Vascular Resistance

The resistance of a blood vessel is directly proportional to the length of the vessel and the viscosity of blood and inversely proportional to the diameter of the vessel ( in fact to the fourth power of the radius).

Resistance ∝ ((vessel length x blood viscosity) / vessel radius⁴)

In normal physiology the vessel length and the blood viscosity do not vary significantly. However the diameter of certain vessels is under physiological control and thus the vascular resistance and hence blood flow may be regulated.

Additional Information

Blood Viscosity
Blood viscosity is determined largely by the concentration of red blood cells which is regulated physiologically. In severe dehydration or at high altitude the red cell count may increase (polycytaemia) and blood viscosity increases.