bulk concentration

Hi

I am working with a 2D axisymmetric geometry of the arterial wall. I am solving transport of diluted species (time dependent).

I would like to compute and represent the partitioning coefficient (k), defined as the tissue concentration at equilibrium normalized by the bulk concentration (k=cT/c_bulk), because I am simulating a test where the arterial wall is incubated in a drug solution at several bulk concentrations for 60 hours.

I do not understand well the concept of "bulk concentration". I know that the tissue concentration is a time dependent concentration, but is the "bulk concentration" a time dependent concentration or is it a constant value??

Thanks in advance,

Javier