In humans air enters the body through the nasal cavity and flows into the following structures which are found in the human thorax:

Ventilation (the process of moving air into and out of the lungs) also requires the following structures:

The respiratory system; nasal cavity, trachea, pleural cavity (filled with fluid), intercostal muscles, ribs, diaphragm, lung, bronchiole, alveoli

The air that enters the nasal cavity flows down the trachea. The trachea has a number of adaptations:

  • cartilage rings in the walls of the trachea help to keep it open
  • ciliated epithelium and goblet cells to clean the air before it reaches the lungs
Cross section of cells in the trachea. Labelled are the Goblet cells and the Cilia.

Goblet cells produce mucus which traps dust, dirt and bacteria to prevent them entering the lungs.

Cilia are small hairs which beat to push the mucus back up the trachea so it can be swallowed and destroyed in the stomach.

Clean air then enters the two bronchi, one bronchus going to each lung. The bronchi in the lungs split into smaller and smaller tubes called bronchioles. These end in microscopic air sacs called alveoli.

Breathing in (inspiration)

When you inhale:

  • the intercostal muscles contract, pulling the ribcage upwards and outwards
  • the diaphragm contracts, pulling downwards
  • volume of the thorax increases and the pressure inside decreases
  • air is drawn into the lungs down a pressure gradient

Breathing out (expiration)

When you exhale:

  • the intercostal muscles relax pulling the ribcage downwards and inwards
  • the diaphragm relaxes, doming upwards
  • volume of the thorax decreases and the pressure inside increases
  • air is pushed out of the lungs
The respiratory system; nasal cavity, trachea, pleural cavity (filled with fluid), intercostal muscles, ribs, diaphragm, lung, bronchiole, alveoli


The elements involved in breathing

The bell jar model

The process of ventilation as a series of changes in pressure within the thorax can be modelled using the bell jar model. Parts of the model represent different parts of the respiratory system as shown here.

2 bell jars labelled a inspiration, and b expiration. Glass tubes represent the trachea and bronchi. Balloons represent the lungs. Rubber sheets at the base of the glass jars represent the diaphragm.

The model, which is air tight, represents the thorax, and air is only able to enter via the glass tube which represents the trachea.

As the rubber sheet is pulled down the volume of the jar increases, the pressure therefore decreases and air is drawn in through the glass tube inflating the balloons, which represent the lungs.

There are a number of similarities and differences between the model and the actual respiratory system.

Glass tube/tracheaAllows air to pass through, splits into twoGlass is rigid and inflexible unlike the cartilage bound trachea
Bell jar/chest cavityAir tightUnable to move, the ribs can move up and out to increase the volume of the thorax
Balloons/lungsCan inflate and deflate, are elastic like the alveoliThe balloons are large open spaces whereas the lungs are made up of millions of individual elastic alveoli
Rubber sheet/diaphragmCan be domed up to decrease the volume in the jarThe diaphragm only flattens, it is not pulled downwards like the rubber sheet