MEDICAL PHYSIOLOGY

Detailed explanation-1: -There are three primary factors that determine the resistance to blood flow within a single vessel: vessel diameter (or radius), vessel length, and viscosity of the blood. Of these three factors, the most important quantitatively and physiologically is vessel diameter.

Detailed explanation-2: -The influence of lumen diameter on resistance is dramatic: A slight increase or decrease in diameter causes a huge decrease or increase in resistance. This is because resistance is inversely proportional to the radius of the blood vessel (one-half of the vessel’s diameter) raised to the fourth power (R = 1/r4).

Detailed explanation-3: -As blood flows, it encounters various factors that resist flow and movement of blood, known as the vascular resistance. The first factor to contribute to vascular resistance is blood viscosity, where you can think of viscosity as the fluid’s thickness, or how sticky it is.