Consequences of the R^4 term in Hagen-Poiseuille relation

I seem to recall from last weeks class discussion that we found dividing a metabolic system into subsystems had an associated cost (a result of the B=kM^3/4 scaling law.) Implicit in this scaling law (as we saw in BIPH2000) are the properties of the circulatory system. In Chapter 5, Nelson uses the Hagen-Poiseuille relation of show that while two pipes with the same cross-sectional area doubles the flow of a single pipe (no surprise there), a single pipe with double the cross-sectional area has 4 times the flow, due to the R^4 term. Thus the relative about of energy needed to "drive" the flow (i.e. establish the pressure differential) will be less for larger pipes. This is perhaps the underlying physical cause for the cost of dividing a metabolic system.

[Not a fully formed idea but I thought I'd put it out there for discussion.]