3.3 Mechanism of Ventilation (breathing)

Ventilation, also known as breathing, is the process of moving air into and out of the lungs. It involves two phases: inspiration (inhaling) and expiration (exhaling). The diaphragm and intercostal muscles work together to change the volume of the thoracic cavity, which creates pressure differences that drive airflow.

Definition

Ventilation (Breathing): Ventilation is the mechanical process of moving air into and out of the lungs. It ensures a continuous supply of oxygen to the alveoli and the removal of carbon dioxide.

Inspiration (Inhalation): Inspiration is the active phase of breathing during which air is taken into the lungs.

Expiration (Exhalation): Expiration is the passive phase of breathing during which air is expelled from the lungs.

Main Idea

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

• Diaphragm contracts → flattens
• External intercostals contract → rib cage moves upward and forward
• Thoracic cavity volume increases
• Pressure decreases below atmospheric pressure
• Air enters the lungs

• Diaphragm relaxes → becomes dome-shaped
• External intercostals relax; internal intercostals contract → rib cage moves downward and backward
• Thoracic cavity volume decreases
• Pressure increases above atmospheric pressure
• Air is expelled from the lungs

Core Concept

• **Ventilation** = breathing
• Inspiration = active phase; air in
• Expiration = passive phase; air out
• Diaphragm = dome-shaped muscle; separates thoracic and abdominal cavities
• Intercostal muscles = between ribs; external and internal
• Thoracic cavity = chest cavity
• Pressure gradient = drives airflow
• Boyle's Law = pressure inversely proportional to volume

Detailed Explanation

Muscles of Breathing

• A large dome of skeletal muscle.
• Separates the thoracic cavity from the abdominal cavity.
• Contracts during inspiration; relaxes during expiration.

• There are two sets of intercostal muscles between each pair of ribs:
• External intercostal muscles
• Internal intercostal muscles
• The muscle fibers run diagonally but in opposite directions in the two sets of muscles.

Inspiration (Inhalation)

Inspiration is the active phase of breathing.

• Contraction of the diaphragm causes its dome shape to flatten (less dome shape).
• Contraction of the external intercostal muscles and relaxation of the internal intercostal muscles causes the rib cage to move upward and forward.
• Both these events result in an increase in the inner space of the thoracic cavity.
• Consequently, the pressure in the thorax and hence in the lungs is reduced to less than atmospheric pressure.
• Air therefore enters the lungs and alveoli become inflated.

Summary of Inspiration:

Expiration (Exhalation)

Expiration is the passive phase of breathing.

• Relaxation of the diaphragm causes it to become more dome-shaped.
• Relaxation of the external intercostal muscles and contraction of the internal intercostal muscles cause the rib cage to move downward and backward.
• Both these events result in a decrease in the inner space of the thoracic cavity.
• Consequently, the pressure in the thorax and hence in the lungs is increased to more than atmospheric pressure.
• Air is therefore forced to be expelled from the lungs.

Summary of Expiration:

Comparison of Inspiration and Expiration

Intermediate Level

Muscles of Ventilation

Pressure Changes During Ventilation

Boyle's Law

Boyle's Law states that the pressure of a gas is inversely proportional to its volume (at constant temperature).

P ∝ 1V

• When thoracic volume increases → pressure decreases → air enters.
• When thoracic volume decreases → pressure increases → air is expelled.

Types of Breathing

Accessory Muscles of Breathing

Formula / Mathematical Form

Boyle's Law: P₁ V₁ = P₂ V₂

• P₁ = initial pressure
• V₁ = initial volume
• P₂ = final pressure
• V₂ = final volume

Solved Example

• Initial volume of lungs = 2.5 L
• Initial pressure = 760 mm Hg
• Final volume = 2.0 L

Required: Final pressure

Formula: P₁ V₁ = P₂ V₂

Substitution: 760 × 2.5 = P₂ × 2.0

Calculation: P₂ = 760 × 2.52.0 = 950 mm Hg

Final Answer: The final pressure is 950 mm Hg. Since this is higher than atmospheric pressure (760 mm Hg), air will be expelled from the lungs.

Important Points

• 1. Ventilation is the movement of air into and out of the lungs.
• 2. Inspiration is the active phase; expiration is passive.
• 3. Diaphragm contracts and flattens during inspiration.
• 4. Diaphragm relaxes and becomes dome-shaped during expiration.
• 5. External intercostals contract during inspiration.
• 6. Internal intercostals contract during expiration.
• 7. Thoracic volume increases during inspiration.
• 8. Thoracic volume decreases during expiration.
• 9. Pressure decreases below atmospheric during inspiration.
• 10. Pressure increases above atmospheric during expiration.
• 11. Air enters lungs during inspiration.
• 12. Air is expelled during expiration.
• 13. Boyle's Law explains pressure-volume relationship.
• 14. Breathing centre is in the medulla.
• 15. Cerebral cortex allows voluntary control of breathing.

Remember

• - **Ventilation** = breathing
• - Inspiration = active phase; air in
• - Expiration = passive phase; air out
• - Diaphragm = dome-shaped muscle
• - External intercostals = contract during inspiration
• - Internal intercostals = contract during expiration
• - Thoracic volume = increases during inspiration
• - Pressure = decreases below atmospheric during inspiration
• - **Boyle's Law** = P ∝ 1V
• - Breathing centre = medulla
• - Cerebral cortex = voluntary control
• - Accessory muscles = used in deep breathing

Common Mistakes

• 1. Confusing inspiration and expiration — Inspiration = air in; expiration = air out.
• 2. Thinking expiration is active — Expiration is passive during quiet breathing.
• 3. Mixing up the intercostal muscles — External contract during inspiration; internal contract during expiration.
• 4. Forgetting the role of the diaphragm — Diaphragm contracts and flattens during inspiration.
• 5. Thinking the lungs draw in air — The lungs do not draw in air; pressure differences drive airflow.
• 6. Mixing up Boyle's Law — Pressure is inversely proportional to volume.
• 7. Believing breathing is only involuntary — Breathing can be voluntarily controlled through the cerebral cortex.
• 8. Confusing the breathing centre — It is in the medulla.
• 9. Forgetting the role of accessory muscles — They are used in deep or labored breathing.
• 10. Thinking air is pushed into the lungs — Air is drawn in by pressure differences.

Quick Revision

Ventilation, or breathing, is the process of moving air into and out of the lungs. It involves two phases: inspiration (inhaling) and expiration (exhaling). Inspiration is the active phase. The diaphragm contracts and flattens, and the external intercostal muscles contract, causing the rib cage to move upward and forward. This increases the volume of the thoracic cavity, decreasing the pressure below atmospheric pressure. Air therefore enters the lungs. Expiration is the passive phase. The diaphragm relaxes and becomes dome-shaped, and the external intercostals relax while the internal intercostals contract, causing the rib cage to move downward and backward. This decreases the volume of the thoracic cavity, increasing the pressure above atmospheric pressure. Air is therefore expelled from the lungs. Boyle's Law explains the relationship between pressure and volume. Breathing is controlled by the breathing centre in the medulla, but can also be voluntarily controlled through the cerebral cortex.

60-Second Revision

• Ventilation = breathing
• Inspiration = active phase; air in
• Expiration = passive phase; air out
• Diaphragm contracts and flattens during inspiration
• Diaphragm relaxes and becomes dome-shaped during expiration
• External intercostals contract during inspiration
• Internal intercostals contract during expiration
• Thoracic volume increases during inspiration
• Pressure decreases below atmospheric during inspiration
• Boyle's Law = P ∝ 1V
• Breathing centre = medulla
• Warning: Expiration is passive during quiet breathing; the lungs do not draw in air

NEXT TOPIC

• Chapter:
• 3. Respiratory System of Man

• Topic:
• 3.4 TRANSPORT OF GASES

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