Mechanics of Breathing

This lesson covers the mechanics of breathing as required by the OCR GCSE PE specification (J587). You need to understand the process of inspiration (breathing in) and expiration (breathing out), the roles of the diaphragm and intercostal muscles, and the key terms breathing rate, tidal volume, and minute ventilation.

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How Breathing Works

Breathing is the process of moving air into and out of the lungs. It involves two phases:

1. Inspiration (inhalation) — breathing in
2. Expiration (exhalation) — breathing out

Breathing is controlled by the diaphragm (a dome-shaped muscle beneath the lungs) and the intercostal muscles (muscles between the ribs).

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Inspiration (Breathing In)

During inspiration, the body increases the volume of the chest cavity (thoracic cavity), which reduces the air pressure inside the lungs below atmospheric pressure. Air then flows in to equalise the pressure.

What Happens

1. The diaphragm contracts and flattens (moves downward).
2. The external intercostal muscles contract, pulling the ribs upward and outward.
3. The chest cavity increases in volume.
4. Air pressure inside the lungs decreases (below atmospheric pressure).
5. Air is drawn into the lungs through the mouth/nose, trachea, bronchi, bronchioles, and into the alveoli.

During Exercise

During vigorous exercise, inspiration becomes more forceful:

• The diaphragm and external intercostals contract more powerfully.
• Additional muscles (such as those in the neck and shoulders) assist with pulling the ribcage upward even further, allowing a greater volume of air to enter.

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Expiration (Breathing Out)

During expiration at rest, the body decreases the volume of the chest cavity, which increases air pressure inside the lungs above atmospheric pressure. Air is then pushed out.

What Happens (at Rest)

1. The diaphragm relaxes and returns to its dome shape (moves upward).
2. The external intercostal muscles relax, allowing the ribs to move downward and inward under gravity.
3. The chest cavity decreases in volume.
4. Air pressure inside the lungs increases (above atmospheric pressure).
5. Air is pushed out of the lungs.

At rest, expiration is largely a passive process — it relies on the elastic recoil of the lungs and the relaxation of the muscles, with no active muscle contraction required.

During Exercise

During exercise, expiration becomes an active process:

• The internal intercostal muscles contract, actively pulling the ribs downward and inward.
• The abdominal muscles contract, pushing the diaphragm upward more forcefully.
• This forces air out of the lungs more quickly and completely, allowing the next breath to be taken sooner.

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Summary of Breathing Mechanics

Phase	Diaphragm	External Intercostals	Internal Intercostals	Ribs	Chest Volume	Air Pressure	Air Movement
Inspiration	Contracts (flattens)	Contract	Relax	Up and out	Increases	Decreases	Air in
Expiration (rest)	Relaxes (domes up)	Relax	Relax	Down and in	Decreases	Increases	Air out
Expiration (exercise)	Relaxes (pushed up by abdominals)	Relax	Contract	Pulled down and in	Decreases more forcefully	Increases more	Air forced out

graph TD
    A["INSPIRATION"] --> B["Diaphragm contracts<br>(flattens)"]
    A --> C["External intercostals<br>contract"]
    B --> D["Chest volume<br>increases"]
    C --> D
    D --> E["Pressure decreases"]
    E --> F["Air drawn IN"]

    G["EXPIRATION"] --> H["Diaphragm relaxes<br>(domes up)"]
    G --> I["Intercostals relax<br>(+ internal intercostals<br>during exercise)"]
    H --> J["Chest volume<br>decreases"]
    I --> J
    J --> K["Pressure increases"]
    K --> L["Air pushed OUT"]

    style A fill:#4a90d9,color:#fff
    style F fill:#27ae60,color:#fff
    style G fill:#e67e22,color:#fff
    style L fill:#e74c3c,color:#fff

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Key Breathing Terms

Breathing Rate

Breathing rate is the number of breaths taken per minute.

Condition	Typical Breathing Rate
At rest	~12-20 breaths per minute
During exercise	Can increase to 40-60+ breaths per minute

Tidal Volume

Tidal volume is the volume of air breathed in or out in one normal breath.

Condition	Typical Tidal Volume
At rest	~500 ml (0.5 litres)
During exercise	Can increase to 2-3+ litres

Minute Ventilation

Minute ventilation is the total volume of air breathed in or out per minute. It is calculated using the equation:

$$\text{Minute Ventilation} = \text{Tidal Volume} \times \text{Breathing Rate}$$Minute Ventilation=Tidal Volume×Breathing Rate

Or: VE = TV x BR (sometimes written as MV = TV x f)

Condition	TV (litres)	BR (breaths/min)	VE (litres/min)
At rest	0.5	15	7.5
During exercise	2.5	40	100

Exam Tip: You MUST be able to use the equation Minute Ventilation = Tidal Volume x Breathing Rate. If given two values, you must calculate the third. For example: if TV = 2 litres and BR = 35, then VE = 2 x 35 = 70 litres/min.

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Worked Examples

Example 1

At rest, a person has a tidal volume of 0.5 litres and a breathing rate of 16 breaths per minute. Calculate their minute ventilation.

VE = TV x BR = 0.5 x 16 = 8 litres/min

Example 2

During exercise, a person's minute ventilation is 90 litres/min and their breathing rate is 45 breaths per minute. Calculate their tidal volume.

TV = VE ÷ BR = 90 ÷ 45 = 2 litres per breath

Example 3

During exercise, a person's tidal volume is 3 litres and their minute ventilation is 120 litres/min. Calculate their breathing rate.

BR = VE ÷ TV = 120 ÷ 3 = 40 breaths per minute

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How Breathing Changes During Exercise

During exercise, the body needs more oxygen and produces more carbon dioxide. The respiratory system responds by:

• Increasing breathing rate — more breaths per minute to move air in and out faster.
• Increasing tidal volume — deeper breaths to move more air with each breath.
• Increasing minute ventilation — the combined effect of faster and deeper breathing.

These changes ensure that more oxygen reaches the alveoli for gaseous exchange and more carbon dioxide is removed.

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Common Exam Mistakes