The Pathway of Air

This lesson covers the structures of the respiratory system and the pathway that air takes from the moment it enters the body to the point where gaseous exchange occurs in the lungs. A clear understanding of respiratory anatomy is essential for the AQA GCSE PE specification (3.1.1.2) and provides the foundation for the lessons on gaseous exchange, breathing mechanics and spirometry that follow.

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Overview of the Respiratory System

The respiratory system is responsible for bringing air into and out of the body so that gaseous exchange can occur — the process by which oxygen enters the blood and carbon dioxide is removed. The respiratory system works closely with the cardiovascular system to ensure that oxygen reaches the muscles and organs, and that carbon dioxide (a waste product of respiration) is expelled.

The main components of the respiratory system are:

Structure	Function
Mouth and nose	Entry points for air
Pharynx (throat)	Passageway connecting the nasal cavity and mouth to the larynx
Larynx (voice box)	Contains the vocal cords; connects the pharynx to the trachea
Trachea (windpipe)	Main airway leading from the larynx towards the lungs
Bronchi (singular: bronchus)	Two branches of the trachea, one leading to each lung
Bronchioles	Smaller branches of the bronchi within the lungs
Alveoli (singular: alveolus)	Tiny air sacs at the ends of bronchioles where gaseous exchange occurs
Diaphragm	Dome-shaped muscle beneath the lungs that assists breathing
Intercostal muscles	Muscles between the ribs that assist breathing
Pleural membranes	Double-layered membranes surrounding each lung, reducing friction during breathing

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The Pathway of Air — Step by Step

When you breathe in (inspiration or inhalation), air follows this pathway:

1. Mouth and Nose

Air enters the body through the mouth or nose. The nose is the preferred route because it performs several important functions:

• Filtering — hairs and mucus in the nasal cavity trap dust, pollen and other particles, preventing them from reaching the lungs.
• Warming — the rich blood supply in the nasal lining warms the air to body temperature, which protects the delicate lung tissue.
• Moistening — mucus adds moisture to the air, preventing the airways from drying out.

During exercise, we often breathe through the mouth as well because it allows a greater volume of air to be inhaled more quickly, even though the mouth does not filter, warm or moisten air as effectively as the nose.

2. Pharynx (Throat)

Air passes from the nose and mouth into the pharynx, a shared passageway for air and food. The epiglottis (a flap of cartilage) closes over the top of the trachea during swallowing to prevent food from entering the airway.

3. Larynx (Voice Box)

Air passes through the larynx, which contains the vocal cords. The larynx sits at the top of the trachea and is sometimes visible as the "Adam's apple" in the neck.

4. Trachea (Windpipe)

The trachea is a tube approximately 10–12 cm long that carries air from the larynx towards the lungs. It has several important features:

• C-shaped rings of cartilage — these keep the trachea open at all times, preventing it from collapsing during inhalation when air pressure inside drops.
• Mucus-producing goblet cells — line the trachea and secrete sticky mucus that traps pathogens and particles.
• Cilia — tiny hair-like structures that line the trachea and beat in a wave-like motion, sweeping mucus (and trapped particles) upwards towards the throat where it can be swallowed or expelled.

5. Bronchi

At the base of the trachea, the airway divides into two bronchi (singular: bronchus) — the left bronchus and the right bronchus. Each bronchus enters one lung. Like the trachea, the bronchi are supported by rings of cartilage and are lined with mucus and cilia.

6. Bronchioles

Inside the lungs, each bronchus divides repeatedly into smaller and smaller tubes called bronchioles. Bronchioles do not have cartilage rings; instead, they have smooth muscle in their walls that can contract or relax to control the diameter of the airway.

• During exercise, the bronchioles dilate (widen) to allow more air to flow into the lungs.
• In conditions such as asthma, the smooth muscle in the bronchiole walls contracts excessively, narrowing the airway and making breathing difficult.

7. Alveoli

At the end of the smallest bronchioles are clusters of tiny, balloon-like air sacs called alveoli (singular: alveolus). There are approximately 300–500 million alveoli in the lungs, providing an enormous surface area for gaseous exchange — estimated at around 70 square metres (about the size of half a tennis court).

The alveoli are the site of gaseous exchange, where oxygen passes from the air into the blood, and carbon dioxide passes from the blood into the air. This process is covered in detail in the next lesson.

graph TD
    A[Mouth / Nose] --> B[Pharynx]
    B --> C[Larynx]
    C --> D[Trachea]
    D --> E[Left Bronchus]
    D --> F[Right Bronchus]
    E --> G[Bronchioles]
    F --> H[Bronchioles]
    G --> I[Alveoli]
    H --> J[Alveoli]
    I --> K[Gaseous Exchange]
    J --> K

    style A fill:#4a90d9,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#2980b9,color:#fff
    style D fill:#1f6dad,color:#fff
    style E fill:#27ae60,color:#fff
    style F fill:#27ae60,color:#fff
    style G fill:#2ecc71,color:#fff
    style H fill:#2ecc71,color:#fff
    style I fill:#f39c12,color:#fff
    style J fill:#f39c12,color:#fff
    style K fill:#e74c3c,color:#fff

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Structure of the Alveoli

The alveoli are perfectly adapted for their role in gaseous exchange. Their key features are:

Feature	How It Aids Gaseous Exchange
Enormous number (300–500 million)	Provides a very large total surface area for exchange
Walls one cell thick	Creates a very short diffusion distance
Moist lining	Gases dissolve in the moisture, aiding diffusion
Dense network of capillaries	Ensures a rich blood supply to maintain a steep concentration gradient
Good blood supply	Constantly brings deoxygenated blood and carries away oxygenated blood, maintaining the concentration gradient
Spherical shape	Maximises surface area relative to volume

Exam Tip: If asked to describe the features of alveoli that make them efficient for gaseous exchange, aim to describe at least four features and explain how each one specifically aids the process. Simply listing features without explanation will not earn full marks.

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The Lungs

The lungs are the two large, spongy organs that house the bronchioles and alveoli. Key facts:

• The right lung has three lobes; the left lung has two lobes (to accommodate the heart, which sits slightly to the left).
• The lungs are surrounded by pleural membranes — two thin layers with a small amount of pleural fluid between them. This fluid reduces friction between the lungs and the chest wall during breathing.
• The lungs themselves do not contain muscle. They are inflated and deflated by the action of the diaphragm and intercostal muscles, which change the volume and pressure inside the thoracic cavity (chest). This is covered in the lesson on breathing mechanics.

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The Thoracic Cavity

The thoracic cavity (chest cavity) is the space inside the ribcage that contains the lungs and heart. It is bounded by:

• The ribs and intercostal muscles on the sides and front
• The spine at the back
• The diaphragm at the bottom

Changes in the volume of the thoracic cavity are what drive air in and out of the lungs during breathing (see the lesson on breathing mechanics).

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Respiratory Structures and Exercise

During exercise, several changes occur in the respiratory system:

Change	Detail
Increased breathing rate	More breaths per minute to bring in more oxygen
Increased tidal volume	Larger volumes of air inhaled and exhaled per breath
Bronchodilation	Bronchioles widen to reduce airway resistance and allow more air flow
Increased blood flow to alveoli	More capillaries open around the alveoli, increasing the surface area for gaseous exchange
Increased depth of breathing	The diaphragm and intercostal muscles work harder; additional muscles (pectorals, sternocleidomastoid, abdominals) are recruited

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Respiratory Conditions Relevant to Sport

Asthma