Quick question about Reynolds number formula

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So, the Reynolds number formula is::

Re=(l)(density)(velocity)/viscosity

Re is the reynolds number.
In the book, it says that l is the "size of the obstacle in the flow". I was wondering if someone can clearify what this length is. Like if a 3-d object is moving through the fluid, which would be the length?

THanks a million!

 

[Charles_Carmichael]

I'm pretty sure that l is supposed to be the diameter of the tube (at least with regards to blood flow), not size of "obstacle in the flow" (whatever that even means).

Edit: Just checked Costanzo's physio book and Re = [(rho)(d)(v)] / (viscosity), where rho is the density of blood, d is the diameter of the vessel, and v is the velocity of blood flow.