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The Mechanism of Ventilation

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The Mechanism of Ventilation
Introduction

The tidal flow of air into and out of the lungs is caused by
differences in pressure between the thorax and the atmosphere.Air
always moves from an area of higher pressure to an area of lower
pressure.A ventilation mechanism ensures that a constant supply of
fresh air is available to provide enough oxygen for gas exchange and
to remove carbon dioxide.

There are two processes of breathing- inhalation and exhalation

Inhalation

Air moves from high to low pressure.Hence the pressure in the lungs is
low.The diaphragm muscle contracts and flattens.The external
intercostals muscles contract and pull the rib cage out and up.The
volume in the lung increases.This reduces the pressure in the lungs
and air moves from the atmosphere(high pressure) to the lungs(low
pressure)

[IMAGE]

Process of breathing in

Exhalation

In this process the pressure in the lungs must be higher than the
pressure in the atmosphere so that air move in an opposite direction
to that in inspiration.The diaphragm muscle relaxes and rises up.The
internal intercostals muscles contract and the ribs are pulled
downwards and inwards.These movements reduced the volume of the
lungs.Since the volume decreases ,pressure increases.Hence air moves
out of the lungs to the atmosphere.

[IMAGE]

Process of breathing out

Ventilation brings oxygen into the lungs for gas exchange and removes
carbon dioxide.Since the amount of oxygen we need varies depending on
activity ,there must be a mechanism which controls the breathing
process.

The rhythmic sequence of inspiration and expiration is brought about
by alternating series of signals from the inspiratory and expiratory
nerve cells in the respiration center of the brain stem(medulla).During
the phase in which the inspiratory nerve cells are activated, the
expiratory nerve cells do not discharge, and conversely.Complex
circuits and networks of nerve brain stem are responsible for this
mechanism. Signals from receptors located in or near the lungs have a
stabilizing influence on this mechanism.At rest regular nerve impulses
are sent to the diaphragm and the intercostals muscles ,causing
rythymic contraction and relaxation of these muscles .This causes the
volume and pressure changes in the thorax which cause breathing.

Mechanism of control of breathing

Breathing is clearly an involuntary process(unless in cases like
forced breathing), and like many involuntary processes (such as heart
rate, coughing and sneezing) it is controlled by a region of the brain
called the medulla. It is located at the top of the spinal cord. The
medulla and its nerves are part of the autonomic nervous system (i.e.
involuntary). The region of the medulla that controls breathing is
called the respiratory centre. It receives inputs from various
receptors around the body and sends output through two nerves to the
muscles around the lungs.Impulses are sent along the phrenic nerve to
the diaphragm and along intercostals nerves to the intercostal
muscles.Unlike the heart, the muscles that

cause breathing cannot contract on their own, but need nerve impulses
from the brain for each breath

Ventilation is controlled by impulses form-

Chemoreceptors - in the medulla, aorta and carotid arteries.

Stretch receptors -in the wall of the bronchi.

[IMAGE]

Nervous control of breathing

Biological Sciences 1 and 2 by D.J Taylor,N.P.O Green and G.W.Stout.)

During inspiaration the lungs get inflated.The inflation causes the
stretch receptors (or proprioceptors) located in the bronchial tree to
get stimulated.More and more nerve impulses are sent via the vagus
nerve to the expiratory centre.The vagus nerve connects the bronchial
tree to the respiratory centre in the brain.Inspiration is temporarily
inhibited.The external intercostals muscles therefore relax,elastic
recoil of the lung tissues occurs and expiration takes place.After
this has occurred the bronchial tree is no longer stimulated
.Therefore the expiratory centre becomes inactive and inspiration can
begin again.The whole cycle is repeated rhythmically throughout the
life of the organism.

The depth and rate of breathing also needs to be regulated,so that
oxygen supply is matched to the body's changing demands.The
respiratory centre detects carbon dioxide concentration in the
blood.If carbon dioxide rises,it sets off deeper and more rapid
breathing .If it falls abnormally low,breaths become shallower and
less frequent.When carbon dioxide level increases chemoreceptors are
stimulated and they send nerve impulses via nerve fibres to the
respiratory centre in the medulla..The control centre in the medulla
gets stimulated and sends out impulses via the external intercostals
and phrenic nerves to the intercostals muscles and diaphragm.This
increases the rate and depth of breathing.This reduces the level of
carbon dioxide in the blood and brings in more oxygen.When the level
of carbon dioxide falls the chemoreceptors are no longer stimulated
and the breathing rate returns to normal.Carbon dioxide can become
harmful if allowed to build up in the body.It dissolves to form acid
which can denature enzymes and proteins.An increase of 0.25 % carbon
dioxide concentration in the air can double the ventilation rate.

The control of breathing by chemoreceptors is an example of negative
feedback mechanism.Negative feedback occurs when any change in the
normal level is detected,corrected and returned to normal level.eg any
change in the carbon dioxide level in the blood is detected ,corrected
and returned to normal level.

Conclusion

The mechanism of control of ventilation is extremely important for the
survival of the organism.As mentioned earlier,this process is
involuntary and takes place automatically.However there are certain
cases where the rate and depth of breathing are under voluntary
control as shown by the ability to hold one's breath.

Voluntary control is used during forced
breathing,speech,singing,sneezing and coughing.When such control is
being exerted,impulses originating in the cerebral hemispheres pass to
the breathing centre which then carries out the appropriate
action.Hyperventilation is forced deep breathing.Breathing in and out
deeply reduces the carbon dioxide level in the blood.Increase in
carbon dioxide concentration triggers breathing.A low level can
suppress the stimulus to breathe.Divers hyperventilate before going
underwater,so that the urge to breathe is reduced.This allows them to
stay underwater for a longer period of time.Hyperventilation on the
other hand could also be dangerous. Hyperventilation during panic or
fear could reduce the urge to breathe which may result in fainting as
the brain is deprived of oxygen.Once the conscious control is

lost ,breathing resumes.In cases when a person has stopped
breathing,it is essential to restart ventilation before permanent
brain damage occurs due to lack of oxygen.Mouth to mouth resuscitation
increases the carbon dioxide concentration in the patient's blood and
hence stimulates their medulla to start the breathing
process.Hospitals have resuscitators which also serve the same
purpose.

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