The Bell Jar Model (WJEC GCSE Biology)
Revision Note
The Bell Jar Model
The process of ventilation can be modelled using a bell jar to illustrate inspiration and expiration
The different parts of the model represent different parts of the respiratory system
The bell jar model diagram
Parts of the bell jar model represent different parts of the human respiratory system
The model is airtight and air is only able to enter via the glass tube, which represents the trachea
As the rubber sheet, representing the diaphragm, is pulled down the volume of the jar increases, so the pressure therefore decreases
Air is drawn in through the glass tube inflating the balloons, which represent the lungs
This models the process of inspiration
When the rubber sheet is released the volume of the jar decreases so the pressure increases and the air is drawn out of the balloons deflating them
This models expiration
Limitations of this model
The glass tube, representing the trachea, is rigid unlike the flexible cartilage rings of the trachea
The bell jar, representing the thorax, is unable to move unlike the chest cavity and ribs which can move up and down
The balloon, representing the lungs, are hollow and air-filled, whereas the lungs are filled with millions of individual alveoli
The rubber sheet, representing the diaphragm, is pulled down whereas the diaphragm only flattens out
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