Which of the following is NOT a determinant of total peripheral resistance?

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Total peripheral resistance is the resistance the systemic vessels offer to blood flow, set mainly by the caliber of the arterioles, blood viscosity, and the total length of the circulation. The radius is the most influential factor because resistance rises dramatically as radius decreases (a small drop in radius greatly increases resistance, per a r^4 relationship). Blood viscosity adds resistance because thicker blood is harder to push, and longer vessel pathways add resistance simply by giving the flow more distance to traverse. Turbulence can raise resistance when flow becomes non-laminar, such as at high velocity or in narrowed segments, but under normal conditions flow is largely laminar and turbulence isn’t a primary determinant. Heart rate, while it affects cardiac output and thus pressure and flow, does not change the intrinsic resistance of the peripheral vessels themselves. So heart rate is not a determinant of total peripheral resistance.