When we breath in, the volume of our lungs increases and when we exhale it decreases. Based on Boyle's Law, we know that when the volume increases, the pressure of the lungs decrease.
This decrease in pressure causes the internal pressure in the lungs to be less than the atmospheric pressure. This gradient allows for air to rush into the lungs, enabling the uptake of oxygen. And the opposite occurs when we breath out.
Thinking about this concept we can examine a problem:
When you are scuba diving you are told not to hold your breath and we will see why.
Imagine you take a deep breath and fill your lungs will 5 liters of air and you are 20 meters below the surface (let's say the tank is filled with normal air). At 20 meters below the surface you experience about 3 atms.
P1*V1 = P2*V2
So, if you were to swim up and hold your breath the volume of air would be 15 liters!