How will an increase in lung size or volume affect the internal pressure?
When the diaphragm contracts it decreases the pressure (transpleural) in the pleural cavity, which, in turn decreases the pressure (transpulmonary) in the lungs drawing in room air until the pressure equalizes with the atmosphere. The greater the pressure gradient the faster and greater the volume change. Pleural pressure, at rest, is around -1 or -2 mmhg. During inspration it might be around -3 or -4 mmhg. It needs to always be negative pressure as this helps hold the lungs open and keeps the transthoracic pressure negative which is essential for blood coming back to the heart in the Vena Cava to flow into the chest cavity. For exhalation there doesn’t need to be the reverse pressure gradient because the lung’s elastic recoil accounts for the flow of air. So, anyway, the two change inversely. As the pressure decreases the volume increases and vice versa.
