Download This is dependent on the volume of blood expelled

CONTROL OF BREATHING
The part of the brain which controls ventilation rate is the respiratory
centre situated in the medulla oblogata part of the brain.
The respiratory centre is divided into an inspiratory area and an
expiratory area.
The respiratory centre is connected to the thoracic area by the nervous
system;
Phrenic. nerve carries impulses from the respiratory centre to the
diaphragm
Intercostal nerve carries impulses from the respiratory centre to the
intercostal muscles.
RESTING VENTILATION
The inspiratory centre sue to an intrinsic excitability discharges impulses
which pass down the phrenic and intercostal nerves resulting in the
diaphragm and external intercostal muscles contracting so increasing the
volume of the thoracic cavity.
When inspiration occurs stretch receptors in the walls of the bronchi and
bronchioles are stimulated. This causes nerve impulses to be sent to the
expiratory centre. The expiratory centre sends impulses to the
inspiratory centre which is then inhibited and stops discharging impulses.
Expiration results from passive recoil of the lungs as the diaphragm and
external intercostal muscles relax.
Summary Diagram
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VENTILATION AND EXERCISE
When you exercise the ventilation rate must increase in order to supply
muscles with more oxygen and to remove excess carbon dioxide.
The control of high ventilation rate is dependent of chemoreceptors,
receptors that are sensitive to chemical changes in the blood, particularly
changes in carbon dioxide and pH. The chemoreceptors are found in
three parts of the body;
The medulla oblongata of the brain central receptor
The aorta
peripheral receptors
The carotid arteries
The medulla oblongata
During exercise, there is an increased rate of respiration in the muscles.
This leads to an increase in the concentration of carbon dioxide and
hydrogen ions. The membranes surrounding the medulla are permeable to
carbon dioxide but not to hydrogen ions. Carbon dioxide diffuses into the
medulla the following reaction occurs
Therefore hydrogen ions form inside the medulla when there is an
increase in the concentration of carbon dioxide. The central
chemoreceptor detects this increase in hydrogen ions and sends impulses
to the inspiratory centre and a group of cells called the ventral group.
These both send impulses to the diaphragm and intercostal muscles which
increase the strength and rate of contraction, increasing the rate and
depth of ventilation.
This lowers the concentration of carbon dioxide in the alveoli and so
increases the concentration gradient for carbon dioxide, therefore
lowering the concentration of carbon dioxide in the blood.
When the carbon dioxide level falls the level of hydrogen ions in the
medulla oblongata fall and the central receptors stop sending impulses.
The ventilation rate returns to normal.
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Peripheral receptors
Some of the peripheral receptors send impulses to the inspiratory centre
if they detect very low concentrations of oxygen in the blood. Others
are sensitive to low pH which is a result of high levels of carbon dioxide
in the blood.
Impulses from the peripheral receptors have the same effect on the
inspiratory centre as impulses from the central receptors. The rate and
depth of ventilation are increased until the pH or oxygen levels return to
normal.
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CONTROL OF HEARTB EAT
The heart has an internal pacemaker called the sino atrial node. This is a
specialised group of cells in the wall of the right atrium that produces
electrical impulses at regular intervals. The specialised muscle tissue of
the heart carries these impulses across the whole heart. When nerve
impulses reach the muscle cell it contracts. The wave of contraction
started by the SAN travels across the atria causing them to contract
(atrial systole)
There is a layer of connective tissue which separates the atria and the
ventricles. This stops the electrical impulses travelling directly to the
ventricles. A group of cells called the atrio ventricular node is found
between the atria and the ventricles and when impulses from the
contracting atria reach the AVN it starts impulses in the purkyne fibres.
These are specialised conductive muscle fibres which are grouped
together to from the bundle of His which passes between the two
ventricles.
At the base of the ventricle the bundle of His divides into two branches.
Fibres fan out from these and as impulses reach them the ventricles
contract from the base upward (ventricular systole)
SUMMARY
1.
2.
3.
4.
5.
6.
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MODIFYING THE HEART RATE
The amount of blood flowing from the heart is called the
This is dependent on the volume of blood expelled at each heart beat (
) and the heart rate.
Starlings Law shows the relationship between these three
One way of controlling cardiac output is by varying the heart rate. The
rate at which the heart beats can be modified considerably.
Resting heart rate
During exercise
During sleep
The heart rate is changed by nerve impulses from two areas of the brain


Both are located in the cardiovascular centre in the medulla oblongata of
the brain. Nerve fibres pass from these two areas to the
……………………………………………….. and the …………………………………………….
CARDIOACCELERATO
RY CENTRE
CARDIOINHIBITORY
CENTRE
BRANCH OF
NERVOUS SYSTEM
NEUROTRANSMITTE
R
EFFECT ON SAN &
AVN
EFFECT ON HEART
RATE
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Stretch receptors and heart rate
Stretch receptors are found in
They are connected to the cardioinhibitory centre and have different
effects on heart rate when stimulated
STRETCH RECEPTOR
EFFECT ON HEART RATE
As the amount of blood passing through these vessels increases the
stretch receptors are stimulated further so the number of impulses to
the cardioinhibitory centre also increases.
For example when exercising body muscles contract strongly and this
increases the rate at which venous blood returns to the heart. The vena
cava is stretched by the large quantities of blood returning to the heart
so the stretch receptors in the vena cava are stimulated, therefore
increasing the heart rate.
The increased stroke volume causes the stretch receptors in the
and
to become stimulated which causes the heart rate to
decrease. This is an automatic fail-safe mechanism to prevent the heart
from beating too fast.
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There are a number of stimuli which act directly on the cardiac muscle or
the SAN.
STIMULUS
LOW pH
HIGH pH
LOW TEMERATURE
HIGH TEMPERATURE
HIGH OXYGEN
LOW OXYGEN
EFFECT ON HEART RATE
Changes of this nature are usually accompanied by and increase in
ventilation rate. Chemoreceptors in the aorta and carotid arteries are
sensitive to changes in oxygen and carbon dioxide concentration, but
these chemoreceptors are not linked to the cardiovascular centre
directly.
Many activities affect the cardiovascular centre, for example emotions
such as anger and blushing. In such cases impulses are transmitted to the
brain where they pass to the cardiovascular centre. The activity of the
cardiovascular centre also fluctuates according to the health and age of
the individual.
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