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