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Family
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Preview of Understanding Heart Surgery
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Every cell in the body needs oxygen
and nutrition to survive, and these essentials are transported around
the body in the bloodstream. The blood carries high concentrations of
oxygen and ‘food’ to the cell, and carries the waste products
of the cell’s activity away from the cell. At the centre of the
bloodstream is the heart, which acts as the pump and is responsible for
the efficient flow of blood to and from the cells. In order to understand
the disease processes that may affect the heart, it is useful to have
a basic understanding of its anatomy, and how it actually functions.
How it is formed
The heart is a muscular organ acting as a pump that
consists of four chambers: the left and right atria and the left and
right ventricles.
Blood enters the heart through the veins into the atria,
and is pumped out of the heart through the arteries from the ventricles.
In fact, the heart is really two separate pumps that are independent
of each other.
The left heart, consisting of its atrium and ventricle,
carries bright red blood which is rich in oxygen and nutrition. This
blood enters the left atrium, a thin-walled collecting chamber, through
the pulmonary veins which come from the lungs. The blood passes from
the atrium to the ventricle, a thick-walled muscular structure, which
pumps it all around the body through the aorta.
The right heart carries dark red blood which is low
in oxygen and rich in waste products. The blood enters the right atrium
(thin-walled) from the rest of the body via the vena cava, and passes
into the muscular right ventricle, from where it is pumped back to the
lungs via the pulmonary artery for disposal.
Thus, a closed circuit exists, consisting of the heart
and the blood vessels, known as the circulatory system. This contains
about five litres of blood, in the average-sized person, which is continuously
recycled by the beating of the heart.
Internal anatomy of the heart.
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The circulatory system works on very simple principles:
the arteries carry blood away from the heart, and the veins carry blood
back to the heart. The heart pumps blood directly into very large arteries,
which keep dividing into slightly smaller arteries as they get further
from the heart. Eventually, they become tiny as they reach the individual
cells, and form part of what is called a capillary. The capillaries surround
all the cells in the body, and are made up of an arterial supply and
a venous drainage. The veins that form a capillary are tiny to start
with, but as they drain back towards the heart, they get bigger and bigger
as they join together, until they become the huge central veins that
drain directly into the heart.
All cells in the body need to be supplied with oxygen
and nutrients, including the cells that form the heart muscle. The blood
vessels that supply this muscle are the coronary arteries, and they come
off the aorta just after it leaves the heart. There are two main coronary
arteries, called the left and the right, but like all other arteries
in the body, they divide into several smaller branches.
Circulatory system.
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There are four valves in the heart. Each atrium is
divided from the ventricle on the same side by a valve, and each ventricle
is divided from its main arterial trunk by a valve. They are named according
to the chart.
THE
HEART VALVES |
Name of
valve
Mitral
Aortic
Tricuspid
Pulmonary |
Side
Left
Left
Right
Right |
Separating
Atrium and ventricle
Ventricle and aorta
Atrium and ventricle
Ventricle and pulmonary artery |
The heart valves only allow blood to flow in one direction.
The mitral and tricuspid valves allow blood to flow from atrium to ventricle,
and the aortic and pulmonary valves allow blood to flow from ventricle
to arterial trunk. This, in combination with the rhythmic pumping of
the heart, facilitates one-way flow around the circulation.
Each valve is made up of either two or three leaflets,
which are thin, membranous structures rather like parachutes. The mitral
and tricuspid valves are attached to the wall of the ventricle that they
serve by tendinous strands called chordae tendinae, similar to parachute
lines. These prevent the leaflets billowing into the atrium when the
ventricle pumps.
As we shall see later, problems with both the coronary
arteries and the valves affect the function of the heart, and in many
cases this can be remedied by surgery.
Anatomy
of the coronary arteries. |
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The heart
valves. |
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How the heart works
In order to pump blood efficiently around the body,
the heart needs to beat at an adequate frequency (heart rate, HR), and
with each beat it needs to pump out an adequate quantity of blood (stroke
volume, SV). The efficiency of the heart can be measured as a number
(cardiac output, CO). The cardiac output can be calculated from the heart
rate and stroke volume using a simple formula:
CO
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=
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HR
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x
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SV
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Cardiac output
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Heart rate
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From this, you can see that anything that reduces either
the heart rate or the stroke volume can lower the cardiac output. The
body tries to compensate for such eventualities, and if, for example,
the heart rate drops, the stroke volume will automatically increase to
give the same overall cardiac output. However, this compen-
sation has a limit, above which the cardiac output will start to fall,
and symptoms will start to appear, such as breathlessness and ankle swelling.
The heart rate is dependent upon an inbuilt pacemaker
in the wall of the right atrium. In the average person, this beats about
72 times per minute. During exercise, when a higher cardiac output is
required by the body, it can increase up to about 200 times per minute.
The stroke volume is dependent upon the muscle of the ventricular wall,
and its ability to contract or squeeze. In the normal person, it is about
70 millilitres of blood per beat.
From the above equation, you can work out that the
average cardiac output is:
HR
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x
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SV
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=
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CO
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72
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x
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70
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=
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5.04
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beats per minute
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millilitres per beat
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litres per minute
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If the ventricle is damaged in any way, such as after
a heart attack, the stroke volume may be reduced accordingly, and the
heart may automatically speed up to compensate.
The blood pressure can be calculated from the cardiac output and the
resistance in the blood vessels around the body. It represents the pressure
of blood in the arteries, and can be measured directly by applying a
pressurised cuff to your arm (called a sphygmomanometer).
Blood pressure is expressed as two numbers, and measured in millimetres
of mercury (mmHg):
where X is called the systolic pressure, and Y is called the diastolic
pressure.
When the heart actually beats and pumps blood into
the arteries, it is called the systolic phase, and after every beat it
relaxes to refill the ventricles, which is the diastolic phase. The normal
blood pressure is about:
The blood pressure can be affected by many things,
as we shall see later, and is also a risk factor for various forms of
heart disease.
Rhythm
of the heart. |
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KEY POINTS
- The heart consists of four chambers
- Arteries take blood away from the heart, and veins
take blood back to the heart
- There are four valves in the heart allowing one-way
blood flow
- The heart rate relies on an in-built pacemaker
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