Vascular Resistance

When blood flows through the vessels, the friction between the vessel walls and blood creates a resistive force called peripheral or vascular resistance.

This resistance is affected by three main factors.

The first factor is blood viscosity or its thickness. It remains constant except in conditions like polycythemia, where an abnormally high RBC count increases the blood viscosity, leading to higher resistance and high blood pressure.

The second factor is the length of the blood vessels; the longer the vessels, the higher the resistance. It remains relatively constant in healthy individuals, except in obesity, where additional vessels in adipose tissue increase resistance and blood pressure.

The third factor, lumen diameter, is variable across the cardiovascular system. The resistance increases when the lumen diameter is small or when a blood vessel constricts.

All the blood vessels in the cardiovascular system have a cumulative resistance known as the total peripheral or systemic vascular resistance — SVR.

Due to their smaller diameter, the arterioles, capillaries, and venules contribute most to SVR and blood pressure.