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.
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. It closes shortly after the Aortic Valve, and this difference in timing can be heard as a split S2.
After the aortic valve closes, the ventricles relax isovolumetrically until ventricular pressure drops below atrial pressure to allow opening of the atrioventricular valves (tricuspid and mitral).
The mitral valve is located in between the left atrium and left ventricle. It opens to allow blood to flow from the atria to ventricles.
The tricuspid valve is between the right atrium and right ventricle, and it opens in response to increased pressure.
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.
At the end of diastole, the SA node initiates an atrial contraction. 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 the last of the atrial blood out into the ventricles.
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