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.

---

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

---

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

---

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.

---

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

• Expiration becomes an active process (no longer passive).
• The internal intercostal muscles contract, actively pulling the ribcage downward and inward.
• The abdominal muscles (including the external obliques) contract, pushing the diaphragm upward more forcefully.
• This forces air out of the lungs more quickly and completely.

Feature	Breathing at Rest	Breathing During Exercise
Inspiration	Diaphragm + external intercostals contract	Same + additional muscles (neck, chest) contract more forcefully
Expiration	Passive (muscles relax, lungs recoil)	Active (internal intercostals + abdominal muscles contract)
Breathing rate	12-20 breaths per minute	Up to 40-60 breaths per minute
Tidal volume	~500 ml per breath	Up to 3,000+ ml per breath
Depth	Shallow	Deep

Exam Tip: A common Edexcel question asks you to explain the difference between breathing at rest and during exercise. The key point is that at rest, expiration is passive, while during exercise, the internal intercostal muscles and abdominal muscles actively contract to force air out more quickly. Make sure you name these muscles specifically.

---

The Role of Pressure Changes

Breathing is driven entirely by pressure changes inside the chest cavity:

Phase	Diaphragm	Intercostals	Chest Volume	Lung Pressure	Air Movement
Inspiration	Contracts (flattens)	External contract (ribcage up/out)	Increases	Decreases (below atmospheric)	Air rushes IN
Expiration	Relaxes (dome shape)	External relax; internal contract during exercise	Decreases	Increases (above atmospheric)	Air pushed OUT

---

The Pleural Membranes and Pleural Fluid

The lungs are surrounded by two thin membranes called the pleural membranes:

• The visceral pleura is attached to the surface of the lungs.
• The parietal pleura is attached to the inside of the chest wall and the top of the diaphragm.

Between them is a thin layer of pleural fluid, which:

• Reduces friction as the lungs expand and contract during breathing.
• Creates a seal that allows the lungs to expand when the chest cavity expands (the lungs are not directly attached to the ribcage — they follow its movement because of the pleural seal).

---

Common Exam Mistakes

• Saying the diaphragm "moves down" during inspiration — be more precise: the diaphragm contracts and flattens (it changes from a dome shape to a flat shape, increasing the volume of the chest cavity).
• Forgetting that expiration at rest is passive — at rest, the internal intercostals and abdominal muscles are NOT used. Expiration happens because the muscles relax and the lungs recoil.
• Not distinguishing rest from exercise — Edexcel questions often specifically ask about one or the other. Make sure your answer matches the context.
• Confusing external and internal intercostals — external intercostals raise the ribcage (inspiration); internal intercostals lower the ribcage (forced expiration during exercise).
• Not mentioning pressure changes — always explain that air moves because of pressure differences between the lungs and the atmosphere.

---

Summary