Describe the cardiac cycle?

The blood enters the heart from the superior and inferior vena cava. It then enters the right atrium. When the atria fill with blood, it's known as atrial systole as they are relaxed. Once the atria have filled with blood, an impulse is fired by the sino-atrial node (SAN) which causes the atria to contract, atrial systole. This causes the blood to move through the tricuspid valve into the right ventricle. The valves then snap shut, which is the 'lub-dub' sound of the heart. When the ventricles fill with blood it's known as ventricular diastole, as they are relaxed. Once full, an impulse is fired by the atrio-vetricular valve (AVN) causing the ventricles to contract from the apex upwards. In the right side of the heart, the blood rushes to the pulmonary artery. Remember, arteries are always away from the heart and veins are always towards the heart.In the left side of the heart, you have the blood arriving from the pulmonary vein, going to the left atrium, through to the left ventricle, through the bicuspid valve and then to the aorta where the blood is pumped around the body. The left side of the heart is therefore more muscular, as it needs to send oxygenated blood at a higher pressure around the whole body.