The Control of Heart Rate
Heart rate is controlled by the autonomic nervous system.
Autonomic means self-governing, the autonomic nervous system
controls the involuntary (or subconscious) workings of the internal muscles and
glands.
It divides into two:
1.
The sympathetic Nervous System
Used to help us cope with stressful
situations by heightening our awareness and preparing us for activity
2.
The Parasympathetic Nervous System
In general, this inhibits effectors and slows
down any activity. The parasympathetic nervous system also controls activities
under normal resting conditions and its purpose is to conserve energy and to
replenish the body’s reserves.
These systems are often antagonistic. If one system
contracts a muscle, then the other relaxes it. Our internal glands and muscles
are therefore regulated by a balance of the two systems.
One such example is heart rate:
·
The resting heart rate of an average human being
is around 70 beats per minute
·
During exercise the resting heart rate may need
to more than double to supply the active tissue with oxygen and food.
Changes to the heart rate are controlled by the Medulla Oblongata
a region of the brain which has two centres.
1)
The first decreases heart rate- Parasympathetic
Nervous System
2)
The second that increases heart rate –
Parasympathetic Nervous System
Both of these centres and linked to the Sino atrial node by
their various nervous systems
The site that is stimulated depends upon the stimuli
received by the receptor. In the case of heart rate it could be either
1)
Chemical changes in the blood, detected by
chemoreceptors
2)
Pressure changes in the blood detected by
Baroreceptors
Control by Chemoreceptors
·
Chemoreceptors detect chemical changes and are
found in the walls of the carotid arteries which serve the brain.
·
These chemoreceptors are sensitive to changes in
the pH of the blood that result from the changes in carbon dioxide
concentration changes.
The process works as follows
1)
When pH falls the chemoreceptors in the walls of
the carotid arteries and aorta detect this and increase the frequency at which
they send nervous impulses to the centre in the medulla oblongata that is
responsible for increasing heart rate.
2)
Via the sympathetic nervous system, the medulla
oblongata in turn increases the rate at which it send nervous impulses to the
Sino atrial node.
3)
The Sino atrial node as a result increases the
rate at which it causes the muscles of the heart to contract.
4)
This means that the blood travels around the
body faster and thus more carbon dioxide is removed from the blood by the
lungs.
5)
With less carbon dioxide in the blood the pH
returns to its normal value.
6)
This rise in pH is detected by the carotid
arteries and they reduce the frequency at which they send impulses to the
medulla oblongata causing the heart rate to return to normal.
Control by the pressure receptors
These receptors are also found in the carotid arteries and walls of the
aorta.
They operate much the same as the chemoreceptors
1)
A higher than normal blood pressure is detected
by the pressure receptors
2)
Nervous impulses are transmitted to the centre
in the medulla oblongata
3)
The medulla oblongata in turn decreases the rate
at which it sends nervous impulses to the Sino atrial node via the parasympathetic
nervous system.
4)
The Sino atrial node stops the heart beating as
fast or as powerful
5)
Blood pressure falls.
