Re: why a low [K+] & a high [Na+] are commonly observed together?

Low [K+] & high [Na+] are commonly observed as ions forming a chemical gradient on either side of the cellular membranes of higher eukaryotes that establish the membrane potential. The membrane potential determines a cell's state of excitability or responsiveness to certain extracellular signalling events (e.g. nerve impulses). Typical skeletal muscle concentrations of Na+ are 145mM extracellular and 12mM intracellular; for K+, its 4mM extracellular and 155 mM intracellular (Hille, 1992 page 15). These precise ionic concentrations are maintained by Na+ and K+ fluxes through ion channels and pumps. This ioninc regulation leads to a resting membrane potential of -90 mV (in skeletal muscle). A more mathematical explaination between ionic gradients and membrane potentials is given by the Goldman equation (Hille, 1992, page 19)

Reference: Hille, B; Ionic Channels of Excitable Membranes, Sinauer Associates Inc.; Sunderland, Massachusetts; 1992

-Dmitri