Mechanics of Breathing

Breathing — or ventilation — is the process by which air is moved in and out of the lungs. This lesson covers the mechanics of inhalation and exhalation at rest, the changes that occur during exercise when additional muscles are recruited, and the role of air pressure in driving breathing. Understanding breathing mechanics is a core requirement of the AQA GCSE PE specification (3.1.1.2).

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Key Terminology

Before exploring the mechanics, it is important to define some key terms:

Term	Definition
Inspiration (inhalation)	Breathing in — drawing air into the lungs
Expiration (exhalation)	Breathing out — pushing air out of the lungs
Ventilation	The process of moving air in and out of the lungs (includes both inspiration and expiration)
Tidal volume	The volume of air breathed in or out in one normal breath
Breathing rate (respiratory rate)	The number of breaths taken per minute
Minute ventilation	The total volume of air breathed in or out per minute (= tidal volume × breathing rate)
Thoracic cavity	The chest cavity containing the lungs and heart
Diaphragm	A dome-shaped muscle below the lungs that is the primary muscle of breathing
Intercostal muscles	Muscles located between the ribs; external intercostals aid inhalation, internal intercostals aid exhalation

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The Muscles of Breathing

At Rest

Two groups of muscles are primarily responsible for breathing at rest:

The Diaphragm

The diaphragm is a large, dome-shaped sheet of muscle that separates the thoracic cavity (chest) from the abdominal cavity. It is the most important muscle of breathing.

• During inhalation: the diaphragm contracts and flattens (moves downwards), increasing the volume of the thoracic cavity.
• During exhalation: the diaphragm relaxes and returns to its dome shape (moves upwards), decreasing the volume of the thoracic cavity.

The External Intercostal Muscles

The external intercostal muscles are located between the ribs. They are the intercostal muscles used during inhalation.

• During inhalation: the external intercostals contract, pulling the ribs upwards and outwards. This further increases the volume of the thoracic cavity.
• During exhalation at rest: the external intercostals relax, and the ribs drop back downwards and inwards under their own weight (and the weight of gravity). This decreases the volume of the thoracic cavity.

The Internal Intercostal Muscles

The internal intercostal muscles are also located between the ribs, but deeper than the external intercostals. At rest, they play a minimal role, but during forced exhalation (e.g., during exercise), they contract to actively pull the ribs downwards and inwards, forcing air out of the lungs more quickly.

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Inhalation at Rest

The process of inhalation at rest follows this sequence:

1. The diaphragm contracts and flattens downwards.
2. The external intercostal muscles contract, pulling the ribs upwards and outwards.
3. These two actions increase the volume of the thoracic cavity.
4. Because the volume increases, the air pressure inside the lungs decreases (it becomes lower than atmospheric pressure outside the body).
5. Air rushes into the lungs through the mouth/nose, down the trachea, through the bronchi and bronchioles, and into the alveoli — moving from high pressure (outside) to low pressure (inside).

graph TD
    A[Diaphragm contracts and flattens] --> C[Volume of thoracic cavity increases]
    B[External intercostals contract - ribs move up and out] --> C
    C --> D[Air pressure inside lungs decreases]
    D --> E[Air pressure inside lungs is less than atmospheric pressure]
    E --> F[Air rushes INTO the lungs]

    style A fill:#3498db,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#2980b9,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#2ecc71,color:#fff
    style F fill:#f39c12,color:#fff

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Exhalation at Rest

At rest, exhalation is largely a passive process — it does not require significant muscular effort:

1. The diaphragm relaxes and returns to its dome shape (moves upwards).
2. The external intercostal muscles relax, and the ribs drop downwards and inwards under their own weight.
3. These two actions decrease the volume of the thoracic cavity.
4. Because the volume decreases, the air pressure inside the lungs increases (it becomes higher than atmospheric pressure outside the body).
5. Air is pushed out of the lungs — moving from high pressure (inside) to low pressure (outside).

Exam Tip: A very common exam question is: "Describe the process of exhalation at rest." The key point is that exhalation at rest is mainly passive — muscles relax, and the natural recoil of the lungs and chest wall pushes air out. No muscles need to contract forcefully. This changes during exercise.

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The Role of Air Pressure

Breathing is driven entirely by pressure differences between the air inside the lungs and the atmospheric air outside the body.

Phase	Lung Volume	Lung Pressure	Compared to Atmospheric Pressure	Air Movement
Inhalation	Increases	Decreases	Lower than atmospheric	Air flows IN
Exhalation	Decreases	Increases	Higher than atmospheric	Air flows OUT

The key principle is Boyle's Law: when the volume of a gas increases, its pressure decreases (and vice versa), assuming temperature remains constant.

• During inhalation, the thoracic cavity expands → lung volume increases → air pressure inside lungs drops → air flows in.
• During exhalation, the thoracic cavity shrinks → lung volume decreases → air pressure inside lungs rises → air flows out.