Hemodynamics refers to the mechanics of blood flow through the cardiovascular system. It is a general study of blood flow, resistance to blood flow, blood pressure, and their interrelationships. 1. Blood flow: the volume of blood flowing through a certain cross-section of a blood vessel per unit of time, which also becomes the volumetric velocity. Blood flow velocity is the linear velocity of a mass of blood moving through a blood vessel. Blood flow velocity is proportional to blood flow and inversely proportional to the cross-sectional area of the blood vessel. Intravascular blood flow includes laminar and turbulent flow. 2. Resistance to blood flow: the resistance encountered by blood flowing through blood vessels, which is mainly generated by the friction between the flowing blood and the walls of the vessels as well as the molecules inside the blood. Friction consumes part of the energy converted into heat, so the energy consumption during blood flow causes a gradual decrease in blood pressure. 3. Blood pressure: the pressure of the blood flowing in the blood vessel on the side wall of the blood vessel, that is, the pressure per unit area is called blood pressure. The unit is Pa, which is customarily expressed in mmHg. The blood pressure usually referred to is arterial blood pressure. The decrease in blood pressure within each segment of a vessel is directly proportional to the amount of resistance to blood flow in that segment. Hemodynamics plays an important role in clinical practice and is the most basic indicator for the diagnosis of diseases of the cardiovascular system.