An equation that models an efficient fluid transport system — Murray’s law states that the radii of vessels will narrow from large vessels to small vessels. The formula for this law is used to find the sum of the cubes of the radii of the branching vessels. This sum should approximate the radius cubed of the larger “parent” vessel. (Teacher note: A diagram of a vein splitting and labeling the “parent” vessel and the smaller vessels may help to convey this idea.) r³ = r₁³ + r₂³ + r₃³ + …