According to Poiseuille's law, the pressure needed to drive a fluid through a tube will increase under which of the following conditions?

1. Increased fluid viscosity
2. Decreased tube length
3. Decreased rate of flow
4. Decreased tube radius
a. 1 and 2
b. 2, 3, and 4
c. 1 and 4
d. 1, 3, and 4

ANS: C
The difference in pressure required to produce a given flow, under conditions of laminar flow through a smooth tube of fixed size, is defined by Poiseuille's law:
P = 8nl/r4,where P is the driving pressure gradient, n is the viscosity of the fluid, l is the tube length, is the fluid flow, r is the tube radius, and 8 is a constant.