Why is the thorax airtight? And why the two lungs are in seperate, airtight compartments?

Question

I've asked many people this question, and it is part of my science homework to find out...
if you know the answer, please tell me!!!=]

and its best if you list the source if you have one =] thanks

Answer 1

The thorax should be airtight so that the change in pressure forces us to breathe air in and out.

For the efficient exchange of gases (uptake of oxygen and release of carbon dioxide), the air around the actual exchange surface must not be allowed to become stale. I.e. the air must be ventilated. This process involves drawing in fresh air, inhalation, and removing some of the used air, exhalation. Air will be pushed into the lungs, which are in an airtight chamber called the thorax, if the pressure inside falls below the pressure outside. This is achieved by increasing the volume of the chest by pulling down the normally domed diaphragm and raising and moving out the ribcage. Both of these are achieved by the contraction of muscles. The air entering the lungs stretches them, and when the intercostal and diaphragm muscles relax the elastic tissues in the lungs recoil, forcing the air out.

Answer 2

Read your books provided for you to answer this homework question.