The Respiratory System and Pathway of Air

This lesson covers the structure of the respiratory system and the pathway of air as required by the OCR GCSE PE specification (J587). You need to know the key structures of the respiratory system, be able to describe the pathway that air follows from the mouth/nose to the alveoli, and understand the function of each structure.

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Overview of the Respiratory System

The respiratory system is responsible for getting oxygen into the body and removing carbon dioxide. It works closely with the cardiovascular system to ensure that working muscles receive the oxygen they need during exercise.

flowchart TD
    A["Mouth / Nose"] --> B["Trachea (windpipe)"]
    B --> C["Bronchi (right and left)"]
    C --> D["Bronchioles"]
    D --> E["Alveoli (air sacs)"]

    style A fill:#4a90d9,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#27ae60,color:#fff

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The Pathway of Air

Air enters the body and travels through the following structures, in order:

1. Mouth and Nose

• Air enters through the mouth and/or nose.
• The nose warms, moistens, and filters the air (removing dust and particles using tiny hairs and mucus).
• During exercise, breathing through the mouth allows a greater volume of air to enter more quickly.

2. Trachea (Windpipe)

• The trachea is the main airway leading from the throat down towards the lungs.
• It is a tube approximately 10-12 cm long and 2 cm in diameter.
• The trachea is held open by C-shaped rings of cartilage, which prevent it from collapsing when pressure changes during breathing.
• The inner lining is covered in cilia (tiny hair-like structures) and mucus, which trap dust and bacteria and move them back up towards the throat to be swallowed.

3. Bronchi

• At the bottom of the trachea, the airway splits into two bronchi (singular: bronchus) — the right bronchus and the left bronchus.
• Each bronchus leads into one lung.
• The bronchi have a similar structure to the trachea, with cartilage rings and a ciliated lining.

4. Bronchioles

• Inside the lungs, the bronchi branch repeatedly into smaller and smaller tubes called bronchioles.
• Bronchioles are much thinner than bronchi and do not have cartilage rings.
• They have smooth muscle in their walls, which can contract or relax to change the diameter of the airway.
• During exercise, the smooth muscle relaxes (bronchodilation), widening the bronchioles to allow more air to flow through to the alveoli.
• In conditions like asthma, the smooth muscle contracts excessively (bronchoconstriction), narrowing the bronchioles and making it difficult to breathe.

5. Alveoli

• At the end of the bronchioles are tiny air sacs called alveoli (singular: alveolus).
• There are approximately 300 million alveoli in each lung, providing an enormous surface area for gaseous exchange (approximately 70 m² — roughly the size of a tennis court).
• The alveoli are where gaseous exchange occurs — oxygen passes from the air in the alveoli into the blood, and carbon dioxide passes from the blood into the alveoli to be breathed out.
• The detailed process of gaseous exchange is covered in the next lesson.

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Summary Table of Structures

Structure	Function	Key Feature
Mouth/Nose	Air entry point	Nose warms, moistens, and filters air
Trachea	Main airway to the lungs	Held open by C-shaped cartilage rings
Bronchi	Airways into each lung (left and right)	Branch from the trachea; have cartilage rings
Bronchioles	Smaller airways within the lungs	Have smooth muscle; can dilate or constrict
Alveoli	Site of gaseous exchange	~300 million; enormous surface area; one cell thick

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The Respiratory System in a Sporting Context

Scenario	Relevant Structure	Explanation
A runner breathing through their mouth during a race	Mouth	Allows a greater volume of air to enter than nose breathing alone
An asthmatic athlete struggling to breathe during exercise	Bronchioles	Bronchoconstriction narrows the airways; an inhaler helps relax the smooth muscle
A swimmer taking a deep breath before a dive	Trachea, bronchi, bronchioles, alveoli	Air travels through the full pathway to fill the lungs
An athlete performing better after a warm-up	Bronchioles	Warm-up causes bronchodilation, increasing airflow
Efficient gaseous exchange during aerobic exercise	Alveoli	Oxygen diffuses into the blood to supply working muscles

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Linking the Respiratory System to the Cardiovascular System

The respiratory and cardiovascular systems work together to deliver oxygen to working muscles:

flowchart LR
    A["Air breathed in"] --> B["Travels to alveoli"]
    B --> C["O₂ diffuses into<br>blood in capillaries"]
    C --> D["Oxygenated blood<br>returns to heart"]
    D --> E["Heart pumps blood<br>to working muscles"]
    E --> F["O₂ used for<br>aerobic respiration"]
    F --> G["CO₂ produced"]
    G --> H["CO₂ carried to lungs<br>in blood"]
    H --> I["CO₂ diffuses into<br>alveoli"]
    I --> J["CO₂ breathed out"]

    style A fill:#4a90d9,color:#fff
    style E fill:#e67e22,color:#fff
    style J fill:#27ae60,color:#fff

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Common Exam Mistakes

• Getting the order of structures wrong — the correct order is: mouth/nose → trachea → bronchi → bronchioles → alveoli. Students sometimes confuse bronchi and bronchioles.
• Forgetting the C-shaped cartilage rings — these are a key feature of the trachea that prevent it from collapsing.
• Confusing bronchi and bronchioles — bronchi are the large branches into each lung; bronchioles are the smaller branches within the lungs.
• Not mentioning bronchodilation during exercise — this is an important adaptation that allows more air to reach the alveoli during physical activity.
• Describing alveoli as "air tubes" — alveoli are tiny air SACS (like grapes on a stalk), not tubes. Their sac-like shape gives them a large surface area.

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Summary