[R-sig-dyn-mod] PBTK model - unexpected behavior

[Johannes Ranke]

Dear Jon,

I saw your mail on the r-sig-dyn-mod mailing list. The attachment did not get 
through. I think you need to either include the R code in the e-mail, or 
upload it to some website and provide a link.

Kind regards,

Johannes Ranke

Am Mittwoch, 17. Juni 2020, 19:04:35 CEST schrieb Haselman, Jon:
> Hello,
> 
> Attached is a very simple model that simulates growth of a larval amphibian
> over a 10 day period with calculation of the volume of distribution of
> toxicant. The apparent volume of distribution constant is 1 for simplicity
> sake and to test whether the model will simulate the true Vd equal to the
> body weight. However, the resulting data suggests the AVD constant is not
> staying constant because the Vd increases by a factor approximately equal
> to the time step in relation to the body weight. I apologize if I am
> overlooking something simple, but could someone please explain why this is
> happening and a possible solution. The body weight data turns out exactly
> as we would expect, but if the rest of our larger, more complex PBTK model
> is to work correctly, we need to see the output data for terms such as Vd
> being more in line with what we would expect.
> 
> I understand that Vd in this case is being treated as a state variable that
> will respond to a 'rate' - the same as growth rate is driving the body
> weight term. If Vd is not defined as a state variable, but is changing over
> time as a function of body weight and driving other equations in the model,
> how do we extract the behavior/data of that term in the model for
> verification purposes? It appears that only state variables can be
> 'tracked' and output by the ode function, but in this case when we track Vd
> as a state variable, it is inappropriately changing because it perhaps
> should not be a state variable.
> 
> Your help is MUCH appreciated.
> 
> Thank you,
> 
> Jon Haselman
> Toxicologist
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