Respiratory pressures are always given in relation to atmospheric pressure (P_atm) which is the pressure exerted by the atmosphere around the body. To standardize numbers, we always set the atmospheric pressure to zero. It doesn't matter whether you are in Rexburg at nearly 5000 ft where atmospheric pressure is about 630 mmHg or at sea level which is 760 mmHg. All other pressures are compared to the atmospheric pressure of zero. Thus, a negative pressure such as -5 mm Hg describing any area in the respiratory system means that the particular respiratory area being described is 5 mm Hg lower than P_atm. When alveolar pressure (P_alv), the pressure inside the alveoli, is -1 mm Hg, inhalation will occur and air will flow into the lungs until alveolar pressure is zero (same as atmospheric pressure). Remember, there must be a pressure difference to have air flow. When P_alv is +1 mm Hg air will flow the opposite direction and exhalation will occur.

Pressure inside the pleural cavity or pleural pressure (also known as intrapleural pressure) is usually -4 mm Hg. Where does this negative pressure in relation to atmospheric come from? The fact that lymphatic vessels constantly suck fluid out of the pleural cavity, essentially creating a partial vacuum, contributes to a lower pleural pressure compared to alveolar pressure. There are also a couple of forces that attempt to pull the lungs away from the thoracic cavity wall and cause the lungs to assume the smallest size possible. The first is the lungs' inward elastic force. Think of a stretched rubber band - it would like to return to its resting state of not being stretched. The lungs are elastic and would like to be as small as possible. Secondly, there is surface tension exerted by water molecules inside the alveoli that are attracted to one another. This force attempts to make the alveoli as small as possible. The fact that our alveoli are placed in an environment (pleural cavity) that has a negative pressure makes it much easier for them to expand.

Imagine filling a balloon with air. In order to make it expand, it would be necessary to vigorously blow up the balloon. This would be an example of how positive-pressure ventilation works, in which you must force air in. In cases where a person is on a ventilator positive-pressure ventilation is used to force oxygen into the lungs. But is it also possible to get a balloon to expand by decreasing the pressure around it? Yes, it is. Put a balloon in a vacuum or take the balloon out to space and see if it expands (as long as the opening to the balloon is at atmospheric pressure). In the same way, it is easier to inflate the alveoli when they are surrounded by a partial vacuum like that in the pleural cavity. If it weren't for this lower pleural cavity pressure compared to alveolar pressure, the lungs would collapse. This is known as negative-pressure ventilation and is the way humans breathe under normal conditions. Unless there is an injury in the lining of the lungs that changes the pressures, there is always an area in the pleural space that is a lower pressure than the alveoli which creates a vacuum for air to flow from a high pressure to a low pressure inside the lungs.

The difference between the pleural pressure and alveolar pressure is called transpulmonary pressure. The larger the transpulmonary pressure the more the alveoli and the lungs will be inflated at any given moment.

Boyle’s Law and Pressures of Ventilation.
Author: T. Orton, BYU-Idaho, Winter 2017. Image modified from OpenStax, Anatomy & Physiology. https://cnx.org/contents/FPtK1zmh@8.80:u67b9E14@8/The-Process-of-Breathing#fig-ch23_03_02 License: Creative Commons Attribution License 4.0