Inulin is used in analysis of
glomerular filtration rate. It is not naturally found in the
body, is
non-toxic, and is usually
infused rather than, say, taken
orally.
The reason it is used in this sort of analysis is that it is freely filtered by the kidneys, and is neither reabsorbed, nor secreted.
The equation used to find the GFR is
GFR = (U * V) / P
Where
GFR is equal to the
urine inulin concentration times the
volume flow of urine, divided by the
plasma inulin
concentration.
Finally, make sure you don't confuse inulin with insulin.
These are my interpretation of my lecture notes, but I may have used some references from Hole's Anatomy and Physiology (Shier, Butler, Lewis) and Human Physiology (Vander, Sherman, Luciano)