The lung is the critical organ that transports oxygen from the atmosphere into the bloodstream and releases carbon dioxide from the bloodstream into the atmosphere. Respiration is driven by muscular action. The diaphragm increases volume and decreases pressure by pulling the cavity that holds the lung downward.

"Unlike amphibians and birds, mammals employ negative pressure breathing - pulling, rather than pushing, air into their lungs. Using muscle contraction to actively expand the thoracic cavity, mammals lower air pressure in their lungs below that of the air outside of their body. ... A mammal breathes by changing the air pressure within its lungs to the pressure of the outside atmosphere."
- Campbell Biology 9th Edition, Chapter 42, Circulation and Gas Exchange, Page 921

To build a lung model, you will connect a balloon to a funnel with tape and place inside a plastic bottle.

Stretch the balloon by inflating and deflating it multiple times.

Tape the balloon to small end of funnel and drop the balloon into the plastic bottle.

Demonstrate the lung exhale by squeezing the plastic bottle.

Demonstrate the lung inhale by releasing the plastic bottle.