Biology
(16 - 18)
Breathing &
Spirometry
© SSER Ltd.
Control of Rhythmic Breathing
The basic breathing rhythm is a reflex action under the control of the nervous system.
The region of the brain controlling this basic rhythm is the medulla oblongata.
The medulla contains a
breathing centre
consisting of two groups
of nerve cells, called the
inspiratory and
expiratory centres.
Nerves arising from these
centres innervate (make
contact with) the intercostal
muscles and the diaphragm.
The thoracic nerves innervate
the intercostal muscles.
The phrenic nerves
innervate the diaphragm.
Impulses travelling
along the thoracic
and phrenic nerves
from the expiratory
centre lead to
relaxation of the
diaphragm and
intercostal muscles.
EXPIRATION
FOLLOWS
The alveoli deflate
and stretch
receptors are no
longer stimulated.
inhibit
stimulate
thoracic nerves
phrenic nerves
A pattern of nerve impulses
travels along the vagus nerve
to the respiratory centres
leading to inhibition of the
inspiratory centre and
stimulation of the
expiratory centre.
Expiratory
centre
Inspiratory
centre
phrenic nerves
stimulate
inhibit
A pattern of nerve impulses
travels along the vagus nerve
to the respiratory centres
leading to stimulation of the
inspiratory centre and
inhibition of the expiratory
centre.
INSPIRATION
FOLLOWS
thoracic
nerves
At the height of an Impulses travelling along
inspiration, the
the thoracic and phrenic
alveoli are inflated
nerves from the
and stretched,
inspiratory centre lead
thus stimulating
to contraction of the
stretch receptors
diaphragm and
in their walls.
intercostal muscles.
Composition of Inspired and Expired Air
The purpose of the breathing rhythm is to ventilate the lungs to allow delivery of oxygen
to the alveoli, and elimination of the waste gas carbon dioxide from the alveoli.
As a consequence of gas exchange at the alveoli, there are differences between the
composition of inhaled and exhaled air.
Another factor that contributes to the differences found between inspired and expired air,
is the dead space content.
The dead space is the
region of the respiratory
tract where no gas
exchange takes place.
trachea
bronchi
Gas exchange only takes
place across the thin walls
of the alveoli.
The air filling the trachea,
bronchi and bronchioles is
unavailable for gas
exchange and is said to
occupy dead space.
A healthy adult, at rest,
inspires approximately
600 cm3 of air of which
about 150 cm3 fills the
airways.
The volume of air actually
reaching the alveoli is
thus about 450 cm3.
bronchioles
As the air passages are never completely emptied of air,
there is only a partial replacement of air in the lungs.
Composition of Inspired and Expired Air
The table below can be used to explain what happens to air as it
enters and leaves the respiratory system.
The Relative Composition (% by Volume) of
Inspired, Expired & Alveolar Air
Gas
Inspired air
%
Expired air
%
Alveolar air
%
Oxygen
20.71
14.6
13.2
Carbon
dioxide
0.04
3.8
5.0
Water
vapour
1.25
6.2
6.2
Nitrogen
78.0
75.4
75.6
It is important to realise that the lungs can never be completely emptied of air; even
following a forced expiration, air remains within the alveoli and this amount of
air is called the residual volume.
Composition of Inspired and Expired Air
Inspired air contains approximately 21% by volume of oxygen gas. As this fresh air is
drawn into the alveoli, it mixes with air already present (the residual volume).
The Relative Composition (% by Volume) of
Inspired, Expired & Alveolar Air
Gas
Inspired air
%
Expired air
%
Alveolar air
%
Oxygen
20.71
14.6
13.2
Carbon
dioxide
0.04
3.8
5.0
Water
vapour
1.25
6.2
6.2
Nitrogen
78.0
75.4
75.6
The residual volume dilutes the fresh air, such that the oxygen content falls to about
67% of that in the atmosphere.
The oxygen content of alveolar air now falls even further as oxygen diffuses
from the alveoli into the blood along its concentration gradient.
Composition of Inspired and Expired Air
The carbon dioxide content of alveolar air increases significantly as gas exchange
proceeds and carbon dioxide diffuses from the blood into the alveoli.
The Relative Composition (% by Volume) of
Inspired, Expired & Alveolar Air
Gas
Inspired air
%
Expired air
%
Alveolar air
%
Oxygen
20.71
14.6
13.2
Carbon
dioxide
0.04
3.8
5.0
Water
vapour
1.25
6.2
6.2
Nitrogen
78.0
75.4
75.6
The oxygen content of expired air is higher than that in the alveoli and is intermediate in
value between that atmospheric air and alveolar air.
This is explained by the fact that expired air from the alveoli mixes with the
dead space air whose oxygen content is the same as that of the atmosphere.
Composition of Inspired and Expired Air
The percent by volume of carbon dioxide in expired air is less than that of alveolar air.
The Relative Composition (% by Volume) of
Inspired, Expired & Alveolar Air
Gas
Inspired air
%
Expired air
%
Alveolar air
%
Oxygen
20.71
14.6
13.2
Carbon
dioxide
0.04
3.8
5.0
Water
vapour
1.25
6.2
6.2
Nitrogen
78.0
75.4
75.6
Again, this is explained by the fact that expired air from the alveoli mixes with
the dead space air containing very low levels of carbon dioxide.
Composition of Inspired and Expired Air
The water vapour content of expired air is significantly higher than that of inspired air.
As air is breathed into the alveoli, water from the lining of the alveoli evaporates into the
alveolar air such that expired air is greater in volume than inspired air.
The Relative Composition (% by Volume) of
Inspired, Expired & Alveolar Air
Gas
Inspired air
%
Expired air
%
Alveolar air
%
Oxygen
20.71
14.6
13.2
Carbon
dioxide
0.04
3.8
5.0
Water
vapour
1.25
6.2
6.2
Nitrogen
78.0
75.4
75.6
Nitrogen gas is neither used or produced by the body and actual amounts of nitrogen in
inspired an expired air do not change.
The slightly larger volume of expired air means that nitrogen forms part of a
larger volume during expiration and so its % by volume decreases.
Measuring Lung Volumes
The volumes of air inspired and expired in different circumstances, can be measured
using an instrument called a spirometer.
The subject breathes in and out through the mouthpiece of the spirometer using a nose
clip to block airflow through the nose.
The spirometer consists of a large tank of water, onto which rests a large, and very light
perspex lid.
A series of pipes lead from
from the air under the lid of
the box to the mouthpiece.
A set of valves ensures that
inspired and expired air
travel along different pipes.
The subject breathes air into
and out of the space under the
lid via the mouthpiece.
Volume changes associated with breathing are recorded with a pen from the lid
onto a rotating drum (kymograph).
Measuring Lung Volumes
counterweight to
help balance box
perspex box
pen
The two-way valve ensures that
inspired air (oxygen) is derived
from under the box and expired
air is delivered back to the box
after passing over soda lime which
prevents the subject inspiring
increasing levels of CO2.
rotating drum
(kymograph)
soda lime to absorb carbon
dioxide from expired air
tank of
water
expired air
inspired
oxygen
valves
The lid (and hence the trace)
moves downwards during an
inspiration, and upwards
during an expiration.
The nose clip
prevents any air
being lost from the
system through
the nose.
Kymograph Recording of Lung Volumes
volume in dm
3
maximal inspiration
time
This graph shows the results of a spirometer recording. It is customary
to display spirometer traces upside down with inspiration curves
moving upwards and expiration curves moving downwards.
The volume of air breathed in an out during one ventilation cycle, or breath,
is called the tidal volume.
volume in dm
3
maximal inspiration
tidal volume
time
The tidal volume is found to vary from 0.4 to 0.6 dm3 in healthy subjects;
following strenuous exercise it can rise to around 3.0 dm3.
The air we normally breathe in and out, does not represent our full capacity for
inspiration or for expiration. If a subject is asked to take as deep a breath as possible,
i.e. force an inspiration, we obtain a trace of the inspiratory capacity.
maximal inspiration
inspiratory
reserve volume
volume in dm
3
inspiratory
capacity
tidal volume
time
In order to achieve their inspiratory capacity, subjects must continue to inhale after
a normal inspiration. The extra amount of air that can be inhaled following a
normal inspiration is called the inspiratory reserve volume.
Subjects can also force an expiration, although the extra volume of air that can be
expired is less than obtained in a forced inspiration.
maximal inspiration
inspiratory
reserve volume
volume in dm
3
inspiratory
capacity
expiratory
capacity
expiratory
reserve volume
tidal volume
time
As with inspiration, we can obtain traces for expiratory capacity and
expiratory reserve volume.
If we add together the inspiratory and expiratory capacities, that is the
maximum volume of air that can be exchanged during one breath
in and out, we have a measure of the vital capacity.
maximal inspiration
volume in dm
3
inspiratory
capacity
inspiratory
reserve volume
vital
capacity
expiratory
capacity
expiratory
reserve volume
time
The average vital capacities are about 5.5 dm3
for a male and 4.25 dm3 for a female.
tidal volume
The lungs cannot be completely emptied and a certain volume of air always remains
in the lungs even following a forced expiration.
This is called the residual volume.
maximal inspiration
volume in dm
3
inspiratory
capacity
inspiratory
reserve volume
vital
capacity
expiratory
capacity
expiratory
reserve volume
tidal volume
residual
volume
time
The residual volume cannot be calculated directly using a spirometer and requires
more sophisticated techniques; a value of about 1.5 dm3 is a typical residual volume.
The total lung capacity is therefore the sum of of the vital capacity
and the residual volume.
maximal inspiration
volume in dm
3
inspiratory
capacity
inspiratory
reserve volume
vital
capacity
expiratory
capacity
expiratory
reserve volume
residual
volume
tidal volume
total
lung capacity
time
Spirometer tracings can be used to determine a variety of physiological
measurements such as metabolic rate, breathing rate and oxygen consumption.
Extension Work
A male subject was connected to the spirometer and asked to breathe normally;
after a few minutes he was asked to undertake a forced inspiration followed by a
forced expiration – the resulting trace is shown on the next slide…
 Determine the tidal volume at one minute
 Determine the vital capacity
 Calculate the oxygen consumption in dm3 per minute
 Determine the tidal volume at one minute
 Determine the vital capacity
 Calculate the oxygen consumption in dm3 per minute
1 dm3
Tidal volume
at one minute
is 1 dm3
Vital capacity
is 4.5 dm3
4.5
dm3
The distance by which the gradient of the trace decreases
in a given time is the rate of oxygen consumption.
0.75 dm3
The gradient of the trace decreases
by 0.75 dm3 (750 cm3) between 1
and 2 minutes; rate of oxygen
consumption is 750 cm3 per minute
Use this information to determine
the metabolic rate of the subject
 Rate of oxygen consumption = 0.75 dm3 per minute
 Rate of oxygen consumption = 45 dm3 per hour
 1 dm3 of oxygen taken into the body produces
20kJ of energy
 Therefore, energy produced by 45 dm3/hr = 900 kJ/hr
900 kJ/hr is the Metabolic rate.
The Basal Metabolic rate (BMR) is expressed
with respect to body surface area.
BMR = Metabolic rate/Body surface area
Our subject weighs 75 kg and is 170 cm in height.
Use the nomogram provided to determine the
surface area of the subject and hence the BMR…


Surface area = 1.88 m2
BMR = 900/1.88 = 479 kJ m-2 hr-1
The BMR for our
subject is 479 kJ m-2 hr-1