Ventilation is a mechanical process.
Gas exchange occurs by diffusion from differential concentrations of O 2 and CO 2 . The intrapleural space is potential in health patients, with the parietal and visceral pleurae wet and sticking to each other, as a result of a subatmospheric pleural pressure . Therefore, if the thoracic wall changes in size, so should the lungs. Hg above atmospheric. Typically, there is a slightly negative interstitial pressure set up by pulmonary capillaries and lymphatics.