 Dear students, in this topic we shall study the changes or fluctuations in pressure and flow of blood during one heart beat. You know that chambers of the heart contract during each heart beat in a sequence. The rhythm of these contractions results in a sequential fluctuation of pressure and flow of blood in the circulatory system. We shall study all the contraction and laxation events of the heart in sequence and see their effect on pressure and flow. First we shall start from the diastole phase. During diastole the aortic valves are closed, ventricles and atria are in relaxed position. The pressure difference between the relaxed chambers and the systemic and pulmonary arteries is large. During these diastole the atria ventricular valves remain open. This is why blood from the venous system flows directly into the ventricles via the atria by venous filling pressure. Venous filling pressure plays an important role in ventricles filling. Now the next phase is the atrial contraction. Then atria contract as a result pressure rises in them. As a result the blood is ejected or flows into the ventricles. The atrial contraction provides only 30% volume of the total ventricular output or ventricular capacity. The rest 70% of the ventricular volume is filled by the venous filling pressure during diastole. So atrial contractions only are simply tops up nearly full ventricles. Now when ventricles are filled and atrial contraction has occurred now ventricular contraction starts. When ventricular muscles begin to contract pressure rises in the chambers. Now atrial ventricular valves close so that when ventricles contract the blood from ventricles into the atria does not come out. Dear students, you know that when ventricular contraction starts then the aortic valves are closed because atrial ventricular valves are closed. So pressure rises in them without a volume change. It means that the ventricular contraction is isometric. Now as the pressure in ventricles exceeds that of the aorta and pulmonary arteries the aortic valves open. Aortic valves opening causes blood is ejected into the aorta and pulmonary arteries. As a result blood flows into the systemic and pulmonary structure circuits and ventricular volume decreases. Dear students, after pumping the ventricles begin to relax. Intraventricular pressure falls. Aortic valves close. Atrio ventricular valves open. Ventricular filling starts again by venous filling pressure and the cycle is repeated.