Two Examples from Biology The Quasi-Steady-State Assumption

Illustration 2.4 Two Examples from Biology The Quasi-Steady-State Assumption 

A well-mixed tissue region surrounding a blood vessel (a) uniform concentration in blood (b) concentration in blood distributed and in quasi-steady state. 

Substitution of Equation 2.13a into Equation 2.13b yields a single expression in either blood or tissue concentrations, which can be integrated in a straightforward fashion by separation of variables. We obtain for Cg in the first instance 

For tire case where a drug is suddenly introduced at a constant rate into the blood, i.e., Cg goes from zero to a constant value at f = 0, integration results in 

A second comment concerns the use of the model solutions. Equation 2.13e and Equation 2.13f. We start by noting that a semilog plot of Equation 2.13e, ln(Cgo - Cg) vs. t, using measured values of the drug concentration C, yields a slope with the value Vj + Vg)/KQ. In many cases of interest, the blood volume is much smaller than that of the surrounding 

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