Chapter 3. Alveolar Ventilation

The reader understands the ventilation of the alveoli.

Defines alveolar ventilation.
Defines the standard lung volumes and understands their measurement.
Predicts the effects of alterations in lung and chest wall mechanics, due to normal or pathologic processes, on the lung volumes.
Defines anatomic dead space and relates the anatomic dead space and the tidal volume to alveolar ventilation.
Understands the measurement of the anatomic dead space and the determination of alveolar ventilation.
Defines physiologic and alveolar dead space and understands their determination.
Predicts the effects of alterations of alveolar ventilation on alveolar carbon dioxide and oxygen levels.
Describes the regional differences in alveolar ventilation found in the normal lung and explains these differences.
Predicts the effects of changes in lung volume, aging, and disease processes on the regional distribution of alveolar ventilation.
Defines the closing volume and explains how it can be demonstrated.
Predicts the effects of changes in pulmonary mechanics on the closing volume.

Alveolar ventilation is the exchange of gas between the alveoli and the external environment. It is the process by which oxygen is brought into the lungs from the atmosphere and by which the carbon dioxide carried into the lungs in the mixed venous blood is expelled from the body. Although alveolar ventilation is usually defined as the volume of fresh air entering the alveoli per minute, a similar volume of alveolar air leaving the body per minute is implicit in this definition.

The volume of gas in the lungs at any instant depends on the mechanics of the lungs and chest wall and the activity of the muscles of inspiration and expiration. The lung volume under any specified set of conditions can be altered by pathologic and normal physiologic processes. Standardization of the conditions under which lung volumes are measured allows comparisons to be made among subjects or patients. The size of a person’s lungs depends on his or her height and weight or body surface area, as well as on his or her age and sex. Therefore, the lung volumes for a patient are usually compared with data in a table of “predicted” lung volumes matched to age, sex, and body size. The lung volumes are normally expressed at the Body Temperature and ambient Pressure and Saturated with water vapor (BTPS).

The Standard Lung Volumes and Capacities

There are 4 standard lung volumes (which are not subdivided) and 4 standard lung capacities, which consist of 2 or more standard lung volumes in combination (Figure 3–1).