The cardiac cycle

===Lecture Details===
Julia Choate; Week 10 MED1022; Physiology
===Lecture Content===
Systole is ventricular contraction and blood ejection, diastole is refilling. Phases of the cardiac cycle are passive filling of ventricles, atrial contraction, isometric ventricular contraction, ventricular ejection, isometric ventricular relaxation.
a) In passive filling of the ventricles the pressure in the atria is higher than that of the ventricles. The pressure of the aorta is higher than that of the left ventricle so the aortic valve is shut. ECG is silent at this time. b) In atrial contraction the SA node fires and the atrium contracts, filling the ventricle. P wave. Pressure in the ventricle raises slightly but the aortic valve is still shut (20% of volume is transferred here).
In isometric ventricular contraction the excitation spreads to the ventricle which begins to contract causing the QRS complex of the ECG. Pressure in the ventricle rises and the AV valve shuts when ventricular pressure increases above atrial pressure causing the first heart sound. Aortic valve is still sealed and there is isovolumetric contraction. Ventricular pressure rises sharply.
When ventricular pressure rises above aortic pressure the blood is ejected to the aorta. AV valve remains shut. When blood has left, ventricular pressure starts to fall again and the ventricle begins to relax, causing the T wave of the ECG.
Isometric ventricular relaxation is when ventricular pressure is less than aortic pressure, causing the aortic valve to snap shut (second heart sound). AV valve is still closed and the ventricle remains sealed causing isovolumetric relaxation. Ventricular pressure falls and when it falls below atrial pressure the cycle repeats.
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