Final answer:
The main variables underlying vascular resistance are viscosity, vessel length, and vessel radius, where changes in vessel radius due to vasoconstriction and vasodilation can quickly alter blood flow and pressure in the body.
Step-by-step explanation:
The variables that underlie vascular resistance (Ve) and affect blood flow and blood pressure are: viscosity, vessel length, and radius. According to Poiseuille's equation, these factors determine the resistance in blood vessels, which in turn impacts blood flow. While viscosity and vessel length change slowly, the radius of a vessel can change quickly due to vasoconstriction and vasodilation, greatly altering resistance and flow because the radius is the fourth power in the equation, making its effects on flow disproportionately large.
Comprehensively, the five main variables that influence blood flow and blood pressure are cardiac output, blood volume, vessel contraction, vessel length, and blood viscosity. Cardiac output and blood volume are both critical for maintaining adequate blood flow and pressure in the body, while vessel contraction influences the radius and thus resistance within the vessels.