First 181 words of the document:
the four chambered heart consists largely of cardiac muscle which is myogenic : It
rhythmically contracts and relaxes of its own accord. This is not due to nervous
The cardiac cycle refers to a sequence of events which takes place during the
beating of the heart.
The sinoatrial node, situated in the wall of the right atrium is spontaneously
active and its excitation spreads out across the two atria, causing them to
contract at the same time
but is prevented from spreading to the ventricles by a thin layer of connective
tissue, acting as electrical insulation.
Excitation spreads via the atrioventricular node, through the Bundle of His to
the apex of the ventricle.
The Bundle branches into Purkinje fibres in the ventricle walls which carry the
wave of excitation upwards through the ventricle muscle.
The impulse eventually causes the ventricle muscles to contract simultaneously
from the apex upwards.
The contraction of the ventricles is therefore delayed after the atria.
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Systole and relaxations:
The cardiac cycle refers to a sequence of events which takes place during one
Atrial systole atria contract, pressure increases, blood forces through the
atrioventricular valves to the ventricles
Ventricular systole ventricles contract, AV valves close
Ventricular diastole ventricles relax and the pressure falls, blood under high
pressure in the arteries causes the semi lunar valves to shut. Blood from the vena cava
and pulmonary veins enters the atria and the process begins again.
Cardiac diastole.…read more