Breathing, also known as ventilation, is how the air moves into and out of the lungs. It consists of repetitive cycles of inspiration, when air flows into the lungs; expiration, when air leaves the lungs; and a brief pause, called the rest period, between these two.
Now, the direction of airflow throughout the breathing cycle depends on the difference between the atmospheric pressure, which is the pressure of the air in the environment, and the alveolar pressure, or the pressure inside the alveoli, which are the tiny sacs of air where gas exchange happens in the lungs.
An additional parameter is the intrapleural pressure, also called the intrathoracic pressure, which is the pressure of the fluid inside the pleural cavity that surrounds the lungs.
Intrapleural pressure is usually negative compared to the alveolar or atmospheric pressure, and this is important because the alveolar pressure minus the intrapleural pressure gives the transmural pressure.
As long as the transmural pressure stays positive, the airways remain open throughout all of the phases of the breathing cycle.
Ok, now, normal, quiet breathing involves inspiration and expiration of a tidal volume, or VT for short, of about 500 mL, which includes the volume of air that fills the alveoli plus the volume of air that fills the airways.
Now, according to what is known as Boyle’s law, at a constant temperature, pressure and volume are inversely related to each other, so when the alveolar pressure decreases, more air will enter the lungs, increasing the air volume
With that in mind, let’s establish the starting point for these variables by looking at the lungs during the rest phase of the breathing cycle.
During rest, the diaphragm is at its balanced position. The alveolar pressure equals the atmospheric pressure to a value of zero centimeters H2O, so there is no pressure gradient, and no air is moving into or out of the lungs.