Through the process of breathing, a combination of oxygen and nitrogen gases enters the respiratory system. As the gases travel the vascular system, carbon dioxide travels the opposite pathway in exchange for oxygen. Carbon dioxide diffuses into the capillary at the venous end and returns to the lungs via the pulmonary artery and arterioles. Once in the pulmonary bed, it diffuses across the alveolar-capillary interspace to enter the alveolus for expiration. Approximately 10% of carbon dioxide travels in its dissolved state in plasma. It is the dissolved CO₂ that is measured as the pCO₂ with a range of 35-45 mm Hg. This range closely resembles that of the partial pressure of gases in the alveoli. Any increase in the amount of CO₂ easily can tilt the pH balance to an acidic state. Just as easily, any decrease in CO₂ can result in an increase in alkalinity. Of the remaining carbon dioxide, 30% is attached to hemoglobin and 60% is transported as bicarbonate. The dissolved CO₂ is carried to the alveoli of the lungs while the remaining diffuses into tissue spaces and capillaries.