Two-compartment irreversible system

Solving (9.17) for hiV, we obtain a time-varying hazard rate 

In the above relationships, u (t) is the step Heaviside function. The Laplace transform of the intravascular retention-time distribution is 

For both cases, the retention-time distribution functions fev (t) and fiv (t) are similar to the input functions vev (t) and Viv (t), respectively, defined for the deterministic models. The only difference is that in the stochastic consideration, the drug amounts are not included is these input functions. 

In conclusion, in order to account for the usual routes of administration, one has to expand the system by artifactual compartments and associated retentiontime distributions corresponding to the extravascular or intravascular routes. So, the expanded system may now be considered without environmental links and the administration protocol is simply expressed by the initial conditions in the input compartments. In this case, at least one of the m compartments must be considered as the input compartment. 

The most frequent situation is the heterogeneous absorption materialized by retention-time distributions Ai Erl(A, v) and A2 Exp ) for compartments 1 and 2, respectively. In this configuration, compartment 1 represents the heterogeneous absorption compartment and compartment 2 represents the distribution compartment that is the sampled compartment. The state probability 

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