Consider a growth factor (e. g. vegf) that is released in the extravascular space such that its concentration co in the extracellular space is constant and equal to 1 um. the goal is to deliver the vegf to the endothelium of a blood vessel passing through this space. the outer radius of the vessel (ro) is 200 um and the inner radius ri is 150 um. the thickness of the endothelial layer is 1 um. the diffusion coefficient of the vegf passing through the vessel wall is 1 x 10-7 cm/s, while the diffusion coefficient of the vegf passing through the endothelium is 1 x 10 cm²/s. you may assume that the concentration of vegf in flowing blood is essentially zero. find the average concentration of vegf in the endothelial lining assuming steady-state conditions.