Work of Breathing

Components

1. Compliance work 65%

(stretching lungs & chest wall)

2. Airways resistance work 30%

3. Moving tissues 5%

Normally <1–3% Total Energy (E)

in exercise, still <3–5% total E output

Compliance = Volume

Pressure

Lung resists stretching:

1. Tissue expansion: Small componentnormally

Fig. shows lung expansion involves an unfolding of elastin and collagen fibers in the alveolar walls. The actual lengths of the individual fibers change little.

dynes / cm

Major component: surface tension

Lung surfactant• protein-phospholipid• from alveolar type 2 cells

Functions

1. surface tension of fluid in alveoli, so lungs able to expand with normal muscle activity.

2. Stabilizes alveoli from collapse

• surface tension is lowered more in small alveoli than in larger alveoli

P = 2T r

Small alveolus

Large alveolus

surfactant molecules more concentrated

surfactant molecules more diluted

Fig. shows surface tension alters alveoli stability. The smaller alveoli generate a greater pressure and cause air to flow into larger units. Surfactant promotes alveolar stability by lowering surface tension proportionately more in the small alveoli.

Compliance work 1. where surfactant ~ premature babies (smoking)

2. where lung tissue fibrotic (scar tissues)~ coal miners ~ asbestosis

3. where chest wall expansion limited~ scoliosis ~ tight bandages

4. at high lung volumes

Fig. shows static expiratory pressurevolume curves of lungs in normal subjects and subjects with pulmonary fibrosis.

compliance work

Airway Resistance

Airway resistance is increased by

• turbulent flow e.g. rapid breathing

• airway narrowing

Gas flow = pressure gradient

resistance

with laminar flow resistance 1

radius 4

Small peripheral airways

• each has small diameter

• but many of them

• so total cross-sectional area is high

• reactive smooth muscle in wall

Fig. shows schematic representation of airway branching in the human lung with approximate dimensions.

Causes of airway narrowing• in expiration vs inspiration

~ expiratory forces tend to push airways shut worse if little support for airways~ emphysema = lung tissue destruction

floppy airways

close during expiration

air trapping

• smooth muscle constriction e.g. asthma• inflammation, mucus, mucosal swellinge.g. chronic bronchitis asthma

Airway resistance is affected by:1. Structure of lungs~ resistance is higher in upper airways (trachea/bronchi etc)~ lower in small airways (large total cross-sectional area, laminar flow)

2. Mechanical factors:

Airway resistance is in expiration vs inspirationexpiratory forces tend to push airways shut worse if little support for airways~ airway resistance is lower at high lung volumes – airways held open~ airway resistance is lower during inspiration vs expiration

3. Smooth muscle tone in small airways~ sympathetic input relaxation~ parasympathetic input constriction

~ immune response e.g. asthma allergen/cold triggers immune response inflammatory mediators released smooth muscle constriction, mucosal swelling and mucus secretion

4. Local reflexes:

Areas with PCO2 cause bronchiolar smooth muscle contraction: important for ventilation-perfusion matching

Mechanical factors cont:

~ airway resistance may be high during expiration due to dynamic airways collapse during expiration (forced expiration, “floppy” airways in emphysema)

Emphysema = lung tissue destruction

floppy airways

close during expiration

air trapping

SpirometryForced expiratory volume in 1 sec

F E V1

~ measure of airways obstruction and/or dynamic airway compression

~ since F E V1 also affected by F V C

(forced vital capacity)

F E V1 / F V C ratio used

N ratio 80%

FEV1 = 3.2L

FVC = 4L

FEV1 = 1.5L

FVC = 3.2L

Peak flow

Overall Work of Breathing

1. Compliance work is greatest at high lung volumes

2. Airways resistance work is greatest at rapid airflow rates ie highest respiratory rate / min

VT = VT x resp rate L / min

minute ventilation

tidal vol

Work of breathing (summary)

Compliance work.

~ surfactant measurement: static compliance curve

spirometry

~ airways resistance work measurement:

alternative: peak flow meter

F E V1

F V C

V P