Negative pressure is created in the lungs by lowering the diaphragm and expanding the ribcage, a mechanism used to draw air into the lungs (called inspiration). This is possible because air follows a pressure gradient moving from an area of higher pressure (atmospheric air) to an area of lower pressure (the air sacs of the lungs (i.e. the intraalveolar space)). A diagram of this process is shown below.
As the diaphragm relaxes and the ribs return to their resting position, the pressure within the lungs becomes greater than the atmospheric pressure outside the body resulting in passive exhaling of the air (called expiration). This is the process by which we (along with many other animals) breathe.
Because it involves muscle contractions, breathing normally requires the use of energy. You can imagine then, that exercise would require more energy as the rate of breathing increases. Kangaroos are pretty special because they actually use less energy to breathe during hopping than used at rest. So how does this work? Imagine watching live x-ray images of a kangaroo in motion…you will see that as it hops, the abdominal organs move up and down within the body but this movement lags slightly behind the hopping motion. As the animal hops up, the abdominal organs remain lower in the body pulling the diaphragm down to create the negative pressure within the lungs needed to inspire. On the descent, the abdominal organs again lag behind and impact the diaphragm producing greater pressure within the lungs than the outside air resulting in expiration.
This article from Advances in Physiology Education describes a pretty neat experiment you can do at home (with the right equipment) to model how kangaroos breathe while hopping.
Giuliodori MJ, Lujan HL, Janbaih H, DiCarlo SE. How Does a Hopping Kangaroo Breathe? Adv Physiol Educ. 34: 228-232, 2010.
Kangaroo Image Credit: Norbert Wu/Getty Images