Describe the process of a cardiac cycle.

Cardiac cycle is the contraction and relaxation of the heart to push the blood to the rest of the body. Systole = contraction (so, atrial systole means contraction of the atrium, ventricular systole means contraction of the ventricle) Diastole = relaxation (complete diastole means relaxation of both ventricle and atrium) 1.    Atrial systole- Blood from pulmonary vein filled up left atrium passively. The pressure in the atrium increases as it is filled up by the blood. Left atrium then contract to push blood into the left ventricle. The atrioventricular (AV) valve is forced open when blood flows into the ventricle. The pressure in the left ventricle increases gradually. 2. Ventricular systole- When the pressure of the ventricle is higher than the atrium, this forces the AV valve to shut to prevent blood flowing backwards. At the same time, the pressure in the left ventricle keeps increasing. When the pressure in the ventricle is higher that in the aorta, ventricle contracts to push blood into the aorta. Semilunar (SL) valve is forced open as blood rushes out of the heart. When the ventricle finishes contracting, the muscle relaxes and the pressure in the ventricle drops greatly. 3. Diastole- Both atria and ventricle are now relaxing. The blood starts flowing into the atrium passively and the pressure in the atrium increases gradually. The cycle starts again.