The pressure of a given quantity of gas is inversely proportional to its volume.  
Increasing volume results in decreasing pressure.  
Decreasing volume results in increasing pressure. 
Increasing volume causes a decrease in pressure, which causes air to rush in as the pressures reach equilibrium.  
Decreasing volume causes an increase in pressure, which causes air to be expelled as pressures reach equilibrium. 




Gas Exchange- 
The process of exchanging gases between the atmosphere and the body's cells is called respiration and involves several events, including breathing. 
As we zoom into the body we can observe the respiratory system during gas exchange. 
Air passes through the alveolar sacs to the alveoli, where gas exchange occurs. 
In this cutaway view of an alveolar sac and its capillary bed, the process of gas exchange is easily observed. 
During inspiration, atmosphere air fills the alveoli and gas exchange occurs between alveolar and capillary epithelia. 
Oxygen in inspired air is represented as a white gas. 
Carbon dioxide in expired air is represented as a blue gas. 
Notice the change in color in the red blood cells during gas exchange. 
Now let's zoom into a higher magnification view of gas exchange at the respiratory membrane. 
-Gas exchange at the respiratory membrane- Red blood cells in the capillary adjacent to the alveolus release carbon dioxide and bind oxygen.