 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, i.e. the arterial system has more pressure and the chambers are relaxed and the pressure is less. 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 the filling of the ventricles. Now, the next phase is the atrial contraction. When atria contract, as a result, pressure rises in them. As a result, the blood is ejected or flows into the ventricles, dear students. The atrial contraction provides only 30% volume of the total ventricular output or ventricular capacity. The remaining 70% of the larger volume of the ventricles 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. This is the time when atrioventricular valves are closed. So that when the ventricles contract, the back flow of blood from the ventricles into the atria does not occur. Dear students. When ventricular contraction starts, the atrioventricular valves are closed. They will also be closed. Ventricles become sealed chambers because of this closing. Because the atrioventricular valves are closed too. Similarly, the atrioventricular valve is not open yet. They are also closed. Therefore, ventricles become closed chambers. 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 ki vaja say blood is ejected into the aorta and pulmonary arteries. As a result, blood flows into the systemic and pulmonary circuits and ventricular volume decreases. Dear students, after pumping, the ventricles begin to relax. Intraventricular pressure falls. Aortic valves close. Atrioventricular valves open. Ventricular filling starts again by venous filling pressure and the cycle is repeated.