Respiration, or breathing, is the process by which gas exchange occurs in the body. When at rest, there is no airflow into, or out of the lungs. The atmospheric pressure, or the pressure of the air around you, is equal to the pressure in your lungs, which is known as the intrapulmonary pressure.
As you breathe in, the diaphragm flattens to allow your lungs to expand as they fill with air. The intrapulmonary pressure falls below the atmospheric pressure, creating a vacuum in the lungs. The lungs suck in air until the atmospheric and intrapulmonary pressures are once again the same.
When the diaphragm relaxes and rises, the intrapulmonary pressure is higher than the atmospheric pressure and expiration occurs.
Let's watch the process of respiration one more time.