Therefore, a highly quantitative concept has been developed to help us understand res-piratory exchange when there is imbalance between alveolar ventilation and alveolar blood flow. In either of these conditions, gas exchange through the respiratory mem-brane is seriously impaired, and the person may suffer severe respiratory distress despite both normal total ventilation and normal total pulmonary blood flow, but with the ventilation and blood flow going to different parts of the lungs. However, even normally to some extent, and especially in many lung diseases, some areas of the lungs are well ventilated but have almost no blood flow, whereas other areas may have excellent blood flow but little or no ventilation. These earlier discussions made the assumption that all the alveoli are ventilated equally and that blood flow through the alveolar capillaries is the same for each alveolus. In the early, we learned that two factors determine the P O 2 and the P CO 2 in the alveoli: (1) the rate of alveolar ventilation and (2) the rate of trans-fer of oxygen and carbon dioxide through the respira-tory membrane. Effect of the Ventilation-Perfusion Ratio on Alveolar Gas Concentration
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