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The Heart…read more

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How the Heart Contracts
The heart is mainly composed of cardiac muscle that has the ability to
generate its own electrical signal for contraction without the need for
nervous stimulation.
The heartbeat is said to be intrinsic as it comes from within the heart
itself, and myogenic because it occurs without nervous stimulation.
The rhythm of the heartbeat is intrinsic, but the timings can be altered
through the sub-conscious autonomic nervous system and the action of
hormones.
The sympathetic nervous system speeds
up heart rate and increases the strength
of heart contraction.
The parasympathetic/vagus nerve causes
the heart rate to decrese and decreases
the strength of heart contraction.…read more

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How the Heart Contracts
The impulse for contraction is stimulated by the sino-atrial node or SAN (small mass of cardiac
muscle found in the wall of the right atrium). The SAN sets the hearts rhythm (heartbeat) and
causes the atria to contract.
The impulse reaches the atrio-ventricular node (AVN), which conducts the impulse from the atria
to the ventricles. There is a delay as the impulse passes through the AVN, and this delay is
important as it gives the atria time to fully contract and fill the ventricles before the ventricles
contract.
The impulse from the AVN passes down the septum through the bundle of His where it branches
off.
The impulse is sent down the left and right bundle branches.
It is then sent to the Purkinje fibres which surround the walls of the ventricles to cause
contraction and push blood out of the aorta…read more

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Regulation of Heart Rate
As exercise intensity increases, the brain (medulla ­ cardiac
control centre) sends an impulse down the sympathetic
branch of the autonomic nervous system, increasing its
firing to the sino-atrial node and increasing the strength of
contraction.
As intensity decreases, the brain (medulla ­ cardiac control
centre) sends an impulse down the parasympathetic branch
of the autonomic nervous system, decreasing its firing to
the sino-atrial node and decreasing the strength of
contraction.
The cardiac control centre receives nerve impulses (the
information about blood acidity) from the chemoreceptor's.…read more

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Heart Function Definitions
Cardiac cycle: the time taken for the heart chambers to go through a
complete cycle of systole and diastole. Last 0.8 seconds.
Stroke volume: volume of blood leaving the left ventricle per beat.
Ejection fraction: proportion of blood in left ventricle pumped out in
each beat.
Cardiac output: volume of blood leaving the left ventricle per minute.
Relationship between stroke volume and cardiac output
Stroke volume x Heart rate = Cardiac output
ml bpm ml/min…read more

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