Chapter 5. Ventilation-Perfusion Relationships

The reader understands the importance of matching of ventilation and perfusion in the lung.

• Predicts the consequences of mismatched ventilation and perfusion.
• Describes the methods used to assess the matching of ventilation and perfusion.
• Describes the methods used to determine the uniformity of the distribution of the inspired gas and pulmonary blood flow.
• Explains the regional differences in the matching of ventilation and perfusion of the normal upright lung.
• Predicts the consequences of the regional differences in the ventilation and perfusion of the normal upright lung.

Gas exchange between the alveoli and the pulmonary capillary blood occurs by diffusion, as will be discussed in the next chapter. Diffusion of oxygen and carbon dioxide occurs passively, according to their concentration differences across the alveolar-capillary barrier. These concentration differences must be maintained by ventilation of the alveoli and perfusion of the pulmonary capillaries.

Alveolar ventilation brings oxygen into the lung and removes carbon dioxide from it. Similarly, the mixed venous blood brings carbon dioxide into the lung and takes up alveolar oxygen. The alveolar and are thus determined by the relationship between alveolar ventilation and pulmonary capillary perfusion. Alterations in the ratio of ventilation to perfusion, called the , will result in changes in the alveolar and , as well as in gas delivery to or removal from the lung.

Alveolar ventilation is normally about 4 to 6 L/min and pulmonary blood flow (which is equal to cardiac output) has a similar range, and so the for the whole lung is in the range of 0.8 to 1.2. However, ventilation and perfusion must be matched on the alveolar-capillary level, and the for the whole lung is really of interest only as an approximation of the situation in all the alveolar-capillary units of the lung. For instance, suppose that all 5 L/min of the cardiac output went to the left lung and all 5 L/min of alveolar ventilation went to the right lung. The whole lung would be 1.0, but there would be no gas exchange because there could be no gas diffusion between the ventilated alveoli and the perfused pulmonary capillaries.

Oxygen is delivered to the alveolus by alveolar ventilation, is removed from the alveolus as it diffuses into the pulmonary capillary blood, and is carried away by blood flow. Similarly, carbon dioxide is delivered to the alveolus in the mixed venous blood and diffuses into the alveolus in the pulmonary capillary. The carbon dioxide is removed from the alveolus by alveolar ventilation. As will be discussed in Chapter 6, at resting cardiac outputs the diffusion of both oxygen and carbon dioxide is normally limited by pulmonary perfusion. Thus, the alveolar partial pressures of both oxygen and carbon dioxide are determined by the . If the in an alveolar-capillary unit increases, the delivery of oxygen relative to its removal will increase, as will the removal ...