I.C.
At the right is shown a pipe with a uniform circular cross-section carrying fluid at a steady flow. The piece of the pipe we are considering has a length, L, and a cross-sectional area, A. The fluid is flowing at a rate of Q. Flow has dimensionality of volume per second ({Q} = L3/T). The pressure at the beginning of the segment is p1, and at the end of the segment is p2, with Δp = p1 - p2. Pressure is force per unit area, so it has dimensionality {p} = M/LT2 (= {F/A} = (ML/T2) / L2). The fluid has a viscosity, μ, which has dimensionality {μ} = M/LT.
In thinking about the flow of blood in an artery, we are interested in how the flow we get is changed as the parameters of the situation change. Let's rewrite the H-P equation that tells the flow produced by a pressure drop. Some equations giving the flow, Q, resulting from a pressure difference, Δp, are shown in the table below. Some have incorrect dimensions and therefore have to be wrong. Some have implausible functional dependence on a parameter and therefore have to be