But because our autonomic nervous system innervates and controls the smooth muscle found inside the blood vessel, the diameter and the radius can easily be controlled by our body. And that's exactly why it's the diameter that actually influences the resistance of blood vessels the most in our body. So the next question is, what exactly is the relationship between the resistance and these three different factors that we mentioned just a moment ago? So the relationship is given by this equation. So our upper case R. The resistance of the blood vessel is equal to eight multiplied by the viscosity multiplied by the length of that blood vessel divided by pi multiplied by R raised to the power of four. Now, from this equation, we see that the resistance depends directly on the viscosity and on the length.