ABSTRACT
Mass transfer lies at the heart of physiology and provides major constraints on the metabolic rates and anatomy [Pries et al., 1996; Bunk, 1998] of living organisms, from the organization of organ networks to intracellular structures. Limitations on mass transport rates are major constraints for nutrient supply, waste elimination and information transmission at all of these levels. The primary functional units of metabolically active tissue, e.g., the Krogh tissue cylinders of muscle and brain, liver lobules and kidney nephrons, have evolved to just eliminate significant mass transfer limitations in the physiological state [Lightfoot, 1974]. Turnover rates of highly regulated enzymes are just on the slow side of diffusional limitations [Weisz, 1973]. Signal transport rates are frequently mass transport limited [Lauffenburger and Linderman, 1993], and very ingenious mechanisms have evolved to speed these processes [Berg and von Hippel, 1985; Bray, 1998; Francis and Palsson, 1997; Valee and Sheets, 1996]. In contrast elaborate membrane barriers organize and control intracellular reactions in even the simplest organisms.
