Respiration Module. Lung mechanics

Lung mechanics
during breathing work is done to
move around the lungs and thorax
move air through the airways

Lungs and thorax
form a mechanical system
lungs tend to collapse
and are held at larger volume by
the pleural seal

Pleural fluid
a thin layer of fluid
between visceral and parietal pleura
ensures that lungs fill thoracic cavity
and change volume as thorax does

Pneumothorax
if the integrity of the pleural seal is broken
lungs will tend to collapse

Equilibrium of forces
lungs pull in and up
thoracic cage pulls out
passive stretch of diaphragm pulls down

At the resting expiratory level
all forces in balance
like a set of springs
if disturbed will spring back to
the resting expiratory level

Breathing in
from resting expiratory level
is active
mainly by contraction of diaphragm
and intercostals

Breathing out
to resting expiratory level
is passive
just stop breathing in

In quiet breathing
inspiration is active
expiration is passive

Forced expiration
breathing out beyond resting expiratory level
requires force
exerted by abdominal muscles
then inspiration to resting expiratory level is passive

Work of breathing
in quiet breathing
most effort required to stretch the lungs
if
diaphragm cannot easily move into abdomen
pregnancy
obesity
corsets

Lung compliance
the stretchiness of the lungs is known as compliance
volume change per unit pressure change
higher compliance means easier to stretch

Elastic properties of the lungs
airways have elastic walls
but elastic tissue cannot explain how stiff the lungs are
compliance reduced by surface tension of lining fluid

Surface tension
interactions between molecules at surface of a liquid
makes the surface resist stretching
the higher the surface tension the harder the lungs are to stretch

Surface tension in the lungs
at low lung volumes much less than expected
if lungs lined with saline
something is reducing surface tension

Detergents
reduce surface tension by disrupting interactions between surface molecules
lung has a mixture of detergents
surfactant
produced by type 2 alveolar cells

Surfactant
reduces surface tension when lungs are deflated
but not when fully inflated
so little breaths are easy
big breaths are hard

Bubbles
formed when a film of fluid surround gas
film shrinks to compress gas
until eventually
equilibrium between tension and pressure

Laplace’s law
pressure = 2 x surface tension/radius
big bubbles have low pressure
little bubbles have high pressure

Law of bubbles
if a big bubble is connected to a small bubble
air will flow from high pressure to low
small bubble collapses into big
‘big bubbles eat little bubbles’

Bubbles in the lung
alveoli form an interconnecting set of bubbles
if Laplace’s law applied
big alveoli would eat little ones
and the lungs become a physical impossibility

Surfactant
as alveoli get bigger
surface tension in their walls increases because surfactant is less effective
so pressure stays high
and stops them eating little alveoli

Respiratory Distress Syndrome
babies born prematurely
have too little surfactant
lungs very stiff
few, large alveoli
breathing and gas exchange compromised

Moving air through airways
remember Poiseulles law
small tubes have high flow resistance
many airways very small
so individual resistance high
but

Over the whole tree of airways
at each branch
the increase in the number of airways
in parallel
compensates for the increase in their resistance

Airway resistance
at normal lung volumes in normal lungs
highest resistance in the trachea
lowest in the small airways
so breathing is easy

Forced expiration
when the lung is compressed
small airways are narrowed
resistance increases dramatically
and air is trapped
in the alveoli

Obstructive airway disease
if the small airways
are narrowed by disease
asthma
chronic bronchitis
resistance increases much earlier in expiration
breathing out can become very difficult