Mechanics of Breathing

This lesson covers the mechanics of breathing — how air is drawn into and pushed out of the lungs — as required by the Edexcel GCSE PE specification (1PE0 — Topic 1). You need to understand the role of the diaphragm, intercostal muscles, and changes in pressure and volume during inspiration (breathing in) and expiration (breathing out), both at rest and during exercise.

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Overview of Breathing

Breathing (also called ventilation) is the physical process of moving air into and out of the lungs. It has two phases:

1. Inspiration (inhalation) — breathing in. Air enters the lungs.
2. Expiration (exhalation) — breathing out. Air leaves the lungs.

Breathing is controlled by two main sets of muscles:

Muscle	Location	Role in Breathing
Diaphragm	A dome-shaped sheet of muscle below the lungs, separating the chest cavity from the abdomen	The primary breathing muscle
External intercostal muscles	Between the ribs (on the outside)	Raise the ribcage during inspiration
Internal intercostal muscles	Between the ribs (on the inside)	Pull the ribcage down during forced expiration

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Inspiration at Rest

During normal, quiet inspiration (breathing in at rest):

1. The diaphragm contracts and flattens (moves downward).
2. The external intercostal muscles contract, pulling the ribcage upward and outward.
3. This increases the volume of the chest cavity (thoracic cavity).
4. As the volume increases, the air pressure inside the lungs decreases (becomes lower than atmospheric pressure outside).
5. Air rushes in through the mouth/nose and down the airways to fill the lungs (air moves from high pressure to low pressure).

graph TD
    A["Diaphragm contracts<br>(flattens)"] --> D["Chest cavity<br>volume increases"]
    B["External intercostals<br>contract"] --> C["Ribcage moves<br>up and out"]
    C --> D
    D --> E["Pressure inside<br>lungs decreases"]
    E --> F["Air rushes IN<br>(inspiration)"]

    style A fill:#4a90d9,color:#fff
    style F fill:#27ae60,color:#fff

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Expiration at Rest

During normal, quiet expiration (breathing out at rest):

1. The diaphragm relaxes and returns to its dome shape (moves upward).
2. The external intercostal muscles relax, and the ribcage drops back downward and inward under its own weight.
3. This decreases the volume of the chest cavity.
4. As the volume decreases, the air pressure inside the lungs increases (becomes higher than atmospheric pressure).
5. Air is pushed out of the lungs through the airways and out of the mouth/nose.

Important: At rest, expiration is largely a passive process — it relies on the relaxation of muscles and the natural elasticity of the lungs (they spring back like an elastic band). The internal intercostal muscles are not used during quiet breathing.

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Breathing During Exercise

During exercise, the body needs to take in more oxygen and expel more carbon dioxide. Both inspiration and expiration become more active and forceful:

Inspiration During Exercise

• The diaphragm contracts more forcefully, flattening further.
• The external intercostals contract more vigorously, raising the ribcage higher.
• Additional muscles are recruited to help, including the neck muscles (sternocleidomastoid) and upper chest muscles (pectoralis minor, scalenes).
• The chest cavity expands more, drawing in a larger volume of air.

Expiration During Exercise