Ventilation-Perfusion Relationships

  • The partial pressure of carbon dioxide and oxygen in an alveolus is largely determined by the relative rates of alveolar ventilation and pulmonary perfusion (perfusion) encountered by that alveolus. Because the partial pressure of CO2 and O2 within alveoli largely determine the tensions of these gases in arterial blood, the relationship between alveolar ventilation and perfusion is of significant physiological importance. We first begin with a highly qualitative discussion of the alveolar air composition followed by a description of how the partial pressures of alveolar carbon dioxide and alveolar oxygen quantitatively vary based on the relative rates of ventilation and perfusion. Finally, we discuss the physiological concept of the Ventilation-Perfusion Ratio and describe how a normal lung displays regional variations of this ratio in Ventilation-Perfusion Ratio Distribution.