Lung Volumes and Capacities
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Direct answer
Lung volumes are the single, non-overlapping fractions of air moved or held by the lungs, while lung capacities are combinations of two or more volumes. The four volumes are tidal volume, inspiratory reserve volume, expiratory reserve volume and residual volume; the four capacities are inspiratory capacity, functional residual capacity, vital capacity and total lung capacity. All except the residual volume — and therefore the capacities containing it — can be measured by simple spirometry.
What you must remember
- Four volumes (adult male values): tidal volume about 500 mL, inspiratory reserve volume about 3000 mL, expiratory reserve volume about 1100 mL and residual volume about 1200 mL; values are roughly 20 to 25 per cent lower in females.
- Four capacities: inspiratory capacity = tidal volume + inspiratory reserve, about 3500 mL; vital capacity = inspiratory reserve + tidal volume + expiratory reserve, about 4600 mL; functional residual capacity = expiratory reserve + residual volume, about 2300 mL; total lung capacity is about 5800 mL.
- Spirometry limits: residual volume, functional residual capacity and total lung capacity cannot be measured by spirometry and need helium dilution or body plethysmography.
- Functional residual capacity: the resting equilibrium volume where the inward recoil of the lungs balances the outward recoil of the chest wall — it keeps the alveoli open between breaths.
- Forced expiration: the FEV1 to FVC ratio is normally more than about 0.8; the mid-expiratory flow rates reflect small airway calibre.
- Obstructive pattern: FEV1 falls more than FVC, so the ratio drops, as in asthma and COPD; the volumes are normal or increased with air trapping.
- Restrictive pattern: both FEV1 and FVC fall proportionately, so the ratio is normal or even high while all volumes are small, as in pulmonary fibrosis, kyphoscoliosis or neuromuscular disease.
Common confusion
Students interchange volume and capacity definitions — a volume is a primary, single component while a capacity always sums at least two volumes. The FRC is mislabelled as a volume when it is a capacity (expiratory reserve plus residual volume). The spirometry rule is also forgotten: any parameter containing residual volume (RV, FRC, TLC) escapes the spirometer. Finally, vital capacity also falls with recumbency, ageing and poor effort, so check the FEV1/FVC ratio before calling a pattern restrictive.
Exam-focused takeaway
In theory, define each volume and capacity with average adult values, draw the spirogram with the subdivisions labelled, and contrast obstructive and restrictive patterns with the FEV1/FVC ratio. In viva, expect why residual volume cannot be expired, the clinical importance of FRC, and which methods measure it. In practicals and OSCE, perform spirometry, derive FEV1, FVC and the ratio from the tracing, and interpret a given table of values.
Frequently asked questions
What are the four lung volumes and their average values?
Tidal volume 500 mL, inspiratory reserve volume 3000 mL, expiratory reserve volume 1100 mL and residual volume 1200 mL in an adult male. Female values are about one-fifth to a quarter lower.
What is the difference between a volume and a capacity?
A volume is a single primary division of lung air, whereas a capacity is the sum of two or more volumes. For example, vital capacity adds inspiratory reserve, tidal and expiratory reserve volumes.
Which parameters cannot be measured by spirometry?
Residual volume and any capacity containing it — functional residual capacity and total lung capacity. These need helium dilution or a body plethysmograph.
What is functional residual capacity and why is it important?
It is the lung volume at the end of quiet expiration, about 2300 mL, where lung and chest wall recoils balance. It prevents alveolar collapse between breaths and keeps gas exchange continuous.