Breathing and Exchange of Gases
Respiratory volumes and capacities
Calculate lung capacities from the volumes.
The amount of air you breathe depends on how hard you breathe. These amounts have names:
- Tidal volume (TV): the air breathed in or out in one normal, quiet breath: about 500 mL. So a healthy person breathes about 6000 to 8000 mL of air a minute.
- Inspiratory reserve volume (IRV): the extra air you can breathe in with a forced deep breath: 2500 to 3000 mL.
- Expiratory reserve volume (ERV): the extra air you can force out after a normal breath out: 1000 to 1100 mL.
- Residual volume (RV): the air that still stays in the lungs even after the strongest breath out: 1100 to 1200 mL. It can never be breathed out; it keeps the alveoli from collapsing.
Adding volumes gives capacities:
- Inspiratory capacity = TV + IRV: all the air you can breathe in after a normal breath out.
- Expiratory capacity = TV + ERV: all the air you can breathe out after a normal breath in.
- Functional residual capacity = ERV + RV: the air left in the lungs after a normal breath out.
- Vital capacity = ERV + TV + IRV: the most air you can breathe in after forcing all you can out (or the most you can breathe out after the deepest breath in).
- Total lung capacity = RV + ERV + TV + IRV, which is vital capacity + residual volume: all the air the lungs can hold.
Doctors measure these capacities to help diagnose lung problems.
Vital capacity = ERV + TV + IRV; total lung capacity = vital capacity + residual volume.
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