1. © SSER Ltd.

2. 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 phrenic nerves innervate the diaphragm
The thoracic nerves innervate the intercostal muscles

3. The alveoli deflate and stretch receptors are no longer stimulated
thoracic nerves
phrenic nerves
Impulses travelling along the thoracic and phrenic nerves from the expiratory centre lead to relaxation of the diaphragm and intercostal muscles
The alveoli deflate and stretch receptors are no longer stimulated
EXPIRATION
FOLLOWS
stimulate
inhibit
Expiratory
centre
Inspiratory
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
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
stimulate
inhibit
phrenic nerves
thoracic
nerves
At the height of an inspiration, the alveoli are inflated and stretched, thus stimulating stretch receptors in their walls
INSPIRATION
FOLLOWS
Impulses travelling along the thoracic and phrenic nerves from the inspiratory centre lead to contraction of the diaphragm and intercostal muscles

4. 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
bronchioles
The air filling the trachea, bronchi and bronchioles is unavailable for gas exchange and is said to occupy dead space
Gas exchange only takes place across the thin walls of the alveoli
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
As the air passages are never completely emptied of air, there is only a partial
replacement of air in the lungs

5. 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
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

6. 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
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 in to the blood along its concentration gradient

7. 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
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

8. 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
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

9. 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
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 percent by volume decreases

10. Measuring Lung Volumes
kymograph
The volumes of air inspired and expired in different circumstances, can be measured using an instrument called a spirometer
The spirometer consists of a large tank of water,
onto which rests a large, and very light
perspex lid
A counterweight
on the edge of
the lid is used to
balance the box,
so that its edges
just rest under
the surface of
the water
counter
-weight
light, perspex
lid
tank of water
A nose clip is placed on the subject
to prevent any air being lost from
the system through the nose
A series of pipes lead
from from the air under
the lid of the box to
the mouthpiece
Expired air is
passed over soda lime to absorb
CO2 gas thus preventing the
subject from inspiring increasing amounts of this gas
Volume changes associated with breathing are recorded with a pen from the lid onto a rotating drum (kymograph)
A set of valves ensures
that inspired and
expired air travel along
different pipes
soda lime
The subject breathes air
into and out of the space
under the lid via
the mouthpiece
mouthpiece
valves

11. As the subject
breathes in through
the mouthpiece,
the lid moves down

12. As the subject
breathes in through
the mouthpiece,
the lid moves down
As the subject
breathes out through
the mouthpiece,
the lid moves up

13. As the subject
breathes in through
the mouthpiece,
the lid moves down
As the subject
breathes out through
the mouthpiece,
the lid moves up

14. Kymograph Recording of Lung Volumes
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

15. is called the TIDAL VOLUME
The volume of air breathed in an out during one ventilation cycle, or breath,
is called the TIDAL VOLUME
time
tidal volume
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

16. for inspiration or for expiration
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
time
inspiratory
capacity
inspiratory
reserve volume
tidal volume
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

17. expired is less than obtained in a forced inspiration
Subjects can also force an expiration, although the extra volume of air that can be
expired is less than obtained in a forced inspiration
time
inspiratory
capacity
inspiratory
reserve volume
expiratory
capacity
tidal volume
expiratory
reserve volume
As with inspiration, we can obtain traces for EXPIRATORY CAPACITY and
EXPIRATORY RESERVE VOLUME

18. The average vital capacities are about 5.5 dm3
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
time
vital
capacity
inspiratory
capacity
inspiratory
reserve volume
expiratory
capacity
tidal volume
expiratory
reserve volume
The average vital capacities are about 5.5 dm3
for a male and 4.25 dm3 for a female

19. in the lungs even following a forced expiration
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
time
vital
capacity
inspiratory
capacity
inspiratory
reserve volume
expiratory
capacity
tidal volume
expiratory
reserve volume
residual
volume
This measurement cannot be made using a spirometer and requires more
sophisticated techniques; values of about 1.5 dm3 are reported for residual volumes

20. The TOTAL LUNG CAPACITY is therefore the sum of of the vital capacity
and the residual volume
time
vital
capacity
inspiratory
capacity
inspiratory
reserve volume
total
lung capacity
expiratory
capacity
tidal volume
expiratory
reserve volume
residual
volume
Spirometer tracings can be used to determine a variety of physiological
measurements such as metabolic rate, breathing rate and oxygen consumption

21. Acknowledgements
Copyright © 2003 SSER Ltd. and its licensors. All rights reserved. All graphics are for viewing purposes only.