Blood circulates through human heart according to the following sequence of events-
I. Diastole of auricles: during diastole of auricles, bold from upper portion of the body enters into right auricle though Superior and interior vena cava. Similarly blood from lower portion enters into the same chamber through inferior vena cava. Blood from cardiac muscles enters into right auricle by coronary sinus. The right auricle receives deoxygenated blood from these three sources. At the same time oxygenated blood from the two lungs enters into left auricle through four pulmonary veins.
II. Systole of auricles: as soon as the auricles become full with oxygenated blood, systole starts. Due to that systole pressure both tricuspid and bicuspid valves at right and left atrioventricular apertures open. After this diastole of ventricles begins.
Systole of ventricles: after being filled by blood, systole of ventricles starts. Both bicuspid and tricuspid valves close to prevent reverse follow of blood to the auricles. So the same time both semilunar caves open and deoxygenated blood from right ventricle goes out of the heat by pulmonary artery. Oxygenated blood from left ventricle flows to all parts of body through the aorta and to cardiac muscles by the coronary artery.
Blood from through the heat due to chronological systole and diastole of auricles and ventricles. The valves here maintain the direction of blood flow. Diastole of ventricle: the moment Systole starts in auricle, both the ventricles start to expand. As a result deoxygenated blood from right auricle and oxygenated blood from left auricle enter into right and left ventricle respectively.
