Question

Why is the intrapleural pressure negative rather than positive?

Answer 1

Intrapleural pressure is the pressure within the pleural cavity. Intrapleural pressure is always negative, which acts like a suction to keep the lungs inflated. ... The surface tension of the alveolar fluid tends to pull each of the alveoli inward and therefore pulls the entire lung inward.

Answer 2

The intrapleural pressure refers to the pressure inside the pleural cavity (space between the lung and pleura). The pressure inside the pleural cavity is usually lesser than the atmospheric pressure, hence we refer to the pleural pressure as negative pressure. The movement of lungs depend upon the pressure gradient between lungs and pleura (transpulmonary pressure, the difference between intra pulmonary pressure and intra pleural pressure). When we breath in, the pressure inside the pleural cavity becomes more negative, trans pulmonary pressure increases, and the lungs expand. During expiration, the pleural pressure increases and the lungs recoil.