First off, I'm a pathology resident, so it's been a long time since I've done real physics. But I really like physics and I try, whenever possible, to at least develop a basic understanding of the physics underlying physiology. With that as prelude, this question was inspired by blood flow but I suspect it has broader scope. So in physiology classes we basically model the flow of blood according to an equation formally analogous to Ohm's law: (Pressure1-Pressure2)=(total resistance)(flow) Where Pressure1 and Pressure2 are the hydrostatic pressures at the beginning and the end of the circulation respectively. Okay, so far so good. But today I was trying to understand why, in general terms, red blood cells drift toward the center of a vessel. The general answer seems to be a sort of "lift" force, which forced me to brush up on Bernoulli's principle and to think about laminar flow and viscosity. Because the fluid velocity is highest in the center, Bernoulli's principle seems to suggest the pressure should be lowest there, so there's a net force on a red cell towards the center (very roughly speaking). But my interest at the moment isn't in the tendency of red cells to drift to the center--my question is how, broadly speaking, I should think about pressures in the flow equation versus pressures in Bernoulli's equation. Imagine a pipe with fluid flowing through it. This pipe has a narrowing in the center. Bernoulli's equation would therefore suggest that the pressure at the center is lower than the pressure on either end. On the other hand, the flow equation suggests the pressure at one end has to be higher than pressure at the other end, in order to account for flow--and moreover would seem to suggest that there shouldn't be flow from low pressure to high pressure, even though Bernoulli's equation looks like it's saying the pressure in the central segment of pipe should be lower than the pressure in the segment immediately after it. I must be thinking about this the wrong way, but it's not obvious to me right now how I should change my thinking to reconcile these two equations. What I'm considering now is whether the problem is that Bernoulli's equation assumes a "lossless" system--that is, no energy is "leaking" from the fluid--while the flow equation model, with resistance and so on, assumes that energy is leaving the fluid as one moves along the pipe. Am I on the right track?