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Heart and Exercise |
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Heart and Exercise |
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Effects of Exercise
The human heart has four chambers: two thin-walled
atria on top, which receive blood, and two thick-walled ventricles
underneath, which pump blood. Veins carry blood into the atria and
arteries carry blood away from the ventricles. Between the atria and the
ventricles are atrioventricular valves, which prevent back-flow of blood
from the ventricles to the atria. The left valve has two flaps and is called the
bicuspid (or mitral) valve, while the right valve has 3
flaps and is called the tricuspid valve. The valves are held in place by
valve tendons (“heart strings”) attached to papillary muscles,
which contract at the same time as the ventricles, holding the vales closed.
There are also two semi-lunar valves in the arteries (the only examples
of valves in arteries) called the pulmonary and aortic valves.
The left and right halves of the heart are separated by
the inter-ventricular septum. The walls of the right ventricle are 3
times thinner than on the left and it produces less force and pressure in the
blood. This is partly because the blood has less far to go (the lungs are right
next to the heart), but also because a lower pressure in the pulmonary
circulation means that less fluid passes from the capillaries to the
alveoli.
The heart is made of cardiac muscle, composed of
cells called myocytes. When myocytes receive an electrical impulse they
contract together, causing a heartbeat. Since myocytes are constantly active,
they have a great requirement for oxygen, so are fed by numerous capillaries
from two coronary arteries. These arise from the aorta as it leaves the
heart. Blood returns via the coronary sinus, which drains directly into
the right atrium.
When the cardiac muscle contracts the volume
in the chamber decrease, so the pressure in the chamber increases, so the blood
is forced out. Cardiac muscle contracts about 75 times per minute, pumping
around 75 cm³ of blood from each ventricle each beat (the stroke volume).
It does this continuously for up to 100 years. There is a complicated sequence
of events at each heartbeat called the cardiac cycle.
Cardiac
muscle is myogenic, which means that it can contract on its own, without
needing nerve impulses. Contractions are initiated within the heart by the
sino-atrial node (SAN, or pacemaker) in the right atrium. This
extraordinary tissue acts as a clock, and contracts spontaneously and
rhythmically about once a second, even when surgically removed from the heart.
The cardiac
cycle has three stages:
1.
Atrial Systole (pronounced sis-toe-lay). The SAN contracts
and transmits electrical impulses throughout the atria, which both
contract, pumping blood into the ventricles. The ventricles are
electrically insulated from the atria, so they do not contract at this time.
2.
Ventricular Systole. The electrical impulse passes to the
ventricles via the atrioventricular node (AVN), the bundle of His
and the Purkinje fibres. These are specialised fibres that do not
contract but pass the electrical impulse to the base of the ventricles, with a
short but important delay of about 0.1s. The ventricles therefore contract
shortly after the atria, from the bottom up, squeezing blood upwards into the
arteries. The blood can't go into the atria because of the atrioventricular
valves, which are forced shut with a loud "lub".
3.
Diastole. The atria and the ventricles relax,
while the atria fill with blood. The semilunar valves in the arteries close as
the arterial blood pushes against them, making a "dup" sound.
The events of
the three stages are shown in the diagram on the next page. The pressure changes
show most clearly what is happening in each chamber. Blood flows because of
pressure differences, and it always flows from a high pressure to a low
pressure, if it can. So during atrial systole the atria contract, making the
atrium pressure higher than the ventricle pressure, so blood flows from the
atrium to the ventricle. The artery pressure is higher still, but blood can’t
flow from the artery back into the heart due to the semi-lunar valves. The
valves are largely passive: they open when blood flows through them the right
way and close when blood tries to flow through them the wrong way.
The PCG
(or phonocardiogram) is a recording of the sounds the heart makes. The cardiac
muscle itself is silent and the sounds are made by the valves closing. The first
sound (lub) is the atrioventricular valves closing and the second (dub) is the
semi-lunar valves closing.
The ECG (or
electrocardiogram) is a recording of the electrical activity of the heart. There
are characteristic waves of electrical activity marking each phase of the
cardiac cycle. Changes in these ECG waves can be used to help diagnose problems
with the heart.
The rate at
which the heart beats and the volume of blood pumped at each beat (the stroke
volume) can both be controlled. The product of these two is called the
cardiac output – the amount of blood flowing in a given time:
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heart |
stroke volume |
cardiac
output |
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at rest |
75 |
75 |
5 600 |
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at exercise |
180 |
120 |
22 000 |
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As the table shows, the cardiac output can
increase dramatically when the body exercises. There are several benefits from
this:
to get
oxygen to the muscles faster
to get
glucose to the muscles faster
to get
carbon dioxide away from the muscles faster
to get
lactate away from the muscles faster
to get heat
away from the muscles faster
But what
makes the heart beat faster? Again, this is an involuntary process and is
controlled a region of the medulla called the cardiovascular centre,
which plays a similar role to the respiratory centre. The cardiovascular centre receives
inputs from various receptors around the body and sends output through two
nerves to the sino-atrial node in the heart.
The
cardiovascular centre can control both the heart rate and the stroke volume.
Since the heart is myogenic, it does not need nerve impulses to initiate each
contraction. But the nerves from the cardiovascular centre can change the
heart rate. There are two separate nerves from the cardiovascular centre to the
sino-atrial node: the sympathetic nerve (accelerator nerve) to speed up
the heart rate and the parasympathetic nerve (vagus nerve) to slow it
down.
The
cardiovascular centre can also change the stroke volume by controlling blood
pressure. It can increase the stroke volume by sending nerve impulses to the
arterioles to cause vasoconstriction, which increases blood pressure so more
blood fills the heart at diastole. Alternatively it can decrease the stroke
volume by causing vasodilation and reducing the blood
pressure.
When the
muscles are active they respire more quickly and cause several changes to the
blood, such as decreased oxygen concentration, increased carbon dioxide
concentration, decreased pH (since the carbon dioxide dissolves to form carbonic
acid) and increased temperature. All of these changes are detected by various
receptor cells around the body, but the pH changes are the most sensitive and
therefore the most important. The main chemoreceptors (receptor cells
that can detect chemical changes) are found in:
The walls
of the aorta (the aortic body), monitoring the blood as it leaves the
heart
The walls
of the carotid arteries (the carotid bodies), monitoring the blood to
the head and brain
The
medulla, monitoring the tissue fluid in the brain
The
chemoreceptors send nerve impulses to the cardiovascular centre indicating that
more respiration is taking place, and the cardiovascular centre responds by
increasing the heart rate.
A similar job
is performed by temperature receptors and stretch receptors in the muscles,
which also detect increased muscle activity.
Exercise affects the rest of the circulation as well as increasing
cardiac output. When there is an
increase in exercise, the muscles respire faster, and therefore need a greater
oxygen supply. This can be achieved
by increasing the amount of blood flowing through the capillaries at the
muscles. A large increase in blood
flowing to one part of the body must be met by a reduction in the amount of
blood supplying other parts of the body, such as the digestive system.
Some organs need a stable blood supply (to supply enough oxygen and
glucose for respiration), to work efficiently what ever the body is doing. The three main organs that require a
constant blood supply are:
The heart needs a constant blood supply otherwise the heart muscle would starve of oxygen and glucose, making it unable to pump more blood, and might cause a heart attack.
The
brain needs a constant blood supply otherwise the brain would reduce
ability to react to danger and might result in unconsciousness/death.
The
kidneys need a constant blood supply otherwise there would be a
build-up of toxins in the blood.
Both the rate
and depth (volume) of breathing can be varied. The product of these two
is called the ventilation rate – the volume air ventilating the lungs
each minute:
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Breathing |
Tidal |
Ventilation
rate |
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at rest |
12 |
500 |
6 000 |
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at exercise |
18 |
1000 |
18 000 |
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When the body exercises the ventilation rate and depth
increases so that
Oxygen can
diffuse from the air to the blood faster
Carbon
dioxide can diffuse from the blood to the air faster
The process is the same as for heart rate, with the
chemoreceptors in the aortic and carotid bodies detecting an increase in
respiration.
Again, the
stretch receptors in the muscles give a more rapid indication of muscular
activity, allowing an anticipatory increase in breathing rate even before the
carbon dioxide concentration the blood has changed.
