Because it is often difficult or impossible to measure the radii of all vessels in a vascular bed, we designate `(8Ln)/(PIr^4)`, the inverse of the term in Poiseuille's law, as the resistance to flow, `R`, which is equal to the pressure difference `(P_1 - P_2)` across a vascular bed divided by the flow rate, `Q`:

`R = (P_1 - P_2)/Q = (8Ln)/(pir^4)`

Notes

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