BIOB33H3 Lecture Notes - Lecture 4: Circulatory System, Atrioventricular Node, Catecholamine

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Cardiac cycle begins mid diastole (referring to ventricular diastole) when the heart is relaxing. Even though the atria are contracting we are focusing on the ventricles. Opening and closing of the valves are due to pressure difference. Semi lunar valves pop open and blood leaves through the aorta. Mid-late diastole we have a conclusion of passive filling of ventricles then contraction of the atria and it pumps the last bit of blood into the ventricles. Throughout this period there is no major change in pressures in atria or ventricles. The imp point is that ventricular pressure is bellow atria pressure which keeps the. Av open, and pressure in the aorta is greater than pressure in the ventricle which keeps the aortic semilunar valve closed. So were simply completing the filling passively and then with atrial contraction. In the isovolumetric phase, the volume of the blood and ventricles is not changing because all the valves are closed.

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