protein binding model

Hi, I am trying to include the protein binding in my model and to estimate the unbound plasma concentration, using the total plasma concentration. The drug was taken orally. It seems that I can write the differential equations as below. A(2) and A(3) represent the unbound and bound amount, respectively. DADT(1)= -KA*A(1) DADT(2)= KA*A(1) – k23*A(2)*(Bmax-A(3))+k32*A(3)-k*A(2) DADT(3)= k23*A(2)*(Bmax-A(3))-k32*A(3) My question is how could I transfer the amount in compartment 3 [A(3)] into concentration since my observation is the sum of the concentration in compartment 2 and 3. Many thanks in advance. Kehua