 Dear students, in this topic we shall discuss in detail the vascular resistance to flow. The resistance that must be overcome to push blood and create flow through the circulatory system is known as vascular resistance. There are two types of vascular resistance. The resistance offered by the systemic circuit is known as systemic vascular resistance and the resistance offered by the arteries or blood vessels of pulmonary circuit is known as pulmonary vascular resistance. The systemic vascular resistance is used in the calculations of blood pressure, blood flow and cardiac functions. Dear students, many factors affect the vascular resistance. The two major factors are the elasticity of vessel wall and the diameter of the vessel. The decrease in blood vessel diameter by vasoconstriction increases vascular resistance. If the diameter of the vessel increases by vasodilation, vascular resistance decreases. Now we shall discuss the method and equation for calculation of vascular resistance. Vascular resistance is calculated by a modified form of Poisewell's equation. It is read as the resistance to blood flow is equal to the 8 L eta divided by pi r raised to power 4, where r is the resistance to blood flow. R is the length of the blood vessel, eta is the viscosity of blood and r is the radius of the blood vessel. From this equation, it is evident that resistance to flow is inversely proportional to the fourth power of radius of the vessel. So, vascular resistance is the lowest in larger vessels, whereas in narrow capillaries, vascular resistance is the highest or maximum.