Negative intrapleural pressure is due to

Pleural pressure is the pressure of the fluid in the narrow space between the lung and the chest wall pleura which is normally a slight suction, which means a slightly negative pressure. There are no attachments between the lung and the walls of the chest cage except where it is suspended at its hilum from the mediastinum. Instead, the lung literally floats in the thoracic cavity. Fuhermore, continual suction of excessive fluid into lymphatic channels maintain a slight suction between the visceral surface of the lung pleura and the parietal pleural surface of the thoracic cavity. Therefore, the lungs are held to the thoracic wall as if glued there, except that they can slide freely, well lubricated as the chest expands and contracts. A concept from Boron&;s physiology/2nd Ed/ 630 The interaction between lungs and chest wall does not occur by direct attachment but through the intrapleural space between the visceral and parietal pleurae. This space is filled with a small amount of pleural fluid and is extremely thin (5 to 30 micrometer). Because the lungs and chest wall pull away from each other on opposite sides of the intrapleural space, the intrapleural pressure (PIP) is less than barometric pressure (PB); that is; the intrapleural space is a relative vacuum. Although the designation P implies that we are referring exclusively to the intrapleural space, this description is not entirely accurate. Indeed, in addition to the intrapleural space, PIP is probably similar to the pressure in several other regions of the chest cavity.The viual space between the chest wall or diaphragm and the parietal pleura.The viual space between the lung and the visceral pleura.The interstitial space that surrounds all pulmonary airways.Around the Hea and Vessels.Around and to the extent that smooth muscle tone can be neglected- inside the Esophagus.Ref: Guyton/10th Ed/ 432