The viscosity of normal (healthy) blood is about three times greater than the viscosity of water. Certain diseases such as polycythemia can cause the viscosity of blood to be as much as three times greater than normal. If the viscosity of a person’s blood increases by a factor of three while the pressures at the ends of an artery stay at their normal levels, by what factor must the diameter of the artery increase to keep the same volume flow rate?