Together, the heart and all the blood
vessels around the body form what’s called the cardiovascular system.
Your heart is actually a pump made of muscle. The
heart is made from a special type of muscle not found anywhere else
in the body, so it doesn’t get tired the way ordinary muscles
do. Its task is to keep your body supplied with the nutrients and oxygen
that are dissolved in your blood. The arteries carry the blood away
from the heart and the veins bring it back; the direction of blood
flow through the heart is controlled by valves which open to let blood
through, then close tightly to prevent it going back the wrong way.
In order to do its job properly, your heart muscle
must be working normally – it can’t pump efficiently if it
is damaged. Damage can be to a part of the muscle, e.g. an area of dead
heart muscle from a heart attack is eventually replaced by a scar that
doesn’t contract when the rest of the muscle does. Damage can also
be widespread involving all of the heart muscle, as happens in dilated
cardiomyopathy. In these cases the individual muscle fibres can’t
function properly. The heart also has to beat regularly at an appropriate
rate, depending on what activity you’re involved in at any given
time. The valves that control the flow of blood must also be in good
condition to keep blood flowing in the right direction.
The heart is divided into separate halves which beat
together but pump blood to different parts of the body. The right side
receives ‘used’ blood from the body and sends it off to the
lungs to be replenished with oxygen. The left side receives the oxygenated
blood from the lungs and pumps it around the body. Each side of the heart
has an upper and lower chamber. The upper ones, called the atria, are
where blood is collected from the veins. When valves leading to the lower
chambers (the ventricles) open, the atria contract, emptying the blood
into the ventricles. Those valves close and the exit valves into the
arteries open. The ventricles then contract in their turn, and the blood
is pumped out into the arteries. The contractions of both the upper and
lower chambers of the heart are controlled by a network of special electrical
tissue throughout the heart. An area in the atria acts as a pacemaker,
ensuring that the heart beats regularly and evenly.
If the heart rhythm becomes irregular (as in atrial
fibrillation), the chambers may not have time to fill properly or empty
completely, so reducing the amount of blood being pumped out. Similarly,
if the heart is beating too fast or too slowly (see palpitations), it
can’t work efficiently.
The average person has about five litres (eight pints)
of blood, which circulates right round the body in one minute while you’re
at rest. When you exert yourself physically, your heart speeds up and
pumps harder, and may pump as much as 25 or 30 litres in a minute. A
failing heart is not capable of doing this, which is why vigorous exercise
of any kind becomes difficult.
When the heart starts to fail . . .
Blood continues to return to the heart but if it is
not working efficiently the heart may not be able to keep up. The pressure
inside the heart rises and it may become enlarged. The pressure backs
up into the circulation too. As a result the tiny blood vessels known
as capillaries begin to leak fluid into the tissues. This can happen
in the lungs or elsewhere in the body, causing swelling. This
is most obvious in the ankles where fluid collects under the influence
of gravity.
As there is less blood reaching the kidneys they can’t
work as efficiently either. They pass out less salt and water than usual
and even more fluid collects in the tissues.
When fluid leaks into the lungs it takes up space which should have been
filled with air, reducing the amount of oxygen reaching the blood, and
so the person becomes short of breath. They may feel tired because
their muscles are not getting all the oxygen and nutrients they require
and waste products are accumulating rather than being carried away in
the bloodstream as they should be. We will look more closely at the symptoms
of heart failure on page 23.
Recent research has identified a variety of hormones
(chemical messengers) and nerves that send signals from a failing heart
to change the way the kidneys and blood vessels work. Some of these changes
help to relieve the load on the heart; others increase it.
There are many different types of medications for treating heart failure
and, as they work in different ways, they can often be combined. Some
work by correcting or controlling the underlying problem, some control
symptoms and some do both. We will look at how some of these medicines
work on page 33.
The
heart and great vessels. |
Internal
anatomy of the heart. Arrows indicate direction of blood flow. |
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Blood
supplying tissues with nutrients and oxygen. |
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KEY POINTS
- Your heart is a pump made of muscle
- The average person has about five litres of blood
circulating round the body in one minute when at rest
- A failing heart cannot pump harder which is why
vigorous exercise is difficult
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