Oxygen is consumed in body tissues or by cells in vitro at a rate that is nearly independent of the O concentration. As a model for either a small tissue region without blood vessels or a spher- oidal aggregate of isolated cells, consider steady O diffusion in a sphere of radius ro that consists of tightly packed cells. Because cell membranes offer negligible resistance to O diffusion, the sphere may be viewed as a homogeneous region in which O is consumed at a constant volumetric rate ko. The O concentration at the edge of the sphere is Co. (a) Determine the O concentration profile C(r), both when O is present throughout the sphere and when there is an anoxic core of radius re. Show how to predict the value of rc. (b) Oxygen consumption and diffusivity values representative of mammalian cells are ko = 30 M/s and D = 2 10 m s . What is the maximum radius ro that will be fully oxygenated if Co= 50 M? What radius will be oxygenated if Co = 200 M? (The lower value of Co is representative of body tissues and the higher one is approximately that for cells in contact with air.)