In a laminar flow, through a horizontal tube, the flow rate is defined as the ratio of pressure difference at two points and flow resistance. Here, resistance is directly proportional to the coefficient of viscosity and tube length, and inversely proportional to the fourth power of tube radius. When combined with the flow rate equation, this relation gives Poiseuille's law for laminar flow. Considering the viscosity to be constant, a slight decrease in the radius of the tube causes a significant reduction in the flow rate. To maintain the same flow rate, the pressure difference has to increase accordingly. Likewise, it is difficult for a more viscous fluid to flow through the tube than a less viscous fluid at constant temperature. The Reynolds number is a value that indicates if the flow of a fluid is laminar or turbulent. If the Reynolds number is below two thousand, the flow is laminar, and turbulent if it is greater than three thousand. Between two and three thousand, the flow is unstable and may show chaotic behavior.