Which factor is NOT included in the Poiseuille's law equation?

The correct answer is C, Temperature of the fluid, because Poiseuille's law specifically relates to the flow rate of a fluid through a cylindrical pipe and is primarily influenced by certain factors: viscosity of the fluid, length of the tube, and radius of the tube.

In the context of Poiseuille's law, the viscosity represents the internal friction of the fluid, which affects how easily the fluid can flow. The length of the tube influences the resistance to flow, as a longer tube means more surface area for drag. The radius of the tube is critical because even small changes in the radius result in significant changes in flow rate; this relationship is proportional to the fourth power of the radius.

Temperature can affect the viscosity of the fluid, but it is not explicitly a component of Poiseuille's law itself. In this particular law's equation, temperature does not have a direct role in determining flow rate, which is why it is the factor that is not included. Understanding these relationships is fundamental in respiratory care, as they can impact airflow in the respiratory system and various medical devices used for respiratory support.