Which factor is not among the five variables influencing blood flow and blood pressure?

1. Five variables govern systemic blood flow and blood pressure. 2. They are cardiac output, compliance, blood volume, blood viscosity, and vessel length and diameter. 3. The number of heart valves is a fixed anatomical count and appears nowhere on that list. 4. Compliance, viscosity and cardiac output are all named members of the five, so each of them is a genuine variable. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 20 "The Cardiovascular System: Blood Vessels and Circulation", section 20.2 Blood Flow, Blood Pressure, and Resistance_