This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiac Output, Subject name: Physiology
Peripheral resistance is inversely propoional to: 
Select and explain. 
A. Radius of the vessel, B. Viscosity of blood, C. Length of the vessel, D. Elasticity of the vessel wall
A: Radius of the vessel
Answer: A: Radius of the vessel 
Explanation: Poiseuille-Hagen equation: (for the flow of a fluid in a rigid cylindrical tube) Q = P x (pr4/8el) Q = flow, P (or P1 - P2) = pressure difference between two points, r = radius of the tube, e = viscosity of the fluid, and l = length of the tube. By Ohm's law: Q = P/R; OR R = P/Q. {R = resistance} Thus, viscous resistance (R) can be given by combining the above two formula: R = (8el)/(pr4) Thus, viscous resistance is propoional to the viscosity (e) of the fluid and the length (l) of the tube. inversely propoional to the fouh power of the radius of the tube (blood vessel). Thus option A is correct.