2011A13 Describe the determinants of work of breathing in an adult human at rest.



·         Intro

·         Graph

·         Elastic forces

·         Inelastic forces

·         Frequency dependence




·   Amount of energy spent on breathing

·   Unit joules

·   = Force x displacement

·    = Pressure work x volume work

Normal value

·   <2% total VO2 = 3mL/min


·   Only 10% (90% -> heat)


·   Inspiration: primary active

o Diaphragm, external intercostals etc

·   Expiration: secondary active at rest (use stored energy), primary active under stress

o Abdominal wall, internal intercostals etc




1: elastic = stored

2: inelastic


1: available

3: used

*Bronchoconstriction -> ↑size of area 3 (and 2)


Elastic (50%)

Surface tension


·   LaPlace’s law: tension = pressure x radius / 4

o Fluid surfaces acquire least surface area due to surface tension

o Alveoli tend to collapse

·   Surfactant:

o Produced by type 2 alveolar cells

o Reduces surface tension at air-water interface

o Effect on small > large alveoli ( [surfactant])

o ->↓ alveolar collapse, ↑compliance, ↓ work of breathing, ↓ transudation

·   Factors increasing surface tension:

o Alveolar oedema

o Surfactant deficiency (e.g. prematurity, SP-B or SP-C deficiency)

Elastic lung tissue


·   Elastic lung fibres e.g. elastin

o e.g. fibrosis -> ↓ elasticity -> ↑WOB


Inelastic (50%):

Resistance to airflow


Flow type:

·   Large airways: turbulent flow – (P1-P2) (density x length) / radius5

·   Small airways: laminar flow – R = (8 x length x viscosity) / (π x radius4)

·   Medium size: transitional flow

Variables: factors ↑resistance

·   ↓Airway radius:
*Most important factor since raised to power of 4 or 5*

o ↓Absolute lung size (e.g. neonate cf. adult)

o ↓Relative lung volume (e.g. diaphragm displacement in pregnancy)

o Intraluminal obstruction (e.g. mucus)

o Luminal obstruction (bronchoconstriction, swelling)

o Extraluminal obstruction (e.g. dynamic airways compression)

·   ↑Density: e.g. reduced with heliox

·   ↑Viscosity: e.g. ↑ with ↑temp

·   ↑Length: not under control

Inelastic lung tissue


·   Deformation of inelastic tissue e.g. collagen

·   Also friction of lung vs pleura, diaphragm vs abdominal viscera

Inertia of air


·   Force = mass x acceleration






Adult with asthma

Elastic resistance


Airflow resistance


Optimal respiratory rate





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