Transcript The Heart - WordPress.com

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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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Heart dissection – follow the
instructions on the sheet.
When you have finished, read about
the cardiac cycle in the textbook and try
to make notes about each of the stages
on the worksheet from last lesson.
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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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Activities
1. Card sort – put the statements into the
correct order to show the events that
occur during the cardiac cycle.
2. Try the questions on your worksheet.
You may wish to use the textbook to help
you interpret the information on the
graph.
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Control of the Heartbeat
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Pacemaker cells of the heart
The heart can beat without any input from the nervous
system as longs as its cells stay alive. This is due to
myogenic contraction.
Muscle cells (myocytes) in the heart have a slight
electrical charge across their membrane. They are
polarized. When the charge is reversed, they are said to
be depolarized and this causes them to contract.
Depolarization is initiated in a region of the heart called the
sinoatrial node (SAN) – also known as the pacemaker –
which is in the wall of the right atrium.
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Myogenic stimulation of the heart
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Interactive heart
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Artificial pacemakers
Artificial pacemakers are devices implanted in people
whose heart’s electrical conduction system is not working
properly.
Problems include the SAN not firing, and the blockage or
disruption of impulses between the SAN and AVN, or in the
bundle of His.
Pacemakers monitor the
heart’s electrical activity and
stimulate the ventricles or atria
to contract when necessary.
Impulses are transmitted
down electrodes implanted in
the muscular walls.
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What are electrocardiograms?
The electrical activity of the heart can be monitored by
an electrocardiograph.
Several electrodes are
attached to specific places
on a person’s chest and
limbs. These detect
changes in polarization in
the heart by measuring
current at the skin surface.
The leads are connected to
a machine that draws an
electrocardiogram (ECG).
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Components of an ECG trace
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ECG in diagnosis
ECGs are used to diagnose problems with the heart, as
variations in different components of the trace can
indicate a disease or other abnormality.
An ECG may be taken
while the patient is relaxed
or it may be taken before,
during and after exercise.
This is called a ‘stress test’
and usually involves the
patient exercising on a
treadmill while attached to
an ECG machine.
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Abnormal ECGs
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