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Alveolar ventilation is defined as the volume of fresh air entering and alveolar air leaving the alveoli per minute. The two volumes are not usually exactly equal because not every molecule of oxygen used by the body results in a molecule of CO2 released by the body, and also because of the heating and humidification of inspired air described in Chapters 25 and 34. Alveolar ventilation is not the same as the ventilation per minute measured at the nose or mouth because not all of the air entering the body per breath reaches the alveoli. A substantial portion of each inspired breath remains in the conducting airways and does not enter the alveoli.


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 respiratory muscles. Standardization of the conditions under which lung volumes are measured allows comparisons to be made between normal subjects and patients. Because the size of a person’s lungs depends on their height and weight or body surface area, as well as age and sex, the lung volumes for a patient are usually compared with data from a table of predicted lung volumes matched to age, sex, and body size. The lung volumes are normally expressed at body temperature, ambient pressure, and saturated with water vapor (BTPS). Note that to obtain most of the standard lung volumes and capacities, the subject must be conscious and cooperative. Subjects or patients unwilling or unable to fully comply with the instructions they are given for these tests will give erroneous data.

There are four standard lung volumes and four standard lung capacities, which are combinations of the standard lung volumes. These are shown in Figure 27-1.

Figure 27-1

The standard lung volumes and capacities. Typical values for a 70-kg adult are shown. (Reproduced with permission from Levitzky MG: Pulmonary Physiology, 9th ed. New York, NY: McGraw Hill; 2018.)

Tidal Volume

The tidal volume (TV, or VT) is the volume of air coming into or out of the nose or mouth per breath. It is determined by the activity of the respiratory control centers in the brain, the contraction of respiratory muscles, and the mechanics of the lung and chest wall. During normal quiet breathing (eupnea), the TV of a 70-kg adult is about 500 ml per breath, but this can be greatly increased, as in exercise.

Residual Volume

The residual volume (RV) is the volume of gas left in the lungs after a maximal forced expiration. It is determined by the force generated by the muscles of expiration acting in concert with the inward elastic recoil of the lungs, ...

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