Error in solving ODE - need help


I have to do a research project for my Systems Biology class and I am finding many difficulties in coding the system of ODEs that I have generated as I am really unexperienced in R.

Here is my code. When I compute it I get an

Error in eval(substitute(expr), data, enclos = parent.frame()) :
object 'rTr' not found

MergeModel <- function(Time, State, Pars) {
  with(as.list(c(State, Pars)), {
    # ------------------------------------------------
    # Treg equation (number 8)
    # ------------------------------------------------
    deathRate		<- rTr*Tr
    productionRate	<- lambdaTrI2*Tb*(I2/(KI2+I2))*(M/(M+KM))*(KTa/(Ta+KTa))
    GCRate		<- a*(1-e^(-g))*Tr

    dTrdt		<- productionRate + GCRate - deathRate
    # ------------------------------------------------
Pars	<- c(rTr = 1.97*10^(-1),
	lambdaTrI2 = 1.76*10^8,
	Tb = 1.55*10^(-7),
	KI2 = 5*10^(-7),
	I2 = 5.2*10^(-7),
	M = 10^(-3),
	KM = 3*10^(-3),
	KTa = 5*10^(-7),
	Ta = 1.56*10^(-7),
	a = 1.0483,
	g = 1 )

y0  <- c(Tr = 5.372*10^(-3))
timespan <- seq(0, 100, by = 1)
MergeModel.sol <- ode(y0, timespan, MergeModel, pars)

I do not understand why the Pars function seems to not be working... My report is due tonight so if somebody could help me out I would greatly appreciate it.
For some background, I am modeling a glucocorticoid treatment of sarcoidosis disease (called Merge Model), the equation presented here corresponds to the effect of this drug in a specific cell population called Treg (Tr).

Thank you very much!

In your code, you define Pars but then pass pars, these are different variables. I guess you have an object called pars in your workspace that doesn't contain Tr.

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