You will observe:The belljarrepresentsthehumanchest cavity (the rib
cage). Therubbersheeting across thebottomrepresentsthe flexible,muscular
diaphragm. Theballoonsrepresentthehumanlungs.
When the rubber sheeting (the diaphragm) is pulled down, the chest
cavity is made larger, and air rushes into the lungs. When we breathein
(inhale) the ribs in anormalchest cavity move outwardand the diaphragm
moves downward. When the rubber sheeting is released it returns to its
originalposition. This representsthereturnof the diaphragm(a musclethat
separatesthe chest from theabdominalcavity) to itsnormalposition. In the
humanbody, the ribs move inward, back to place at the same time. This
actionsqueezes the air outofthe lungs and webreatheout(exhale).
The wastecarbondioxide in the bloodstimulatesa breathingcenterin
the brain, the medulla. Inturn, the medulla stimulatesthe muscles ofthe
diaphragmto move back upwardinto their normalposition. The ribs, too,
move back into place,makingthe chest cavity decrease in size and putting
pressure on the filled lungs. When webreatheoutor exhale, ourlungs are
squeezedtogether(asillustratedby the collapse of theballoonsonyourmodel).