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.

Exam Tip: If AQA asks you to explain the mechanism of breathing, always include the role of pressure changes. Many students describe the muscle movements but forget to explain that air moves because of the pressure difference this creates. Linking muscle action to pressure change to air movement is what earns full marks.

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Breathing During Exercise

During exercise, breathing becomes much more active and forceful. The body needs to move a much larger volume of air in and out of the lungs per minute to meet the increased oxygen demand of the working muscles. Several additional changes occur:

Changes to Inhalation During Exercise

In addition to the diaphragm and external intercostal muscles, the following additional muscles are recruited to increase the depth and force of inhalation:

Muscle	Action During Exercise
Sternocleidomastoid	Lifts the sternum (breastbone) upwards, further increasing chest volume
Scalenes	Elevate the upper ribs
Pectoralis minor	Lifts the upper ribs outwards

These muscles help to expand the thoracic cavity even further, allowing a much greater volume of air to be inhaled (increased tidal volume).

Changes to Exhalation During Exercise

At rest, exhalation is passive. During exercise, exhalation becomes an active process, requiring muscular contraction:

Muscle	Action During Exercise
Internal intercostal muscles	Contract to actively pull the ribs downwards and inwards
Abdominal muscles (rectus abdominis, obliques)	Contract to push the diaphragm upwards, forcing air out of the lungs more rapidly

By actively contracting these muscles, the body can exhale more forcefully and quickly, allowing for a faster breathing rate and a greater volume of air to be moved per minute.

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Comparison: Breathing at Rest vs. During Exercise

Feature	At Rest	During Exercise
Breathing rate	12–15 breaths per minute	Up to 40–60 breaths per minute
Tidal volume	~500 ml (0.5 litres)	Up to 3 litres or more
Minute ventilation	~6–7.5 litres per minute	Up to 120–180 litres per minute
Inhalation	Diaphragm and external intercostals only	Diaphragm, external intercostals, sternocleidomastoid, scalenes, pectoralis minor
Exhalation	Passive (muscles relax)	Active — internal intercostals and abdominal muscles contract
Depth of breathing	Shallow	Deep
Effort	Minimal	Significant

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Minute Ventilation

Minute ventilation (VE) is the total volume of air breathed in (or out) per minute. It is calculated using the formula:

VE = Tidal Volume × Breathing Rate

Worked Example 1

At rest, a student has a tidal volume of 500 ml and a breathing rate of 14 breaths per minute.

VE = 500 × 14 = 7,000 ml/min = 7 l/min

Worked Example 2

During exercise, the same student has a tidal volume of 2,500 ml and a breathing rate of 40 breaths per minute.

VE = 2,500 × 40 = 100,000 ml/min = 100 l/min

This demonstrates how dramatically minute ventilation increases during exercise — from 7 l/min at rest to 100 l/min during exercise in this example.