1. Poiseuille's Law is used to determine flow via various lengths, widths and pressure gradients. Flow is ultimately dependent on radius and pressure gradient because viscosity and length relatively remain constant. Small changesi n radius lead to drastic changes in resistance.
2. Parallel Circulation. Vasoconstriction serves to shunt blood flow to other areas with much less significant effects on pressure. The total resistance is inversely proportional.
3. Series Circulation. Vasoconstriction increases back pressure and decreases downstream flow and pressure.