carbon dioxide.
• The increase (from 0.04% to 4%) is due to carbon dioxide produced by cells during cellular respiration.

Simple explanation: When you breathe in, the air has very little carbon dioxide (0.04%). Your body produces carbon dioxide as a waste product, so when you breathe out, the air has much more carbon dioxide (4%).

Why this is important:

• Carbon dioxide is a waste product of cellular respiration.
• It must be removed from the body to maintain proper pH.

3. Nitrogen

What happens:

• Nitrogen is not used or produced by the body.
• Its percentage remains the same in inspired and expired air.

Simple explanation: Nitrogen goes in and out without any change. The body does not use it.

Why this is important:

• Nitrogen is an inert gas.
• It does not participate in gas exchange.

4. Water Vapour

What happens:

• Inspired air contains a variable amount of water vapour (depending on humidity).
• Expired air is saturated with water vapour.
• This is because the air is moistened as it passes through the respiratory passages.

Simple explanation: When you breathe out, the air is full of water vapour. This is why you can see your breath on a cold day.

Why this is important:

• The respiratory system adds moisture to the air.
• This helps keep the alveoli moist for gas exchange.

5. Temperature

What happens:

• Inspired air has a variable temperature (depending on the environment).
• Expired air is warmed to about body temperature (37°C).
• This is because the air is warmed as it passes through the respiratory passages.

Simple explanation: When you breathe in cold air, it gets warmed up in your nose and throat. When you breathe out, the air is warm.

Why this is important:

• Warming the air protects the lungs from cold damage.
• It also helps maintain body temperature.

6. Dust Particles

What happens:

• Inspired air may contain dust particles (depending on the environment).
• Expired air contains little, if any, dust particles.
• This is because the dust particles are trapped by the hairs and mucus in the nasal cavity and respiratory passages.

Simple explanation: When you breathe in dusty air, the dust gets trapped in your nose and airways. When you breathe out, the air is mostly clean.

Why this is important:

• Filtering the air protects the lungs from damage.
• It reduces the risk of respiratory infections.

Intermediate Level

The composition of inspired and expired air differs because of gas exchange in the lungs:

Key points:

• Oxygen is used by the body for cellular respiration.
• Carbon dioxide is produced by the body as a waste product.
• Nitrogen is not used or produced.
• Air is warmed, moistened, and filtered as it passes through the respiratory passages.

Important Points

• - Inspired air contains 21% oxygen.
• - Expired air contains 16% oxygen.
• - Inspired air contains 0.04% carbon dioxide.
• - Expired air contains 4% carbon dioxide.
• - Nitrogen is 79% in both inspired and expired air.
• - Water vapour is variable in inspired air and saturated in expired air.
• - Temperature is variable in inspired air and about body temperature in expired air.
• - Dust particles are variable in inspired air and little, if any, in expired air.
• - The difference in oxygen and carbon dioxide is due to cellular respiration.
• - The difference in water vapour and temperature is due to air conditioning in the respiratory passages.
• - The difference in dust particles is due to filtering by the respiratory passages.

Remember

• - Inspired air = 21% O₂, 0.04% CO₂, 79% N₂
• - Expired air = 16% O₂, 4% CO₂, 79% N₂
• - Oxygen decreases by 5% (used by body)
• - Carbon dioxide increases by ~4% (produced by body)
• - Nitrogen remains the same (not used or produced)
• - Water vapour increases (air is moistened)
• - Temperature increases (air is warmed)
• - Dust particles decrease (air is filtered)

Common Mistakes

• 1. Thinking oxygen is completely removed from expired air.
• - Expired air still contains 16% oxygen. Not all oxygen is used.
• 2. Believing carbon dioxide is the only gas that changes.
• - Oxygen also changes (decreases), and water vapour increases.
• 3. Thinking nitrogen is used by the body.
• - Nitrogen is inert and is not used or produced by the body.
• 4. Forgetting that expired air is warmer and more moist.
• - Expired air is warmed to body temperature and saturated with water vapour.
• 5. Mixing up the percentages of oxygen and carbon dioxide.
• - Inspired air: 21% O₂, 0.04% CO₂.
• - Expired air: 16% O₂, 4% CO₂.
• 6. Believing dust particles are completely removed in inspired air.
• - Dust particles are trapped in the respiratory passages, but some may still reach the lungs.
• 7. Thinking the composition of expired air is always the same.
• - It can vary with exercise, altitude, and health status.

Quick Revision

60-Second Revision

• **Inspired air** = 21% O₂, 0.04% CO₂, 79% N₂
• **Expired air** = 16% O₂, 4% CO₂, 79% N₂
• Oxygen decreases by 5% (used by body)
• Carbon dioxide increases by ~4% (produced by body)
• Nitrogen remains the same (not used or produced)
• Water vapour increases (air is moistened)
• Temperature increases (air is warmed)
• Dust particles decrease (air is filtered)
• Warning: Do not think that expired air contains no oxygen. It still has 16% oxygen.