Pulmonary Surface Area 46. Make a hole in the chest wall, eg. The inward pull of the lungs on the chest wall causes a negative intrapleural pressure. To breathe out just relax, elastic recoil of the stretched lungs does the rest. In alveolar air the PO2 is about 100 mm Hg.
Gases diffuse from regions of higher pressure to lower pressure.
So blood leaves the lungs with oxygen in equilibrium with alveolar air. So arterial blood PCO2 regulates ventilation. The body makes only a weak response to lack of oxygen. The respiratory system provides O2 to the body and removes CO2 produced by tissues and transported by the bloood. Not all air that you breathe enters alveoli for gas exchange.