By doubling our radius, we not only double our denominator, we actually increase our denominator by a factor of 16. And that's because the radius is raised to the power of four, and two to the power of four is 16. So if the radius is doubled but everything else is allowed to stay the same, then we decrease our resistance by a factor of 16. And that means, in general, changing the radius will affect the resistance much more than if we change the length or if we change our viscosity. So from this discussion, we can basically conclude that our body actually changes the resistance of our blood vessels not by changing the viscosity or the blind length, the blood vessel length, but rather by changing the radius, the diameter of our blood vessel. So it's no surprise that our body uses the blood vessel's diameter to control the resistance.