参考答案:
The sinoatrial node (SA node /”pacemaker”) initiates the heartbeat.
(OR The SA node depolarises to initiate a heartbeat.)
The wave of (atrial) depolarisation passes across the atria, causing the contraction of the atria.
The contraction of the atriua pushes blood into the ventricles (OR the left ventricle).
The wave of depolarisation passes down the septum.
The time taken for the passage of the depolarisation allows the ventricles time to fill with blood.
The ventricle contracts from the base towards the top of the heart.
The left ventricle has a thick muscular wall and it creates a high pressure
The blood being pushed out of the ventricle closes the atrio-ventricular valve.
The closure of the valve prevents backflow (from the ventricle to the atrium).
The blood flows at high pressure into the aorta opening the semilunar valves.
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