Absorption site Blood Biologic membrane For practical purposes the value of C1-C2 may be taken simply as that of C and the equation written in the standard form for a first order rate equation: dc/dt=PC1For practical purposes the value of C1 -C2 may be taken simply as that of C1 and the equation written in the standard form for a first order rate equation: -dc/dt=PC1 Absorption site C1 Blood C2 Biologic membrane