Intrapulmonary pressure is the pressure within the lungs. This pressure varies between inspiration and expiration. During inspiration intrapulmonary pressure becomes less than atmospheric pressure, and during expiration it becomes greater than atmospheric pressure, according to McGraw-Hill Higher Education.
When a person takes a breath, air is moved into the lungs because the diaphragm and the intercostal muscles contract, causing an increase in the thoracic volume. According to Boyle’s law there is an inverse relationship between the volume and pressure of gases; therefore, an increase in the volume inside of the thorax causes a decrease in the pressure inside of the thorax, according to NASA.
This decrease in pressure creates a difference in pressures, or pressure gradient, between the intrapulmonary pressure in the lung and the outside atmospheric pressure, according to McGraw-Hill Higher Education. This can be thought of as pulling a balloon from all sides at once, causing air to rush into the balloon. It is this pressure difference that becomes the driving force for air movement, causing air to enter the lungs from the outside.
When a person exhales, the muscles of respiration relax, causing an elastic recoil of the lungs, like a balloon full of air being opened. The volume inside of the thorax decreases and conversely the pressure inside of the lungs increases, becoming greater than the atmospheric pressure, causing air to move out of the lungs.