IT is forced into your lungs by the contraction of your diaphragm because the hair you breath is from your lungs and it passes through the diaphragm which causes the contraction
Diaphragm is a term used to name a muscle located between the chest and abdominal cavities.
In respiratory movements, the diaphragm has a primordial function, since the movement of this muscle significantly alters the vertical size of the chest. The fibers of the diaphragm muscle converge to a tendon region, called the tendon center, located in the center of the dome. When the muscle contracts on inspiration, the dome is pulled and the structure is depressed. When relaxation on exhalation occurs, the diaphragm rises.
With that, we can conclude that when you take a deep breath before diving underwater, you are inhaling and pulling the air into your lungs. This is only possible by contraction of the diaphragm.
