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The Cardiac Cycle We will begin the cycle during mid to late ventricular diastole, when the heart is completely relaxed. We will focus on the left side of the heart; remember that pressure differences are also occurring on the right side (they are a lot less, however). 1. Mid to late Diastole The pressure in the atrium is increased and greater than the ventricular pressure because blood is returning to the atria. This increased atrial pressure forces the AV valves open and the left ventricle fills passively as the atrial blood drips into the ventricle (flow from higher to lower pressure). This dripping fills the ventricle to 70% of its capacity! 2. Atrial Systole The atria contract and rising atrial pressure pushes blood into the ventricles through the open AV valves This atrial contraction "tops off" the ventricles, adding another 30 % of the total volume of blood to the 70% of the volume that has passively "dripped". The ventricles now contain the maximum amount of blood that they will contain (about 130 ml). This is the end diastolic volume (EDV). 3. Early Ventricular Systole The ventricles contract isometrically, generating tension and a rise in ventricular pressure. This is the period of isovolumetric contraction. As the pressure inside the ventricles rises above that in the atria, the AV valves shut (LUB!) No blood can flow because the semilunar and AV valves are both closed. 4. Late Ventricular Systole Once pressure in the ventricles exceeds the pressure in the aorta, the semilunar valve opens and blood flows into the aorta. This is the start of ventricular ejection. About 60 - 70 ml of blood is ejected. This ejected blood is called the stroke volume. Ventricular pressures then rapidly fall and aortic blood begins to flow back towards the semilunar valves. This closes the valve (DUP!) and the aortic pressure slightly increases as the elastic aortic walls recoil, forming the "bump" in pressure that is called the dichrotic notch. The amount of blood remaining in the ventricle when the semilunar valve closes is called the end systolic volume (ESV). 5. Early Ventricular Diastole All the heart valves are now closed and the ventricles are relaxing. Ventricular pressure is still higher than atrial pressure. This is the period of isovolumetric relaxation. When the ventricular pressure falls below the atrial pressure, the AV valves open and blood "drips" into the ventricles. Both the atria and the ventricles are now in diastole, but the ventricular pressures continue to fall.