The major function of the lungs is to arterialize the venous blood. This requires the addition of sufficient oxygen to maintain normal saturation of hemoglobin in the arterial blood (about 97%-98%) and the removal of enough carbon dioxide to maintain the arterial CO2 tension at about 40 mm. Hg and the arterial pH between 7.39 and 7.41.
The lung provides a magnificent mechanism for the exchange of gases between pulmonary capillary blood and air. The surface area of the pulmonary alveoli is estimated to be as high as 90 square meters, and gas in the air sacs is separated only by alveolar-capillary membranes as thin as 0.1μ from venous blood in a capillary bed estimated to have a surface area as high as 50 square meters.
There are only a few requirements for adequate pulmonary oxygen exchange1:
1. A volume of air must be drawn past the conducting
COMROE JH. Causes and Differential Diagnosis of Cyanosis of Cardiopulmonary Origin. AMA Arch Intern Med. 1958;102(6):862-865. doi:10.1001/archinte.1958.00260230008002