Question

what does each part of the constructed lung model represent

Answer 1

You forgot to include the diagram but I will still answer this question. The Diagram is supposed the function of lungs as we breathe, and goes to show the movement of the lung and it's diaphragm as we inhale and exhale oxygen.
The balloons go to show how when we inhale air, the lungs fill up, while when he exhale the lungs get empty, shown in the diagram through a deflated balloon 
In order for this model to work, a human is physically pulling and pushing the diaphragm.

Answer 2

"In the lung model, respiration process is explained with the help of some objects like plastic bottles, plastics tube, small balloons, straws, and also big balloons. These objects are arranged in such a way that they make a complete respiration system.

In the model,

1. Plastic bottle represents chest cavity.
2. Straws represents bronchi.
3. Lungs are represented by small balloons.
4. The diaphragm is represented by the plastics tube which is connected to the big balloons.

So, this model is constructed in order to make things easy to understand."