Added to make certain structure easier to see.
It would help if you had question or attached an image.
Answer: 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.
Not applicable ): in all situations
