3.2 Mechanism of Breathing

Introduction

Breathing is a mechanical process that moves air into and out of the lungs. It is essential for bringing oxygen into the body and removing carbon dioxide. The lungs themselves cannot draw in or push out air — they depend on the diaphragm and intercostal muscles to expand and contract the chest cavity. This topic explains the mechanism of breathing, including inspiration (inhalation) and expiration (exhalation).

Definition

Breathing is the mechanical process of moving air into and out of the lungs. It is also called ventilation.

The mechanism of breathing consists of two phases:

• Inspiration (inhalation) — taking air into the lungs.
• Expiration (exhalation) — pushing air out of the lungs.

Main Idea

Breathing is a mechanical process. It is the movement of air into and out of the lungs. The force that drives air into the lungs comes from ordinary air pressure.

The lungs themselves neither draw in air nor push it out. The expansion and contraction of the lungs is accomplished by the diaphragm and by intercostal muscles.

Core Concept

The Two Phases of Breathing

Detailed Explanation

1. Inspiration (Inhalation)

When you breathe in or inhale, these changes occur:

(1) The muscles in the diaphragm contract.

• This pulls the diaphragm downwards.
• The diaphragm becomes flattened.

(2) The intercostal muscles contract.

- This pulls the ribs upwards and outwards.

(3) The lungs expand.

- When the volume increases, the pressure decreases in the lungs.

(4) Air moves into the lungs from the nose/mouth.

- Air flows from an area of higher pressure (outside) to an area of lower pressure (inside the lungs).

Simple explanation: When you breathe in, your diaphragm moves down and your ribs move up and out. This makes the chest cavity bigger. The lungs expand, and air rushes in to fill the space.

Why this happens:

• The diaphragm and intercostal muscles work together to increase the volume of the thoracic cavity.
• Increased volume = decreased pressure.
• Air moves from high pressure (outside) to low pressure (inside).

2. Expiration (Exhalation)

When you breathe out or exhale, these changes occur:

(1) The diaphragm muscles relax.

- This allows the diaphragm to become normal, dome-shaped.

(2) The intercostal muscles relax.

- This allows the ribs to move downwards and inwards.

(3) The lungs are compressed.

- When the volume decreases, the pressure increases in the lungs.

(4) Air is forced out from the lungs to the surrounding air.

- Air flows from an area of higher pressure (inside the lungs) to an area of lower pressure (outside).

Simple explanation: When you breathe out, your diaphragm relaxes and moves up, and your ribs move down and in. This makes the chest cavity smaller. The lungs are squeezed, and air is pushed out.

Why this happens:

• The diaphragm and intercostal muscles relax, decreasing the volume of the thoracic cavity.
• Decreased volume = increased pressure.
• Air moves from high pressure (inside) to low pressure (outside).

Summary Table

Intermediate Level

The mechanism of breathing involves pressure changes:

Inspiration:

• Diaphragm contracts → moves down.
• Intercostal muscles contract → ribs move up and out.
• Thoracic volume increases.
• Pressure in lungs decreases.
• Air rushes in.

Expiration:

• Diaphragm relaxes → moves up.
• Intercostal muscles relax → ribs move down and in.
• Thoracic volume decreases.
• Pressure in lungs increases.
• Air is pushed out.

Key principle:

• Air always moves from an area of higher pressure to an area of lower pressure.
• Volume and pressure are inversely related (Boyle's Law).

Important Points

• - Breathing is a mechanical process.
• - It is the movement of air into and out of the lungs.
• - The force that drives air into the lungs comes from ordinary air pressure.
• - The lungs themselves neither draw in air nor push it out.
• - The expansion and contraction of the lungs is accomplished by the diaphragm and by intercostal muscles.
• - The mechanism of breathing consists of two phases: inspiration and expiration.
• - During inspiration, the diaphragm contracts and moves down.
• - During inspiration, the intercostal muscles contract and pull the ribs upwards and outwards.
• - The lungs expand, volume increases, and pressure decreases.
• - Air moves into the lungs from the nose/mouth.
• - During expiration, the diaphragm muscles relax and the diaphragm becomes dome-shaped.
• - During expiration, the intercostal muscles relax and the ribs move downwards and inwards.
• - The lungs are compressed, volume decreases, and pressure increases.
• - Air is forced out from the lungs to the surrounding air.

Remember

• - Breathing = movement of air into and out of lungs
• - Inspiration = inhalation (air in)
• - Expiration = exhalation (air out)
• - Diaphragm contracts → moves down → inspiration
• - Diaphragm relaxes → moves up → expiration
• - Intercostal muscles contract → ribs up and out → inspiration
• - Intercostal muscles relax → ribs down and in → expiration
• - Volume increases → pressure decreases → air in
• - Volume decreases → pressure increases → air out
• - Air moves from high pressure to low pressure

Common Mistakes

• 1. Thinking the lungs actively draw in air.
• - The lungs do not draw in air. The diaphragm and intercostal muscles create pressure changes that move air.
• 2. Confusing inspiration and expiration.
• - Inspiration = breathing in.
• - Expiration = breathing out.
• 3. Mixing up the diaphragm's movement.
• - Inspiration: diaphragm contracts and moves down.
• - Expiration: diaphragm relaxes and moves up.
• 4. Forgetting the role of intercostal muscles.
• - Intercostal muscles move the ribs during breathing.
• 5. Thinking air is pushed into the lungs.
• - Air is drawn into the lungs by negative pressure (lower pressure inside).
• 6. Believing expiration is always active.
• - At rest, expiration is passive. It becomes active during forced expiration.
• 7. Confusing breathing with respiration.
• - Breathing = mechanical process.
• - Respiration = chemical process inside cells.

Quick Revision

60-Second Revision

• Breathing = mechanical process of moving air into and out of lungs
• Inspiration = inhalation; diaphragm contracts and moves down; ribs move up and out; volume increases; pressure decreases; air rushes in
• Expiration = exhalation; diaphragm relaxes and moves up; ribs move down and in; volume decreases; pressure increases; air is pushed out
• Key principle = air moves from high pressure to low pressure
• Muscles involved = diaphragm and intercostal muscles
• Warning: Do not confuse breathing (mechanical) with respiration (chemical). The lungs do not actively draw in air; pressure changes do.