[mug3n]

I don't really see this fit under any of the other specialty forums and this is a fairly common course in undergrad curricula, so forgive me if this is the wrong place.  Mods may feel free to move if this is inappropriate.

I haven't taken a chem course in a while so forgive me for sounding totally idiotic.  It's not an assignment question or anything, just a general understanding clarification.  For concentration curves (with time being the independent variable), let's say in the urine, so it's an excretion curve.  Why are the curves showing cumulative concentration of the drug/substance in the urine?  Why is it not possible to graph the instantaneous concentration at t=x?  

I've been a bit confused by this so I'd appreciate any insight.  For those confused I'm talking about something like this: http://en.wikipedia.org/wiki/Pharmacokinetics#Plasma_concentration_curves

[fledarmus]

I'm not sure I understand the question. The curves in the Wiki article you pointed to ARE showing instantaneous concentration of drug in the plasma at t=x. To find the cumulative concentration of the compound, you have to calculate the area under the curve (the integral). That gives you the total exposure to the drug.

In the urine, you aren't typically interested in the instantaneous concentration, but this is critical in the plasma. In urine, you are usually more interested in how fast you are excreting the drug, so the curve that you are plotting is the integral of the instantaneous concentration, and a point on that curve will tell you how much of the drug you administered has been excreted through the urine. There isn't any reason you can't plot the instantaneous concentration, it just isn't a very useful plot.