Vascular resistance

Vascular resistance is the opposition to blood flow within the circulation. It arises mainly from friction in vessels and is strongly influenced by vessel radius, blood viscosity, and vessel length.

Poiseuille's law: A fluid-flow relation showing how flow through a cylindrical tube depends on pressure difference, radius, length, and viscosity. It explains why small changes in vessel radius can produce large changes in resistance.
Blood flow: The movement of blood through the heart and blood vessels. Resistance describes what opposes this movement.
Systemic vascular resistance: The total resistance opposing blood flow through the systemic circulation. It summarizes the resistance faced by the left ventricle in supplying the body.
Vascular impedance: The opposition to pulsatile blood flow, incorporating resistance and effects of vessel elasticity and blood inertia. It extends resistance to account for the heartbeat's changing pressure and flow.
Vessel radius: The distance from a blood vessel's center to its inner wall. Radius has a particularly strong effect on resistance in small arteries and arterioles.
Blood pressure: The force exerted by circulating blood on the walls of blood vessels. Pressure differences drive flow against vascular resistance.
Pulmonary vascular resistance: The resistance opposing blood flow through the pulmonary circulation. It is used to assess the load on the right ventricle and pulmonary vessels.
Total peripheral resistance: The aggregate resistance to blood flow across the systemic circulation, often used as a synonym for systemic vascular resistance. Its common clinical use can obscure the distinction between systemic and local resistance.
Blood viscosity: A measure of how strongly blood resists deformation and flow. Greater viscosity increases friction and raises resistance at a given vessel geometry.
Pressure gradient: A change in pressure over distance that drives fluid movement. A pressure gradient is the driving force paired with resistance in blood flow.

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