USMLE Step 1 Respiratory Physiology Review 60 10 Relationship between Lung Capacities and Volumes

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Play USMLE Audio MP3 60 10 Relationship between Lung Capacities and Volumes Below

Begin 60 10 Relationship between Lung Capacities and Volumes Transcription

As a rule, any lung capacity can be expressed as the sum of two or more of the lung volumes. For example, how can total lung capacity be expressed in terms of lung volume?

• TLC is the sum of all four lung volumes. That is, tidal volume plus IRV plus ERV plus residual volume.

For each of the following lung volumes, give the percent of normal total lung capacity that it represents.
First, normal tidal volume.

• About 8.3% of total lung capacity.

Second, normal inspiratory reserve volume.

•  About 55% of total lung capacity. 

Third, normal expiratory reserve volume.

• About 16.7% of total lung capacity.

Fourth, normal residual volume.

• About 20% of total lung capacity.

How can the following lung capacities be defined in terms of lung volumes?
First, inspiratory capacity.

• Inspiratory capacity is tidal volume plus inspiratory reserve volume.

Second, vital capacity.

• Vital capacity is expiratory reserve volume plus tidal volume plus inspiratory reserve volume.

Third, functional residual capacity.

• FRC is residual volume plus expiratory reserve volume.

Why is residual volume, that is, the air that can never be expelled included in functional residual capacity?

•  Residual volume mixes with incoming air via turbulence and so is reasonably well oxygenated. Another way of looking at vital capacity is that it’s everything but residual volume.

Student doctor please pause the tape and summarize what we’ve discussed about the relationship between lung capacities and lung volumes.

• Any of the four lung capacities can be expressed as the sum of two or more of the lung volumes. For example, total lung capacity is the sum of all four lung volumes – tidal volume plus inspiratory reserve volume plus expiratory reserve volume plus residual volume. Normal tidal volume is about 8.3% of total lung capacity. Normal inspiratory reserve volume about 55%. Normal expiratory reserve volume about 16.7% and normal residual volume is about 20% of total lung capacity. Inspiratory capacity is tidal volume plus inspiratory reserve volume. Functional residual capacity is residual volume plus expiratory reserve volume. Residual volume is included in functional residual capacity because even though the residual volume is the volume of air that can never be expelled, it is mixed with incoming air via turbulence and so is reasonably well oxygenated. Vital capacity is expiratory reserve volume plus tidal volume plus inspiratory reserve volume. Another way of looking at vital capacity is that it is everything but residual volume.

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