Negative intrapleural pressure is due to:

absorption by lymphatics "A negative force is always required on the outside of the lungs to keep the lungs expanded. This is provided by negative pressure in the normal pleural space. The basic cause of this negative pressure is pumping of fluid from the space by the lymphatics (which is also the basis of the negative pressure found in most tissue spaces of the body)."- Guyton 11/e p489 The intrapleural pressure is usually about -7 mm Hg (a few millimeters of mercury more negative than the collapse pressure of the lungs). The intrapleural pressure is more negative at the apex of lung than at bases.