Lung Function
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Direct answer
One quiet breath moves about 500 mL of air (tidal volume) in and out of roughly 2300 mL sitting in the lungs at end-expiration — the functional residual capacity, the equilibrium point where lung recoil and chest-wall recoil balance, keeping alveolar gas tensions steady between breaths. Spirometry traces every volume except residual volume, and therefore every capacity that contains it — FRC and total lung capacity need helium dilution or body plethysmography. The FEV1/FVC ratio sorts the two great patterns of lung disease: below 0.7 means obstruction (asthma, COPD), while small volumes with a normal or high ratio point to restriction, confirmed by a reduced TLC.
What you must remember
- Adult male volumes: tidal 500 mL, inspiratory reserve 3000, expiratory reserve 1100-1200, residual 1200; capacities: vital capacity about 4600, inspiratory capacity 3500, FRC 2300-2400, TLC 5800 mL — female values roughly 20-25 per cent lower.
- RV, FRC and TLC are invisible to the spirometer because the subject cannot exhale below residual volume; helium dilution, nitrogen washout or plethysmography supply them.
- Pattern rules: obstruction lowers FEV1/FVC below 0.7 with air trapping (raised RV and RV/TLC); restriction shrinks all volumes but keeps the ratio normal or raised.
- Bronchodilator reversibility above 12 per cent and 200 mL separates asthma from largely irreversible COPD; peak expiratory flow tracked at home gauges asthma control.
- Compliance of lungs plus chest wall is about 0.1 L per cm of water: reduced in fibrosis, pulmonary oedema, ARDS and surfactant deficiency, increased in emphysema and old age as elastin is lost.
- Surfactant is dipalmitoyl phosphatidylcholine from type II pneumocytes, appearing from about 24-28 weeks; lecithin-sphingomyelin ratio of 2 in amniotic fluid signals maturity by 34-35 weeks, and its absence explains neonatal respiratory distress syndrome — prevented by antenatal steroids.
- Dead space: anatomic 150 mL measured by Fowler's method; alveolar ventilation is (500 minus 150) times 15, near 5.25 L/min against a minute ventilation of 7.5 L/min.
Interpreting a spirometry report step by step
Read every report in a fixed order and the pattern falls out. First the ratio: a smoker of thirty years reports FEV1 45 per cent of predicted with FVC 80 per cent, ratio near 0.55 — obstructed. Second, the volumes you cannot exhale: residual volume and TLC are raised, confirming hyperinflation with air trapping rather than restriction. Third, the diffusing capacity: DLCO collapsed in emphysema because alveolar-capillary membrane is destroyed, but normal in pure asthma where the membrane is intact — the cleanest physiological separator of the two obstructive diseases.
Now run the contrast case: a woman with progressive dry breathlessness shows FEV1 58 per cent and FVC 62 per cent of predicted — ratio near 0.9, both small. That proportionate reduction is the restrictive signature, and the reduced TLC on plethysmography confirms it; a collapsed DLCO then localises the cause to the interstitium (fibrosis) rather than the chest wall or neuromuscular weakness, which reduce lung volumes with a normal membrane. One more shape completes the set: the flow-volume loop. Scooping of the expiratory limb means obstruction; a tiny symmetrical loop means restriction; flattening of both limbs at a fixed ceiling means a stenosis in the trachea or upper airway that no lower-airway disease produces.
Where students slip
Three slips repeat in every batch. Candidates forget why FRC changes: emphysema raises it because elastic recoil is lost, obesity lowers it because the chest wall is loaded — a sedated obese patient supine can dip below closing volume and desaturate, the physiology behind post-operative hypoxaemia. They call RV raised only in obstruction, missing that in restriction every volume including RV is proportionately small. And they quote FEV1 alone as severity in isolation, forgetting the ratio is the classifier and the DLCO is the localiser — a low DLCO with normal spirometry should send you looking for anaemia, pulmonary embolism or early interstitial disease, not reassurance.
Frequently asked questions
Which volumes cannot be measured by spirometry and why?
Residual volume plus FRC and TLC, because they contain air that can never be exhaled; helium dilution or plethysmography measures them.
What defines obstruction on pulmonary function testing?
FEV1/FVC below 0.7 with reduced FEV1 and raised residual volume; bronchodilator response above 12 per cent and 200 mL favours asthma.
What does surfactant do and when does it mature?
Dipalmitoyl phosphatidylcholine from type II pneumocytes lowers surface tension by Laplace's law, stabilising alveoli; production matures around 34-35 weeks, marked by a lecithin-sphingomyelin ratio of 2.
How do obstructive and restrictive patterns differ?
Obstruction: low ratio, air trapping, hyperinflation, often low DLCO; restriction: proportionately small volumes, normal or high ratio, confirmed by reduced TLC.
Why is alveolar ventilation less than minute ventilation?
About 150 mL of each 500 mL breath only fills the anatomic dead space, so alveolar ventilation is roughly 5.25 L/min against 7.5 L/min of minute ventilation.
What raises functional residual capacity and what lowers it?
Loss of elastic recoil (emphysema, ageing) raises FRC; obesity, supine posture and pregnancy lower it by loading or splinting the chest wall.