The mechanics of
breathing and
Respiratory Volumes
Pass and
Merit
Overview
• Breathing is regulated by:
• the respiratory centre,
located in the brain.
• Receptors in the air
passages and lungs
• Breathing in = Inspiration
• Breathing out = Expiration
• To breathe the thorax must
increase and then
decrease in size
Inspiration
• In order to breathe in the volume of the
chest cavity needs to increase.
• This increase in size of the chest cavity,
causes a decrease in pressure within the
lungs
• Boyle’s Law states that a volume of gas is
inversely proportional to its pressure.
• This means that the increase in volume in the
lungs causes a decrease in pressure.
• Gases flow from a high pressure area to a low
pressure area
• In this situation the ambient air is the high
pressure area and the lungs are the low
pressure area, so the air flows into the lungs
Inspiration
• Inspiration – Breathing in
• Diaphragm contracts
• It flattens and pulls down
• This is an active process
• External intercostal muscles contract
• The sternum moves up and out, with the • These two actions cause
the volume of the thoracic
lungs following
cavity to increase
• The lungs are attached to the pleural sac
(containing pleural fluid), which in turn is
attached to the thoracic cage
• As the chest expands, the surface
tension, created by the film of pleural
fluid causes the lungs to be pulled
outwards, with the chest
• According to Boyles Law
this increase in volume
causes a decrease in
pressure
• Air flows into the lungs
• As gas flows from high
pressure to low pressure.
Expiration
• To breathe out – Expiration:
• Diaphragm relaxes
• It moves back up and into the thoracic cavity
• This is a passive process
• The external intercostal muscles relax
• The ribs/sternum moves down.
• The lungs, sternum and rib cage are elastic structures that naturally 'spring'
back to their resting positions once the forces of the inspiratory muscles
are removed. So expiration is a passive process.
• The volume of the thoracic cavity decreases causing the air to
move out of the lungs.
• This is because air pressure in the lungs is now higher than atmospheric
pressure, according to Boyles Law, so the air is forced out of the lungs to
equate the pressure in and out of the body.
The muscles of breathing
Respiratory
volumes
Lung Volumes
• Lung volumes: refers to physical differences in lung volume, while
lung capacities represent different combinations of lung volumes,
usually in relation to inhalation and exhalation.
• The average pair of human lungs can hold about 6 litres of air, but
only a small amount of this capacity is used during normal breathing.
Spirometer trace
Respiratory
volumes
• Tidal Volume
– The volume of air inspired
or expired per breath
(Approx 500ml at rest)
• Inspiratory Reserve Volume
– The amount of space that is
available to draw in more air
• Eg; Breathe in normally, then
breathe in more. This extra
capacity is your IRV
• Expiratory Reserve Volume
– The amount of space that is available to breathe out, once you have exhaled
normally
• Eg: Breathe out normally, then force out more air. This is your ERV.
Respiratory
volumes
•
• Residual Volume
– Breathe out as much as
•
possible
• There is always
some air left in
your lungs
• This is your RV
(Approx 1200ml)
Vital Capacity
– Breathe in as much as you can,
and then force as much air out of
your lungs as possible.
• This is your IRV+ERV+TV,
and is your Vital Capacity
Total Lung
Capacity
– Take in as
much breath
as possible
– This is your
total lung
capacity
– ERV+IRV+TV
+RV (Approx
6000ml)
Spirometer trace