The work of breathing !

Concept
Exam priority: medium

Breathing costs energy: the lung must be stretched and airway resistance overcome. In asthma or obesity it costs far more.

Updated 2026-11-01

Also called: Cost of breathing

See it in 3D: Diaphragm · External intercostals · Internal intercostals

What it is

The work of breathing is the energy the inspiratory muscles spend moving air. It has an elastic part (stretching lung and chest wall) and a resistive part (overcoming airway and tissue resistance). At rest expiration is passive and breathing uses a small share of the body's oxygen; in disease that cost can become the cause of muscle fatigue and respiratory failure.

How it works, step by step

To inhale, the muscles generate pressure that must overcome elastic recoil and resistance to flow.
If compliance falls (fibrosis, edema, obesity), each milliliter of volume needs more pressure: elastic work rises.
If resistance rises (bronchospasm, mucus), each liter per second of flow needs more pressure: resistive work rises.
In obstruction, expiration does not finish; air is trapped and the next breath starts from a fuller lung.
The flattened, shortened diaphragm generates less force; muscle O₂ use rises and fatigue sets in.
When load exceeds muscle capacity, ventilation falls and PaCO₂ rises: hypercapnic failure.

Lung volumes and capacities

TLC 6.7 LVC 4.1 LFRC 3.5 LRV 2.6 LFEV1/FVC 0.45

Obstructive (COPD, asthma): air is trapped: ↑ RV and ↑ FRC, TLC normal or ↑, VC ↓. FEV1 falls more than FVC: FEV1/FVC < 0.70.

Tap a volume, a capacity or FEV1.

IRV · Inspiratory reserve volume
TV · Tidal volume
ERV · Expiratory reserve volume
RV · Residual volume
Capacity (sum of volumes)
Current forced expiration
Normal

Compare the obstructive pattern with normal: the high residual volume is trapped air that forces extra work.

How the body minimizes the work

Situation	Dominant component	Pattern adopted
Restrictive disease	Elastic	Rapid and shallow
Asthma or COPD	Resistive	Slow and deep, prolonged expiration
Obesity	Elastic (chest wall)	Rapid, small volumes, worse lying flat

Clinical notes

Asthma attack: a child who stops wheezing and looks tired may be tiring out, not improving.
Weaning from mechanical ventilation: a high rate-to-tidal-volume ratio (rapid shallow breathing) predicts failure.
Nursing: count the respiratory rate for a full minute and look for sternocleidomastoid use and retractions.

Test yourself

Think of the answer, then tap the question to see it.

Why does a patient with pulmonary fibrosis breathe rapidly and shallowly?

With a stiff lung, each deep breath needs a lot of elastic pressure; spreading ventilation over many small breaths lowers total work.

How does dynamic hyperinflation increase the work of breathing in COPD?

Trapped air leaves a positive end-expiratory pressure that must be overcome before air flows in, and it flattens the diaphragm, which loses force.

Why does expiration, passive at rest, become active in an asthma attack?

Elastic recoil can no longer empty the lung through narrowed airways; abdominal and internal intercostal muscles push, which adds work and compresses the airways.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

← All of physiology

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 22.3 The Process of Breathing (OpenStax (Rice University), 2022); Asthma (StatPearls (NCBI Bookshelf), 2026); Chronic Obstructive Pulmonary Disease (COPD) (StatPearls (NCBI Bookshelf), 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 38). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.