Before deoxygenated blood coming from the inferior and superior vena cava fills the right atrium, it is assumed that the pressure in the right atrium is 0mmHg. The blood flowing from the right atrium flows passively into the right ventricle because the atrioventricular (AV) valve (or bicupsid) between them is open. This is called diastole because the heart is relaxed and not contracting yet. Systole (contraction) begns to occur when the pressure in the right atrium increases by a little bit to push the rest of its contents into the right ventricle. This causes the pressure in the right ventricle to exceed the pressure of the atrium above it and the AV valve to close shut with the help of the chordae tendineae and papillary muscles. The right ventricle pressure has to exceed the pressure in the aortic valve in order for the aortic valve to open and blood can go to the lungs to become oxygenated. The aortic pressure is the determinant our blood pressure and has a normal range of 80-120 mmHg. As the ventricle builds up in pressure by isometric contraction, it surpasses the aortic pressure and the aortic valve opens. As the right ventricular pressure begins to decrease due to ejection of blood into the aorta, it reaches below 80 mmHg and the aortic valve closes. The ventricle will cease systole and go into diastole. The ventricle ejects 70 mL of blood into the aorta. It's pressure will decrease to a point lower than the pressure in the right atrium, the AV valve will open again and the right ventricle will be filled passively once again.This similar process occurs on the left side of the heart as the left atrium fills with oxygenated blood that came from the lungs.
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