The pulmonary |arterioles| and |venules| wind along the |bronchioles|, then branch out to form dense webs of |capillaries| that surround each aveolus. Some 300 million |alveoli| fill the two |lungs|. Enveloped in a thin film of water, the |alveoli| and their |capillaries| make up the respiratory membrane. The tiny |capillaries| of the |lungs| hold just three ounces of ~blood~, which is spread thinly, making it possible for oxygen and carbon dioxide to be exchanged in just a quarter of a second. As ~blood~ flows through the |lung| |capillaries|, carbon dioxide leaves it and oxygen enters it. Venous ~blood~ enters |lung| |capillaries| (these are the only |capillaries| where this occurs. Arterial ~blood~ enters all other |capillaries|). The hemoglobin in the red |blood cells| of venous ~blood~ is combined with carbon dioxide rather than oxygen, and is called carbaminohemoglobin. As the ~blood~ flows through the |lung| |capillaries| the carbaminohemoglobin is broken down into carbon dioxide and hemoglobin. The hemoglobin picks up oxygen molecules, while the carbon dioxide molecules move out of the ~blood~ and are expelled from the |lungs|. This process changes the venous ~blood~ to arterial ~blood~, which is then circulated through the body.
