Physiology chapters
Lung mechanics, volumes and pulmonary function tests
How we breathe in and out, the lung volumes and capacities, dead space and ventilation, compliance and surfactant, and how spirometry separates obstructive from restrictive lung disease.
- Normal values (adult male): tidal volume about 500 mL, residual volume about 1,200 mL, vital capacity about 4,600 mL, total lung capacity about 5,800 mL; FRC is where lung recoil and chest wall recoil balance.
- Alveolar ventilation = (tidal volume − dead space) × rate; dead space is about 150 mL.
- Obstructive disease: FEV1/FVC low (under 0.7). Restrictive: both fall, ratio normal or high.
On this page
Overview
Breathing moves air in and out of the lungs. It depends on pressure differences created by the respiratory muscles, the elasticity of the lungs and chest wall, and the resistance of the airways.
Spirometry measures lung volumes and flows. It is the main test used to separate obstructive from restrictive lung disease.
Conducting and respiratory zones
- Conducting zone: trachea, bronchi, bronchioles down to the terminal bronchioles. Carries, warms and humidifies air; no gas exchange. This is the anatomical dead space.
- Respiratory zone: respiratory bronchioles, alveolar ducts and alveoli. Gas exchange happens here.
Muscles of respiration
- Quiet inspiration: mainly the diaphragm; it contracts and moves down. The external intercostals help lift the ribs.
- Quiet expiration: passive, from elastic recoil of the lungs and chest wall.
- Forced inspiration: accessory muscles: sternocleidomastoid, scalenes, serratus anterior.
- Forced expiration: abdominal muscles and internal intercostals.
Quiet expiration is passive. Active expiration uses the abdominal muscles.
Pressures
- Intrapleural (pleural) pressure: slightly negative, about −5 cmH2O at the start of inspiration, falling to about −7.5 cmH2O during a normal breath. It keeps the lungs expanded.
- Alveolar pressure: zero (atmospheric) between breaths; slightly negative during inspiration and positive during expiration.
- Transpulmonary pressure: alveolar minus pleural pressure; the force holding the lungs open.
If air enters the pleural space (pneumothorax), pleural pressure becomes atmospheric and the lung collapses.
Lung volumes and capacities
| Volume | Definition | Approx. value (adult male) |
|---|---|---|
| Tidal volume (TV) | Air in or out in a normal breath | 500 mL |
| Inspiratory reserve volume (IRV) | Extra air that can be breathed in after a normal inspiration | 3,000 mL |
| Expiratory reserve volume (ERV) | Extra air that can be breathed out after a normal expiration | 1,100 mL |
| Residual volume (RV) | Air left after a maximal expiration | 1,200 mL |
| Capacity | Made of | Approx. value |
|---|---|---|
| Inspiratory capacity | TV + IRV | 3,500 mL |
| Functional residual capacity (FRC) | ERV + RV | 2,300 mL |
| Vital capacity (VC) | IRV + TV + ERV | 4,600 mL |
| Total lung capacity (TLC) | VC + RV | 5,800 mL |
Values are about 20–25% lower in women.
Residual volume, FRC and TLC cannot be measured by simple spirometry because they include air that cannot be breathed out. They need helium dilution, nitrogen washout or body plethysmography.
FRC is the resting volume, where the inward recoil of the lungs balances the outward recoil of the chest wall.
Ventilation and dead space
- Minute ventilation = tidal volume × respiratory rate = about 500 mL × 12 = 6 L/min.
- Anatomical dead space: about 150 mL (about 1 mL per pound body weight).
- Physiological dead space: anatomical dead space plus alveoli that are ventilated but not perfused. Normally almost equal to anatomical; larger in lung disease.
- Alveolar ventilation = (tidal volume − dead space) × rate = (500 − 150) × 12 = about 4.2 L/min.
Rapid shallow breathing wastes much of each breath on dead space, so alveolar ventilation falls even if minute ventilation stays the same.
Compliance and elasticity
Compliance is the change in lung volume per unit change in pressure (how easily the lungs stretch). Normal lung compliance is about 200 mL/cmH2O (lungs alone).
- Low compliance (stiff lungs): pulmonary fibrosis, pulmonary oedema, lack of surfactant.
- High compliance (floppy lungs): emphysema (loss of elastic tissue), old age.
Two forces create the lungs' elastic recoil:
- Elastic fibres in the lung tissue (about one third).
- Surface tension of the fluid lining the alveoli (about two thirds).
Surfactant
Surfactant is a mixture of phospholipids (mainly dipalmitoyl phosphatidylcholine) and proteins made by type II alveolar cells.
- Lowers surface tension, so the lungs are easier to inflate (higher compliance).
- Stops small alveoli collapsing into large ones (by the law of Laplace, pressure = 2 × tension ÷ radius).
- Helps keep the alveoli dry.
Surfactant secretion starts only between the sixth and seventh months of pregnancy, so premature babies often lack it and develop respiratory distress syndrome. Steroids given to the mother before preterm delivery speed surfactant production.
Airway resistance
- Most resistance is in the medium-sized bronchi. The many small airways in parallel have a large total cross-section, so they add little resistance in health.
- Sympathetic (β2) stimulation dilates airways; parasympathetic (vagus, ACh) and histamine constrict them.
- Resistance falls as lung volume rises (airways are pulled open).
Pulmonary function tests
Forced vital capacity (FVC): the total air breathed out as hard and fast as possible after a full breath in.
FEV1: the volume breathed out in the first second of that forced breath. Normally about 80% of FVC.
| Feature | Obstructive | Restrictive |
|---|---|---|
| Examples | Asthma, COPD (emphysema, chronic bronchitis), bronchiectasis | Pulmonary fibrosis, chest wall deformity, neuromuscular weakness, obesity |
| FEV1 | Much reduced | Reduced |
| FVC | Normal or reduced | Reduced |
| FEV1/FVC | Low (under 0.7) | Normal or high |
| TLC | Normal or increased (air trapping) | Reduced |
| RV | Increased | Reduced |
The FEV1/FVC ratio is the key: low in obstruction, normal or high in restriction.
In asthma the obstruction improves after a bronchodilator (reversible); in COPD it improves little.
Clinical relevance
Emphysema
How it happens: destruction of alveolar walls and elastic tissue (smoking).
What you see: high compliance, air trapping, raised TLC and RV, low FEV1/FVC, barrel chest.
Pulmonary fibrosis
How it happens: scarring of lung tissue.
What you see: stiff lungs (low compliance), small volumes, normal or high FEV1/FVC, breathlessness.
Expect: normal lung volumes and which cannot be measured by spirometry, the definition of FRC, dead space and alveolar ventilation calculations, compliance in emphysema and fibrosis, the role and source of surfactant, the main site of airway resistance, and the FEV1/FVC pattern in obstructive vs restrictive disease.
Quick revision
- TV 500 mL; RV 1,200; VC 4,600; TLC 5,800 mL.
- FRC = ERV + RV; balance point of lung and chest wall recoil.
- RV, FRC, TLC need helium dilution or plethysmography.
- Alveolar ventilation = (TV − 150) × rate.
- Surfactant from type II cells lowers surface tension.
- Most resistance: medium-sized bronchi.
- Obstructive: FEV1/FVC under 0.7. Restrictive: ratio normal or high.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
What is functional residual capacity?
The volume of air left in the lungs after a normal expiration; the sum of expiratory reserve volume and residual volume.
Which lung volume cannot be measured by spirometry?
Residual volume, and therefore FRC and total lung capacity.
What does surfactant do?
It lowers surface tension in the alveoli, making the lungs easier to inflate and preventing small alveoli from collapsing.
How do obstructive and restrictive diseases differ on spirometry?
In obstructive disease the FEV1/FVC ratio is low; in restrictive disease both are reduced and the ratio is normal or high.