Which of the following factors is NOT included in the resistance equation?

The resistance equation in the context of physiology, particularly within the circulatory system, is often described by Poiseuille's law. This equation states that resistance to blood flow is influenced by several factors, specifically vessel length, viscosity of the blood, and vessel radius.

Systolic blood pressure, however, does not directly influence the resistance in the way that the other factors do. Instead, systolic blood pressure is a measure of the pressure in the arteries during the contraction of the heart. While it certainly relates to blood flow, it does not factor into the equation when calculating resistance specifically.

In summary, vessel length, viscosity, and vessel radius are critical components that determine the frictional resistance encountered by blood as it flows through the vascular system, making them integral to resistance calculations. Systolic blood pressure, while relevant in understanding overall cardiovascular dynamics, is not part of the resistance equation itself.