The initiating event in diastole is the sinoatrial node firing. The impulse initiating at the sinoatrial node occurs independently, without neuronal input.
The sinoatrial node spreads a depolarization wave causing the two atria to contract simultaneously, pushing blood out into the ventricles.
The tricuspid valve is between the right atrium and right ventricle, and it opens in response to increased pressure.
The mitral valve is located in between the left atrium and left ventricle, and it also opens because of pressure from the atrium contracting.
The ventricles fill with blood due to atrial contraction, as blood flows through the atrioventricular valves (AV valves), which are the tricuspid and mitral valves.
The pulmonary valve is between the right ventricle and pulmonary artery, and it is closed during diastole. This allows blood to fill the ventricles properly, and also prevents backflow into the ventricle from the previous systolic cycle.
The aortic valve is between the left ventricle and aorta. It is closed to allow the left ventricle to fill properly, as well as prevent backflow from the previous systolic cycle.
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