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© Boardworks Ltd 2008
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© Boardworks Ltd 2008
The human heart
The heart is a muscular
organ located between the
lungs in the centre of the
chest (thorax), and is about
the size of a fist.
It pumps blood continuously
around the body. An organism
can lose consciousness within
just a few seconds if the brain
is deprived of blood.
In foetuses, the heart begins
beating about 5–6 weeks after
conception.
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Cardiac muscle
The heart mainly consists of cardiac muscle tissue, which
like smooth muscle (but not skeletal muscle), contracts
involuntarily.
Cardiac muscle is
made up of cells that
are connected by
cytoplasmic bridges.
This enables electrical
impulses to pass
through the tissue.
It contains large numbers of mitochondria and
myoglobin molecules.
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Structure of the heart
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What structure?
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Blood flow through the heart
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Preventing backflow
Blood always flows in the same direction as it moves
through the heart during each circulation of the body.
Why is it important that blood does not flow backwards?
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Heart valves
The chambers of the heart are separated by valves
which prevent blood from flowing in the wrong direction.
Semilunar valve
Atrioventricular
valve
Semilunar valve
Atrioventricular
valve
There are valves between the atria and the ventricles…
…and there are valves leading out of the ventricles.
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How are valves held in place?
The valves between the atria and ventricles are connected
to the inner walls of the heart by tough tendons.
valve open
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How are valves held in place?
The tendons allow the valves to close and hold the valve
flaps in place. They prevent the valves from flipping up
and turning inside out. Why is this important?
valve open
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valve closed
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The cardiac cycle
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Interactive heart
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Cardiac output
The amount of blood pumped around the body is called the
cardiac output, and depends on two factors:

the stroke volume – the volume of blood pumped by
the left ventricle in each heart beat. A typical value for an
adult at rest is 75 ml.

the heart rate – the number of times the heart beats per
minute. A typical value for an adult at rest is 70 bpm.
cardiac output = stroke volume × heart rate
A typical resting cardiac output is 4–6 litres per minute.
This can rise to as much as 40 litres per minute in highly
trained endurance athletes.
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