The air in the lungs is divided into volumes, and capacities are sums of two or more volumes. These numbers help sort lung disease into obstructive and restrictive patterns.
Volumes
Tidal volume is the air moved in a normal breath. The inspiratory and expiratory reserve volumes are the extra air you can breathe in or out with effort, and the residual volume is the air that always stays in the lungs.
Capacities
Vital capacity is the most air you can breathe out after a full breath in, and total lung capacity adds the residual volume. Functional residual capacity is the air left after a normal breath out; it keeps oxygenating the blood between breaths.
Spirometry
Spirometry measures how much air can be blown out and how fast. FEV1 is the volume blown out in the first second and FVC is the total forced volume; residual volume cannot be measured by spirometry alone.
Obstructive and restrictive patterns
In obstructive disease such as asthma or COPD, air comes out slowly, so the FEV1/FVC ratio falls below about 0.70. In restrictive disease such as pulmonary fibrosis, the lungs hold less air but the ratio stays normal or high.
| Measure | What it is | Typical young adult |
|---|---|---|
| Tidal volume | Air moved in a normal breath | About 500 mL |
| Residual volume | Air left after a full breath out | About 1200 mL |
| Functional residual capacity | Air left after a normal breath out | About 2400 mL |
| Vital capacity | Largest breath out after a full breath in | About 4800 mL |
| Total lung capacity | All the air the lungs can hold | About 6000 mL |
At the bedside
Coach spirometry with a deep full breath in, a hard fast blast and steady blowing for at least 6 seconds. A poor effort makes FVC look low and can mimic a restrictive pattern.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).