 Dear students in this topic we shall introduce you to the basic principles of hemodynamics. Hemodynamics is the study of physical laws that explain the relationship between pressure and flow of blood through blood vessels of circulatory system. We shall discuss a few important principles of hemodynamics. The first one is that the animals with a closed circulatory system have the blood flow that moves or flows in a continuous circuit. Here the implication of closed circulation is that the blood returned to the heart is equal to the blood pumped by the heart. This volume is equal because blood is an incompressible fluid. So the volume of blood returning back to the heart each minute must be equal to the cardiac output that is the volume that is pumped each minute. Dear students hemodynamics also studies the velocity of flow in the circulatory system. The velocity of flow of blood at any point is inversely proportional to the total cross sectional area of the blood vessel. The equation that describes the velocity of flow says that velocity of flow is equal to the blood flow divided by the cross sectional area of the blood vessel. It implies that blood flow velocity is highest where cross sectional area is smallest. That is arteries which are comparatively larger arteries, their cross sectional area is smaller and the more velocity of blood flow there is. Whereas lowest velocities are seen where the cross sectional area is largest. Capillaries are blood vessels where cross sectional area is largest and therefore the flow of blood is minimum. Highest velocities occur in the aorta and pulmonary arteries which are the largest arteries of the body whose cross sectional area is the least. When this blood flow occurs in the capillaries then velocity of flow falls or decreases. However, when blood comes in veins from the capillaries, then its velocity increases here because the cross sectional area of the veins is also less. Capillaries with less blood flow have very important functional implications or significance because slow flow of blood takes time and the exchange of materials which occurs in the tissues also requires time. Therefore, when blood flow is slow from capillaries, then the time consuming exchange of materials between the blood and tissues through capillaries that can happen with success. Hemodynamics also explains the vascular resistance. Vascular resistance is the resistance to flow offered by the blood vessels. This vascular resistance must be overcome to create flow through the circulatory system. Vascular resistance is increased by vasoconstriction which reduces the diameter of the blood vessels and increases the viscosity of the blood in the vascular resistance. Whereas visodilation, i.e. the dilation of the blood vessels or the lower viscosity of the blood i.e. the blood cation, these two factors decrease the vascular resistance to flow. Dear students, vascular resistance is related to the vessel radius, vessel length and blood viscosity and this is used in the calculations of blood pressure, blood flow and cardiac function. Dear students, another important topic and principle of hemodynamics is the blood pressure that is produced due to the pumping action of the heart. As the pumping action of heart is pulsatile, so is the blood pressure. Blood pressure in systemic arteries varies during each heart beat that is it is pulsatile and in one phase this pressure is high high and is known as systolic blood pressure. In the other phase it is low and is known as diastolic blood pressure.