Which factor limits the application of Pouiselle's Law in cardiopulmonary bypass (CPB)?

The application of Poiseuille's Law in the context of cardiopulmonary bypass (CPB) is predominantly limited by its accuracy for laminar flow conditions. Poiseuille's Law describes the flow of an incompressible and Newtonian fluid through a cylindrical pipe, and it applies strictly under laminar flow conditions where the flow is smooth and orderly.

In CPB, the flow typically involves a combination of laminar and turbulent flow due to the complex geometry of the circuit and variations in the flow rates. When turbulent flow occurs, the assumptions of Poiseuille's Law do not hold true, and the relationship it provides between flow rate, pressure drop, viscosities, and pipe dimensions becomes less reliable. This limitation places constraints on the practical use of Poiseuille's Law for predicting fluid behavior in a CPB setting.

The other options do not accurately address the foundational aspects of Poiseuille's Law. Although the law does apply to Newtonian fluids, it's critical to recognize that the primary concern in CPB is the flow regime rather than the characteristics of the fluid. Length and flexibility of tubes also play a role, but the essential limitation in this context remains tied explicitly to the laminar flow condition.