Lung Capacity
Lung capacity refers to the volumes of air associated with the different phases of the respiratory cycle:
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Total lung capacity – Volume of air in the lungs after a maximal inhalation (~ 6 litres in a normal adult male)
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Vital capacity – Volume of air that can be exchanged by the lungs via a maximal inhalation and exhalation
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Residual volume – Volume of air that is always present in the lungs (~ 20% of total lung capacity)
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Tidal volume – Volume of air that is exchanged via normal breathing (~ 500 ml per breath)
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Inspiratory Reserve Volume – Volume of air able to be inspired with maximum effort after a normal inhalation
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Expiratory Reserve Volume – Volume of air able to be expired with maximum effort after a normal exhalation
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Inspiratory Capacity – The sum of the inspiratory reserve volume (IRV) and tidal volume (VT)
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Functional Residiual Capacity – Sum of the expiratory reserve volume (ERV) and residual volume (RV)
The ventilation rate (breathing frequency) for a typical adult at rest is roughly 12 – 16 breaths per minute
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Ventilation rate can be substantially increased with physical activity (maximal increase is roughly 5 – 6 fold over normal)
A number of factors contribute to an individual’s total lung capacity and ventilation rate, including:
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Height (taller people tend to have larger chests and hence larger total lung capacities)
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Location (people living at high altitudes tend to have larger capacities to compensate for the lower atmospheric pressure)
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Lifestyle (obese people and smokers tend to have lower capacities and higher rates of ventilation)
Measuring Ventilation
Ventilation in humans can be monitored in a number of ways:
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Via simple observation (counting number of breaths per minute)
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Chest belts and pressure meters (recording the rise and fall of the chest)
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Spirometers (recording the volume of gas expelled per breath)
Spirometry involves measuring the amount (volume) and / or speed (flow) at which air can be inhaled or exhaled
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A spirometer is a device that detects the changes in ventilation and presents the data on a digital display
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A more simplistic method involves breathing into a balloon and measuring the volume of air in a single breath
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The volume of air can be determined by submerging the balloon in water and measuring the volume displaced (1ml = 1cm3)
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Overview of Lung Capacities
