The flow, F, of a fluid through a rigid tube is directly proportional to the pressure difference and the fourth power of the radius, and inversely proportional to the tube length and the fluid viscosity:
F = ∂Pπr⁴ / 8Lη
During acute homeostatic maintenance of blood flow to tissues (e. g., during exercise or hemorrhage), which of the following variables in the Hagen-Poiseuille equation are unlikely to be regulated by the autonomic nervous system? (Note: More than one answer may be correct.)

a. Flow
b. Pressure gradient
c. Tube radius
d. Tube length
e. Fluid viscosity