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 air that enters the nasal cavity flows down the trachea. The trachea has a number of adaptations:
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.
When you inhale:
When you exhale:
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.
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/trachea||Allows air to pass through, splits into two||Glass is rigid and inflexible unlike the cartilage bound trachea|
|Bell jar/chest cavity||Air tight||Unable to move, the ribs can move up and out to increase the volume of the thorax|
|Balloons/lungs||Can inflate and deflate, are elastic like the alveoli||The balloons are large open spaces whereas the lungs are made up of millions of individual elastic alveoli|
|Rubber sheet/diaphragm||Can be domed up to decrease the volume in the jar||The diaphragm only flattens, it is not pulled downwards like the rubber sheet|