© sser ltd.. the basic breathing rhythm is a reflex action under the control of the nervous system...

The basic breathing rhythm is a reflex action under the control of the nervous systemThe region of the brain controlling this basic rhythm is the medulla oblongata

The medulla contains abreathing 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

Control of Rhythmic BreathingControl of Rhythmic Breathing

At the height of an inspiration, the

alveoli are inflated and stretched,

thus stimulating stretch receptors

in their walls

A pattern of nerve impulses travels along the vagus nerve

to the respiratory centres leading to inhibition of the

inspiratory centre and stimulation of theexpiratory centre

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

EXPIRATIONFOLLOWS

Expiratorycentre

Inspiratorycentre

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

thoracic nerves

phrenic nerves

stimulate

inhibit

Impulses travelling along the thoracic and phrenic

nerves from the inspiratory centre lead to

contraction of the diaphragm and

intercostal muscles

INSPIRATIONFOLLOWS

phrenic nerves

thoracic nerves

The purpose of the breathing rhythm is to ventilate the lungs to allow delivery of oxygento 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 thecomposition 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

Gas exchange only takes place across the thin walls

of the alveoli

trachea

bronchi

bronchioles

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

As the air passages are never completely emptied of air, there is only a partialreplacement of air in the lungs

Composition of Inspired and Expired AirComposition of Inspired and Expired Air

The table below can be used to explain what happens to air as itenters and leaves the respiratory system

It is important to realise that the lungs can never be completely emptied of air; evenfollowing a forced expiration, air remains within the alveoli and this amount of

air is called the residual volume


The Relative Composition (% by Volume) ofInspired, Expired & Alveolar Air

GasInspired 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

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 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 diffusesfrom the alveoli in to the blood along its concentration gradient


GasInspired 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 carbon dioxide content of alveolar air increases significantly as gas exchange proceeds and carbon dioxide diffuses from the blood into the alveoli

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 thedead space air whose oxygen content is the same as that of the atmosphere


GasInspired 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 percent by volume of carbon dioxide in expired air is less than that of alveolar air

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


GasInspired 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



GasInspired 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 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

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 alarger volume during expiration and so its percent by volume decreases


The volumes of air inspired and expired in different circumstances, canbe 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 nose clip is placed on the subjectto prevent any air being lost from

the system through the nose

tank of water

light, perspexlid

A counterweighton the edge of

the lid is used tobalance the box,so that its edgesjust rest underthe surface of

the water

counter-weight

A series of pipes leadfrom from the air under

the lid of the box tothe mouthpiece

mouthpiece

A set of valves ensuresthat inspired and

expired air travel alongdifferent pipes

valvesThe subject breathes airinto and out of the space

under the lid viathe mouthpiece

Expired air ispassed 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)

kymograph

soda lime

Measuring Lung VolumesMeasuring Lung Volumes

As the subject breathes in through

the mouthpiece,the lid moves down

As the subject breathes in through

the mouthpiece,the lid moves down

As the subject breathes out through

the mouthpiece,the lid moves up

m axim al inspirationv

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This graph shows the results of a spirometer recording; it is customaryto display spirometer traces upside down with inspiration curves

moving upwards and expiration curves moving downwards

Kymograph Recording of Lung VolumesKymograph Recording of Lung Volumes


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

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


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The air we normally breathe in and out, does not represent our full capacityfor inspiration or for expiration

tidal volume

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

inspiratorycapacity

In order to achieve their inspiratory capacity, subjects must continue to inhale aftera normal inspiration

The extra amount of air that can be inhaled following a normal inspirationis called the INSPIRATORY RESERVE VOLUME

inspiratoryreserve volume


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Subjects can also force an expiration, although the extra volume of air that can beexpired is less than obtained in a forced inspiration

tidal volume

inspiratorycapacity

As with inspiration, we can obtain traces for EXPIRATORY CAPACITY andEXPIRATORY RESERVE VOLUME

expiratorycapacity

expiratoryreserve volume



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If we add together the inspiratory and expiratory capacities, that is the maximum volumeof air that can be exchanged during one breath in and out, we have a measure of the

VITAL CAPACITY

tidal volume

inspiratorycapacity

The average vital capacities are about 5.5 dm3

for a male and 4.25 dm3 for a female

expiratorycapacity



vitalcapacity


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The lungs cannot be completely emptied and a certain volume of air always remainsin the lungs even following a forced expiration

tidal volume

inspiratorycapacity

This measurement cannot be made using a spirometer and requires moresophisticated techniques; values of about 1.5 dm3 are reported for residual volumes

expiratorycapacity



vitalcapacity

This is called the RESIDUAL VOLUME

residualvolume


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The TOTAL LUNG CAPACITY is therefore the sum of of the vital capacityand the residual volume

tidal volume

inspiratorycapacity

expiratorycapacity



vitalcapacity

residualvolume

Spirometer tracings can be used to determine a variety of physiologicalmeasurements such as metabolic rate, breathing rate and oxygen consumption

totallung capacity

© sser ltd.. the basic breathing rhythm is a reflex action under the control of the nervous system...

Documents

fresh air

cm3 of air

air evenfollowing

air passages

partialreplacement of

airwaysthe volume of

volume of oxygen gas

expired airthe table